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2005-08-24 | Enhancing the Alloy Analyzer with Patterns of Analysis | Formal techniques have been shown to be useful in the development of correct
software. But the level of expertise required of practitioners of these
techniques prohibits their widespread adoption. Formal techniques need to be
tailored to the commercial software developer. Alloy is a lightweight
specification language supported by the Alloy Analyzer (AA), a tool based on
off-the-shelf SAT technology. The tool allows a user to check interactively
whether given properties are consistent or valid with respect to a high-level
specification, providing an environment in which the correctness of such a
specification may be established. However, Alloy is not particularly suited to
expressing program specifications and the feedback provided by AA can be
misleading where the specification under analysis or the property being checked
contains inconsistencies. In this paper, we address these two shortcomings.
Firstly, we present a lightweight language called "Loy", tailored to the
specification of object-oriented programs. An encoding of Loy into Alloy is
provided so that AA can be used for automated analysis of Loy program
specifications. Secondly, we present some "patterns of analysis" that guide a
developer through the analysis of a Loy specification in order to establish its
correctness before implementation. | 0508109v1 |
2008-02-21 | First principles calculations of the electronic and geometric structure of $Ag_{27}Cu_{7}$ nanoalloy | \emph{Ab initio} calculations of the structure and electronic density of
states (DOS) of the perfect core-shell $Ag_{27}Cu_{7}$ nanoalloy attest to its
$D_{5h}$ symmetry and confirm that it has only 6 non-equivalent (2 $Cu$ and 4
$Ag$) atoms. Analysis of bond-length, average formation energy, heat of
formation of $Ag_{27}Cu_{7}$ and $L1_2$ $Ag-Cu$ alloys provide an explanation
for the relative stability of the former with respect to the other nanoalloys
in the same family. The HOMO-LUMO gap is found to be 0.77 eV, in agreement with
previous results. Analysis of the DOS of $Ag_{27}Cu_{7}$, $L1_2$ $Ag-Cu$ alloys
and related systems provides insight into the effects of low coordination,
contraction/expansion and the presence of foreign atoms on the DOS of $Cu$ and
$Ag$. While some characteristics of the DOS are reminiscent of those of the
phonon-stable $L1_2$ $Ag-Cu$ alloys, the $Cu$ and $Ag$ states hybridize
significantly in $Ag_{27}Cu_{7}$, compensating the $d$-band narrowing that each
atom undergoes and hindering the dip in the DOS found in the bulk alloys.
Charge density plots of $Ag_{27}Cu_{7}$ provide further insights into the
relative strengths of the various interatomic bonds. Our results for the
electronic and geometric structure of this nanoalloy can be explained in terms
of length and strength hierarchies of the bonds, which may have implications
also for the stability of alloy in any phase or size. | 0802.2987v1 |
2008-03-24 | Effect of static and dynamic disorder on electronic transport of $RCo_2$ compounds: a study of $Ho(Al_xCo_{1-x})_2$ alloys | We present experimental results on thermoelectric power ({\em S}) and
electrical resistivity ($\rho $) of pseudobinary alloys
Ho(Al$_x$Co$_{1-x}$)$_2$ ($0 \leq x \leq 0.1 $), in the temperature range 4.2 K
to 300 K. The work focuses on the effects of static (induced by alloying) and
dynamic (induced by temperature) disorder on the magnetic state and electronic
transport in a metallic system with itinerant metamagnetic instability. Spatial
fluctuations of the local magnetic susceptibility in the alloys lead to a
development of a partially ordered magnetic ground state of the itinerant 3d
electron system. This results in a strong increase of the residual resistivity
and a suppression of the temperature-dependent resistivity. Thermopower
exhibits a complex temperature variation in both the magnetically ordered and
in the paramagnetic state. This complex temperature variation is referred to
the electronic density of states features in vicinity of Fermi energy and to
the interplay of magnetic and impurity scattering. Our results indicate that
the magnetic enhancement of the Co 3d-band in RCo$_{\rm 2}$--based alloys upon
a substitution of Co by non-magnetic elements is mainly related to a
progressive localization of the Co -- 3d electrons caused by disorder. We show
that the magnitude of the resistivity jump at the Curie temperature for
RCo$_{\rm 2}$ compounds exhibiting a first order phase transition is a
non-monotonic function of the Curie temperature due to a saturation of the
3d--band spin fluctuation magnitude at high temperatures. | 0803.3380v3 |
2010-03-18 | Ab-initio description of heterostructural alloys: Thermodynamic and structural properties of Mg_x Zn_{1-x} O and Cd_x Zn_{1-x} O | Pseudobinary heterostructural alloys of ZnO with MgO or CdO are studied by
composing the system locally of clusters with varying ratio of cations. We
investigate fourfold (wurtzite structure) and sixfold (rocksalt structure)
coordination of the atoms. By means of density functional theory we study a
total number of 256 16-atom clusters divided into 22 classes for the wurtzite
structure and 16 classes for the rocksalt structure for each of the alloy
systems. The fraction with which each cluster contributes to the alloy is
determined for a given temperature T and composition x within (i) the
generalized quasi-chemical approximation, (ii) the model of a strict-regular
solution, and (iii) the model of microscopic decomposition. From the cluster
fractions we derive conclusions about the miscibility and the critical
compositions at which the average crystal structure changes. Thermodynamic
properties such as the mixing free energy and the mixing entropy are
investigated for the three different statistics. We discuss the consequences of
the two different local lattice structures for characteristic atomic distances,
cohesive energies, and the alloys' elasticities. The differences in the
properties of Mg_x Zn_{1-x} O and Cd_x Zn_{1-x} O are explained and discussed. | 1003.3614v2 |
2010-09-16 | First-principles calculations of phase transition, elasticity, and thermodynamic properties for TiZr alloy | tructural transformation, pressure dependent elasticity behaviors, phonon,
and thermodynamic properties of the equiatomic TiZr alloy are investigated by
using first-principles density-functional theory. Our calculated lattice
parameters and equation of state for $\alpha$ and $\omega$ phases as well as
the phase transition sequence of
$\alpha$$\mathtt{\rightarrow}$$\omega$$\mathtt{\rightarrow}$$\beta$ are
consistent well with experiments. Elastic constants of $\alpha$ and $\omega$
phases indicate that they are mechanically stable. For cubic $\beta$ phase,
however, it is mechanically unstable at zero pressure and the critical pressure
for its mechanical stability is predicted to equal to 2.19 GPa. We find that
the moduli, elastic sound velocities, and Debye temperature all increase with
pressure for three phases of TiZr alloy. The relatively large $B/G$ values
illustrate that the TiZr alloy is rather ductile and its ductility is more
predominant than that of element Zr, especially in $\beta$ phase. Elastic wave
velocities and Debye temperature have abrupt increase behaviors upon the
$\alpha$$\mathtt{\rightarrow}$$\omega$ transition at around 10 GPa and exhibit
abrupt decrease feature upon the $\omega$$\mathtt{\rightarrow}$$\beta$
transition at higher pressure. Through Mulliken population analysis, we
illustrate that the increase of the \emph{d}-band occupancy will stabilize the
cubic $\beta$ phase. Phonon dispersions for three phases of TiZr alloy are
firstly presented and the $\beta$ phase phonons clearly indicate its
dynamically unstable nature under ambient condition. Thermodynamics of Gibbs
free energy, entropy, and heat capacity are obtained by quasiharmonic
approximation and Debye model. | 1009.3073v3 |
2011-03-25 | Anisotropic intrinsic anomalous Hall effect in ordered 3dPt alloys | By performing first principles calculations we investigate the intrinsic
anomalous Hall conductivity (AHC) and its anisotropy in ordered L1o FePt, CoPt
and NiPt ferromagnets, and their intermediate alloys. We demonstrate that the
AHC in this family of compounds depends strongly on the direction of the
magnetization in the crystal. We predict that such pronounced orientational
dependence in combination with the general decreasing trend of the AHC when
going from FePt to NiPt leads to a sign change of the AHC upon rotating the
magnetization direction in the crystal of CoPt alloy. We also suggest that for
a range of concentration x in Co(x)Ni(1-x)Pt alloy it is possible to achieve a
complete quenching of the anomalous Hall current for a certain direction of the
magnetization in the crystal. By analyzing the spin-resolved AHC in 3dPt alloys
we endeavor to relate the overall trend of the AHC in these compounds to the
changes in their densities of d-states around the Fermi energy upon varying the
atomic number. Moreover, we show the generality of the phenomenon of
anisotropic anomalous Hall effect by demonstrating its occurrence within the
three-band tight-binding model. | 1103.4941v2 |
2012-10-10 | Determination of the crystal structures of In70-Ni30 and In70-Pd30 using perturbed angular correlation | According to phase diagrams based on x-ray measurements, In70-Pt30 has the
cubic Sn7Ir3 crystal structure (D8f, cI40) but the alloys In70-Ni30 and
In70-Pd30 have been variously reported to have either a cubic gamma-brass
(D81-3, cI52) or the Sn7Ir3 structures. In this study, hyperfine interaction
measurements are applied as an alternate method to identify phases. Perturbed
angular correlation (PAC) measurements were made of characteristic nuclear
quadrupole interactions of 111In/Cd probe atoms, and demonstrated a common,
characteristic "signature" of the Sn7Ir3 structure in all three alloys. The
Sn7Ir3 structure has two inequivalent Sn-sites with a 3:4 ratio of atoms and
point symmetries indicate that the electric-field gradients at both sites
should be axially symmetric. Measured perturbation functions for all three
alloys exhibited two axially symmetric quadrupole interaction signals having
the expected 3:4 ratio of amplitudes, as expected for the structure.
Furthermore, ratios of the two quadrupole interaction frequencies in each alloy
were characteristically large, with frequencies for probe atoms on In(3) sites
roughly five times greater than on In(4) sites. Taken together, these
observations confirm that all three phases have the Sn7Ir3 structure.
Quadrupole interaction frequencies are also reported for isostructural alloys
of gallium with Pt, Pd and Ni. Negligible inhomogeneous broadening was observed
in measurements near room temperature in all six phases, indicating excellent
atomic ordering at the stoichiometric 70:30 compositions. | 1210.3076v1 |
2012-11-16 | Localization for alloy-type models with non-monotone potentials | We consider a family of self-adjoint operators [H_\omega = - \Delta + \lambda
V_\omega, \quad \omega \in \Omega = \bigtimes_{k \in \ZZ^d} \RR,] on the
Hilbert space $\ell^2 (\ZZ^d)$ or $L^2 (\RR^d)$. Here $\Delta$ denotes the
Laplace operator (discrete or continuous), $V_\omega$ is a multiplication
operator given by the function $$V_\omega (x) = \sum_{k \in \ZZ^d} \omega_k
u(x-k) on $\ZZ^d$, or \quad V_\omega (x) = \sum_{k \in \ZZ^d} \omega_k U(x-k)
on $\RR^d$,$$ and $\lambda > 0$ is a real parameter modeling the strength of
the disorder present in the model. The functions $u:\ZZ^d \to \RR$ and $U:\RR^d
\to \RR$ are called single-site potential. Moreover, there is a probability
measure $\PP$ on $\Omega$ modeling the distribution of the individual
configurations $\omega \in \Omega$. The measure $\PP = \prod_{k \in \ZZ^d} \mu$
is a product measure where $\mu$ is some probability measure on $\RR$
satisfying certain regularity assumptions. The operator on $L^2 (\RR^d)$ is
called alloy-type model, and its analogue on $\ell^2 (\ZZ^d)$ discrete
alloy-type model.
This thesis refines the methods of multiscale analysis and fractional moments
in the case where the single-site potential is allowed to change its sign. In
particular, we develop the fractional moment method and prove exponential
localization for the discrete alloy-type model in the case where the support of
$u$ is finite and $u$ has fixed sign at the boundary of its support. We also
prove a Wegner estimate for the discrete alloy-type model in the case of
exponentially decaying but not necessarily finitely supported single-site
potentials. This Wegner estimate is applicable for a proof of localization via
multiscale analysis. | 1211.3891v1 |
2012-12-17 | Alloying-related trends from first principles: An application to the Ti--Al--X--N system | Tailoring and improving material properties by alloying is a long-known and
used concept. Recent research has demonstrated the potential of ab initio
calculations in understanding the material properties at the nanoscale. Here we
present a systematic overview of alloying trends when early-transition metals
(Y, Zr, Nb, Hf, Ta) are added in the Ti$_{1-x}$Al$_x$N system, routinely used
as a protective hard coating. The alloy lattice parameters tend to be larger
than the corresponding linearised Vegard's estimation, with the largest
deviation more than 2.5% obtained for Y$_{0.5}$Al$_{0.5}$N. The chemical
strengthening is most pronounced for Ta and Nb, although also causing smallest
elastic distortions of the lattice due to their atomic radii being comparable
with Ti and Al. This is further supported by the analysis of the electronic
density of states. Finally, mixing enthalpy as a measure of the driving force
for decomposition into the stable constituents, is enhanced by adding Y, Zr and
Nb, suggesting that the onset of spinodal decomposition will appear in these
cases for lower thermal loads than for Hf and Ta alloyed Ti$_{1-x}$Al$_x$N. | 1212.4052v2 |
2013-11-18 | High-Temperature Activated AB2 Nanopowders for Metal Hydride Hydrogen Compression | A reliable process for compressing hydrogen and for removing all contaminants
is that of the metal hydride thermal compression. The use of metal hydride
technology in hydrogen compression applications though, requires thorough
structural characterization of the alloys and investigation of their sorption
properties. The samples have been synthesized by induction - levitation melting
and characterized by Rietveld analysis of the X-Ray diffraction (XRD) patterns.
Volumetric PCI (Pressure-Composition Isotherm) measurements have been conducted
at 20, 60 and 90 oC, in order to investigate the maximum pressure that can be
reached from the selected alloys using water of 90oC. Experimental evidence
shows that the maximum hydrogen uptake is low since all the alloys are
consisted of Laves phases, but it is of minor importance if they have fast
kinetics, given a constant volumetric hydrogen flow. Hysteresis is almost
absent while all the alloys release nearly all the absorbed hydrogen during
desorption. Due to hardware restrictions, the maximum hydrogen pressure for the
measurements was limited at 100 bars. Practically, the maximum pressure that
can be reached from the last alloy is more than 150 bars. | 1311.4465v1 |
2014-09-11 | Microscopic, first-principles model of strain-induced interaction in concentrated size-mismatched alloys | The harmonic Kanzaki-Krivoglaz-Khachaturyan model of strain-induced
interaction is generalized to concentrated size-mismatched alloys and adapted
to first-principles calculations. The configuration dependence of both Kanzaki
forces and force constants is represented by real-space cluster expansions that
can be constructed based on the calculated forces. The model is implemented for
the fcc lattice and applied to Cu$_{1-x}$Au$_x$ and Fe$_{1-x}$Pt$_x$ alloys for
concentrations $x=0.25$, 0.5, and 0.75. The asymmetry between the $3d$ and $5d$
elements leads to large quadratic terms in the occupation-number expansion of
the Kanzaki forces and thereby to strongly non-pairwise long-range interaction.
The main advantage of the full configuration-dependent lattice deformation
model is its ability to capture this singular many-body interaction. The roles
of ordering striction and anharmonicity in Cu-Au and Fe-Pt alloys are assessed.
Although the harmonic force constants defined with respect to the unrelaxed
lattice are unsuitable for the calculation of the vibrational entropies, the
phonon spectra for ordered and disordered alloys are found to be in good
agreement with experimental data. The model is further adapted to concentration
wave analysis and Monte Carlo simulations by means of an auxiliary
multi-parametric real-space cluster expansion, which is used to find the
ordering temperatures. Good agreement with experiment is found for all systems
except CuAu$_3$ (due to the known failure of the generalized gradient
approximation) and FePt$_3$, where the discrepancy is likely due to the neglect
of magnetic disorder. | 1409.3596v1 |
2015-02-18 | Anomalous electrical conductivity in rapidly crystallized Cu${}_{50-x}$Zr${}_{x}$ (x = 50 - 66.6) alloys | Cu${}_{50-x}$Zr${}_{x}$ (x = 50, 54, 60 and 66.6) polycrystalline alloys were
prepared by arc-melting. The crystal structure of the ingots has been examined
by X-ray diffraction. Non-equilibrium martensitic phases with monoclinic
structure were detected in all the alloys except Cu${}_{33.4}$Zr${}_{66.6}$.
Temperature dependencies of electrical resistivity in the temperature range of
T = 4 - 300 K have been measured as well as room temperature values of Hall
coefficients and thermal conductivity. Electrical resistivity demonstrates
anomalous behavior. At the temperatures lower than 20 K, their temperature
dependencies are non-monotonous with pronounced minima. At elevated
temperatures they have sufficiently non-linear character which cannot be
described within framework of the standard Bloch--Gr\"{u}neisen model. We
propose generalized Bloch--Gr\"{u}neisen model with variable Debye temperature
which describes experimental resistivity dependencies with high accuracy. We
found that both the electrical resistivity and the Hall coefficients reveal
metallic-type conductivity in the Cu-Zr alloys. The estimated values of both
the charge carrier mobility and the phonon contribution to thermal and electric
conductivity indicate the strong lattice defects and structure disorder. | 1502.05297v1 |
2015-05-28 | High-throughput in-situ characterization and modelling of precipitation kinetics in compositionally graded alloys | The development of new engineering alloy chemistries is a time consuming and
iterative process. A necessary step is characterization of the
nano/microstructure to provide a link between the processing and properties of
each alloy chemistry considered. One approach to accelerate the identification
of optimal chemistries is to use samples containing a gradient in composition,
ie. combinatorial samples, and to investigate many different chemistries at the
same time. However, for engineering alloys, the final properties depend not
only on chemistry but also on the path of microstructure development which
necessitates characterization of microstructure evolution for each chemistry.
In this contribution we demonstrate an approach that allows for the in-situ,
nanoscale characterization of the precipitate structures in alloys, as a
function of aging time, in combinatorial samples containing a composition
gradient. The approach uses small angle x-ray scattering (SAXS) at a
synchrotron beamline. The Cu-Co system is used for the proof-of-concept and the
combinatorial samples prepared contain a gradient in Co from 0% to 2%. These
samples are aged at temperatures between 450{\textdegree}C and
550{\textdegree}C and the precipitate structures (precipitate size, volume
fraction and number density) all along the composition gradient are
simultaneously monitored as a function of time. This large dataset is used to
test the applicability and robustness of a conventional class model for
precipitation that considers concurrent nucleation, growth and coarsening and
the ability of the model to describe such a large dataset. | 1505.07658v2 |
2015-06-23 | Atomic-scale investigation of creep behavior in nanocrystalline Mg and Mg-Y alloys | Magnesium (Mg) and its alloys offer great potential for reducing vehicular
mass and energy consumption due to their inherently low densities.
Historically, widespread applicability has been limited by low strength
properties compared to other structural Al-, Ti- and Fe-based alloys. However,
recent studies have demonstrated high-specific-strength in a number of
nanocrystalline Mg-alloys. Even so, applications of these alloys would be
restricted to low-temperature automotive components due to microstructural
instability under high temperature creep loading. Hence, this work aims to gain
a better understanding of creep and associated deformation behavior of columnar
nanocrystalline Mg and Mg-yttrium (Y) (up to 3at.%Y(10wt.%Y)) with a grain size
of 5 nm and 10 nm. Using molecular dynamics (MD) simulations, nanocrystalline
magnesium with and without local concentrations of yttrium is subjected to
constant-stress loading ranging from 0 to 500 MPa at different initial
temperatures ranging from 473 to 723 K. In pure Mg, the analyses of the
diffusion coefficient and energy barrier reveal that at lower temperatures
(i.e., T < ~423K) the contribution of grain boundary diffusion to the overall
creep deformation is stronger that the contribution of lattice diffusion.
However, at higher temperatures (T > ~423K) lattice diffusion dominates the
overall creep behavior. Interestingly, for the first time, we have shown that
the(101-1),(101-2),(101-3) and (101-6) boundary sliding energy is reduced with
the addition of yttrium. This softening effect in the presence of yttrium
suggests that the experimentally observed high temperature beneficial effects
of yttrium addition is likely to be attributed to some combination of other
reported creep strengthening mechanisms or phenomena such as formation of
stable yttrium oxides at grain boundaries or increased forest dislocation-based
hardening. | 1506.07149v1 |
2015-08-12 | Localization of Fe d-states in Ni-Fe-Cu alloys and implications for ultrafast demagnetization | Ni$_{80}$Fe$_{20}$ (Py) and Py-Cu exhibit intriguing ultrafast
demagnetization behavior, where the Ni magnetic moment shows a delayed response
relative to the Fe [S. Mathias et al., PNAS {\bf 109}, 4792 (2012)]. To unravel
the mechanism responsible for this behavior, we have studied Py-Cu alloys for a
wide range of Cu concentrations using X-ray magnetic circular dichroism (XMCD).
The magnetic moments of Fe and Ni are found to respond very differently to Cu
alloying: Fe becomes a strong ferromagnet in Py, with the magnetic moment
largely unaffected by Cu alloying. In contrast, the Ni magnetic moment
decreases continuously with increasing Cu concentration. Our results are
corroborated by ab-initio calculations of the electronic structure, which we
discuss in the framework of virtual bound states (VBSs). For high Cu
concentrations, Ni exhibits VBSs below the Fermi level, which are likely
responsible for an increased orbital/spin magnetic ratio at high Cu
concentrations. Fe exhibits VBSs in the minority band, approximately 1 eV above
the Fermi level in pure Py, that move closer to the Fermi level upon Cu
alloying. A strong interaction between the VBSs and excited electrons above the
Fermi level enhances the formation of localized magnons at Fe sites, which
explains the different behavior between Fe and Ni during ultrafast
demagnetization. | 1508.03015v1 |
2015-12-17 | The statistical physics of multi-component alloys using KKR-CPA | We apply variational principles from statistical physics and the Landau
theory of phase transitions to multicomponent alloys using the
multiple-scattering theory of Korringa-Kohn-Rostoker (KKR) and the coherent
potential approximation (CPA). This theory is a multicomponent generalization
of the $S^{(2)}$ theory of binary alloys developed by G. M. Stocks, J. B.
Staunton, D. D. Johnson and others. It is highly relevant to the chemical phase
stability of high-entropy alloys as it predicts the kind and size of
finite-temperature chemical fluctuations. In doing so it includes effects of
rearranging charge and other electronics due to changing site occupancies. When
chemical fluctuations grow without bound an absolute instability occurs and a
second-order order-disorder phase transition may be inferred. The S$^{(2)}$
theory is predicated on the fluctuation-dissipation theorem; thus we derive the
linear response of the CPA medium to perturbations in site-dependent chemical
potentials in great detail. The theory lends itself to a natural interpretation
in terms of competing effects: entropy driving disorder and favorable pair
interactions driving atomic ordering. To further clarify interpretation we
present results for representative ternary alloys CuAgAu, NiPdPt, RhPdAg, and
CoNiCu within a frozen charge (or band-only) approximation. These results
include the so-called Onsager mean field correction that extends the
temperature range for which the theory is valid. | 1512.05797v1 |
2016-02-12 | Suppressing diborane production during the hydrogen release of metal borohydrides: The example of alloyed Al(BH$_4$)$_3$ | Aluminum borohydride (Al(BH$_4$)$_3$) is an example of a promising hydrogen
storage material with exceptional hydrogen densities by weight and volume and a
low hydrogen desorption temperature. But, unfortunately, its production of
diborane (B$_2$H$_6$) gases upon heating to release the hydrogen restricts its
practical use. To elucidate this issue, we investigate the properties of a
number of metal borohydrides with the same problem and find that the
electronegativity of the metal cation is not the best descriptor of diborane
production. We show that, instead, the closely related formation enthalpy is a
better descriptor and we find that diborane production is an exponential
function thereof. We conclude that diborane production is sufficiently
suppressed for formation enthalpies of $-$80 kJ/mol BH$_4$ or lower, providing
specific design guidelines to tune existing metal borohydrides or synthesize
new ones. We then use first-principles methods to study the effects of Sc
alloying in Al(BH$_4$)$_3$. Our results for the thermodynamic properties of the
Al$_{1-x}$Sc$_x$(BH$_4$)$_3$ alloy clearly show the stabilizing effect of Sc
alloying and thus the suppression of diborane production. We conclude that
stabilizing Al(BH$_4$)$_3$ and similar borohydrides via alloying or other means
is a promising route to suppress diborane production and thus develop viable
hydrogen storage materials. | 1602.04185v1 |
2016-11-02 | Nature of low dimensional structural modulations and relative phase stability in MoS2/WS2-ReS2 transition metal dichalcogenide alloys | We report on the various types of Peierls like two dimensional structural
modulations and relative phase stability of 2H and 1T poly-types in MoS2-ReS2
and WS2-ReS2 alloy system. Theoretical calculation predicts a polytype phase
transition cross over at ~50 at.% of Mo and W in ReS2 in both monolayer and
bulk form, respectively. Experimentally, two different types of structural
modulations at 50% and a modulation corresponding to trimerization at 75% alloy
composition is observed for MoS2-ReS2 and only one type of modulation is
observed at 50% WS2-ReS2 alloy system. The 50% alloy system is found to be a
suitable monolithic candidate for metal semiconductor transition with minute
external perturbation. ReS2 is known to be in 2D Peierls distorted 1Td
structure and forms a chain like superstructure. Incorporation of Mo and W
atoms in the ReS2 lattice modifies the metal-metal hybridization between the
cations and influences the structural modulation and electronic property of the
system. The results offer yet another effective way to tune the electronic
structure and poly-type phases of this class of materials other than
intercalation, strain, and vertical stacking arrangement. | 1611.00478v1 |
2017-01-25 | Composition dependent band offsets of ZnO and its ternary alloys | We report the calculated fundamental band gaps of \emph{wurtzite} ternary
alloys Zn$_{1-x}$M$_x$O (M=Mg, Cd) and the band offsets of the
ZnO/Zn$_{1-x}$M$_x$O heterojunctions, these II-VI materials are important for
electronics and optoelectronics. Our calculation is based on density functional
theory within the linear muffin-tin orbital (LMTO) approach where the modified
Becke-Johnson (MBJ) semi-local exchange is used to accurately produce the band
gaps, and the coherent potential approximation (CPA) is applied to deal with
configurational average for the ternary alloys. The combined LMTO-MBJ-CPA
approach allows one to simultaneously determine both the conduction band and
valence band offsets of the heterojunctions. The calculated band gap data of
the ZnO alloys scale as $E_g=3.35+2.33x$ and $E_g=3.36-2.33x+1.77x^2$ for
Zn$_{1-x}$Mg$_x$O and Zn$_{1-x}$Cd$_x$O, respectively, where $x$ being the
impurity concentration. These scaling as well as the composition dependent band
offsets are quantitatively compared to the available experimental data. The
capability of predicting the band parameters and band alignments of ZnO and its
ternary alloys with the LMTO-CPA-MBJ approach indicate the promising
application of this method in the design of emerging electronics and
optoelectronics. | 1701.07147v1 |
2017-03-07 | Thermodynamic Stabilization of Precipitates through Interface Segregation: Chemical Effects | Precipitation hardening, which relies on a high density of intermetallic
precipitates, is a commonly utilized technique for strengthening structural
alloys. Structural alloys are commonly strengthened through a high density of
small size intermetallic precipitates. At high temperatures, however, the
precipitates coarsen to reduce the excess energy of the interface, resulting in
a significant reduction in the strengthening provided by the precipitates. In
certain ternary alloys, the secondary solute segregates to the interface and
results in the formation of a high density of nanosize precipitates that
provide enhanced strength and are resistant to coarsening. To understand the
chemical effects involved, and to identify such systems, we develop a
thermodynamic model using the framework of the regular nanocrystalline solution
model. For various global compositions, temperatures and thermodynamic
parameters, equilibrium configuration of Mg-Sn-Zn alloy is evaluated by
minimizing the Gibbs free energy function with respect to the region-specific
(bulk solid-solution, interface and precipitate) concentrations and sizes. The
results show that Mg$_2$Sn precipitates can be stabilized to nanoscale sizes
through Zn segregation to Mg/Mg$_2$Sn interface, and the precipitates can be
stabilized against coarsening at high-temperatures by providing a larger Zn
concentration in the system. Together with the inclusion of elastic strain
energy effects and the input of computationally informed interface
thermodynamic parameters in the future, the model is expected to provide a more
realistic prediction of segregation and precipitate stabilization in ternary
alloys of structural importance. | 1703.02621v2 |
2017-07-13 | Anomalous random correlations of force constants on the lattice dynamical properties of disordered Au-Fe alloys | Au-Fe alloys are of immense interest due to their biocompatibility, anomalous
hall conductivity, and applications in various medical treatment. However,
irrespective of the method of preparation, they often exhibit a high-level of
disorder, with properties sensitive to the thermal or magnetic annealing
temperatures. We calculate lattice dynamical properties of Au$_{1-x}$Fe$_x$
alloys using density functional theory methods, where, being a multisite
property, reliable interatomic force constant (IFC) calculations in disordered
alloys remain a challenge. We follow a two fold approach: (1) an accurate IFC
calculation in an environment with nominally zero chemical pair correlations to
mimic the homogeneously disordered alloy; and (2) a configurational averaging
for the desired phonon properties (e.g., dispersion, density of states, and
entropy). We find an anomalous change in the IFC's and phonon dispersion (split
bands) near $x$=0.19, which is attributed to the local stiffening of the Au-Au
bonds when Au is in the vicinity of Fe. Other results based on mechanical and
thermo-physical properties reflect a similar anomaly: Phonon entropy, e.g.,
becomes negative below $x$=0.19, suggesting a tendency for chemical unmixing,
reflecting the onset of miscibility gap in the phase diagram. Our results match
fairly well with reported data, wherever available. | 1707.04060v1 |
2017-07-13 | Microstructural and magnetic property evolution with different heat-treatment conditions in an alnico alloy | Further property enhancement of alnico, an attractive near-term,
non-rare-earth permanent magnet alloy system, primarily composed of Al, Ni, Co,
and Fe, relies on improved morphology control and size refinement of its
complex spinodally decomposed nanostructure that forms during heat-treatment.
Using a combination of transmission electron microscopy and atom probe
tomography techniques, this study evaluates the magnetic properties and
microstructures of an isotropic 32.4Fe-38.1Co-12.9Ni-7.3Al-6.4Ti-3.0Cu
(wt.$\%$) alloy in terms of processing parameters such as annealing
temperature, annealing time, application of an external magnetic field, as well
as low-temperature "draw" annealing. Optimal spinodal morphology and spacing is
formed within a narrow temperature and time range ($\sim 840 \unicode{x2103}$
and 10 min during thermal-magnetic annealing (MA). The ideal morphology is a
mosaic structure consisting of periodically arrayed $\sim 40$ nm diameter
(Fe-Co)-rich rods ($\alpha_1$ phase) embedded in an (Al-Ni)-rich ($\alpha_2$
phase) matrix. A Cu-enriched phase with a size of $\sim$ 3-5 nm is located at
the corners of two adjacent $\{110\}$ facets of the $\alpha_1$ phase. The MA
process significantly increased remanence ($B_\text{r}$) ($\sim$ 40-70 $\%$) of
the alloy due to biased elongation of the $\alpha_1$ phase along the
$\langle100\rangle$ crystallographic direction, which is closest in orientation
to the applied magnetic field. The optimum magnetic properties of the alloy
with an intrinsic coercivity ($H_\text{cj}$) of 1845 Oe and a maximum energy
product ($BH_\text{max}$) of 5.9 MGOe were attributed to the uniformity of the
mosaic structure. | 1707.04165v1 |
2017-08-09 | Materials selection rules for amorphous complexion formation in binary metallic alloys | Complexions are phase-like interfacial features that can influence a wide
variety of properties, but the ability to predict which material systems can
sustain these features remains limited. Amorphous complexions are of particular
interest due to their ability to enhance diffusion and damage tolerance
mechanisms, as a result of the excess free volume present in these structures.
In this paper, we propose a set of materials selection rules aimed at
predicting the formation of amorphous complexions, with an emphasis on (1)
encouraging the segregation of dopants to the interfaces and (2) lowering the
formation energy for a glassy structure. To validate these predictions, binary
Cu-rich metallic alloys encompassing a range of thermodynamic parameter values
were created using sputter deposition and subsequently heat treated to allow
for segregation and transformation of the boundary structure. All of the alloys
studied here experienced dopant segregation to the grain boundary, but
exhibited different interfacial structures. Cu-Zr and Cu-Hf formed nanoscale
amorphous intergranular complexions while Cu-Nb and Cu-Mo retained crystalline
order at their grain boundaries, which can mainly be attributed to differences
in the enthalpy of mixing. Finally, using our newly formed materials selection
rules, we extend our scope to a Ni-based alloy to further validate our
hypothesis, as well as make predictions for a wide variety of transition metal
alloys. | 1708.02971v2 |
2017-11-29 | Mechanical consequences of dynamically loaded NiTi wires under typical actuator conditions in rehabilitation and neuroscience | In the field of rehabilitation and neuroscience shape memory alloys play a
crucial role as lightweight actuators. Devices are exploiting the shape memory
effect by transforming heat into mechanical work. In rehabilitation
applications, dynamic loading of the respective device occurs, which in turn
influences the mechanical consequences of the phase transforming alloy. Hence
in this work, dynamic thermomechanical material behavior of temperature
triggered phase transforming NiTi shape memory alloy wires with different
chemical compositions and geometries is experimentally investigated. Storage
modulus and mechanical loss factor of NiTi alloys at different temperatures and
loading frequencies are analyzed under force controlled conditions.
Counterintuitive storage modulus and loss factor dependent trends regarding the
loading frequency dependency of the mechanical properties on the materials
composition and geometry are hence obtained. It could be revealed that loss
factors show a pronounced loading frequency dependency, whereas the storage
modulus was not affected. It is shown that force controlled conditions lead to
a lower storage modulus than expected. Further it turned out that a simple
empirical relation can capture the characteristic temperature dependency of the
storage modulus, which is an important input relation for modeling the
rehabilitation device behavior under different dynamic and temperature loading
conditions, taking directly into account the material behavior of the shape
memory alloy. | 1711.11079v1 |
2018-01-16 | Magnetic irreversibility and pinning force density in the Mo$_{100-x}$Re$_x$ alloy superconductors | We have measured the isothermal field dependence of magnetization of the
Mo$_{100-x}$Re$_x$ (15 $\leq$ x $\leq$ 48) alloys, and have estimated the
critical current and pinning force density from these measurements. We have
performed structural characterization of the above alloys using standard
techniques, and analyzed the field dependence of critical current and pinning
force density using existing theories. Our results indicate that dislocation
networks and point defects like voids and interstitial imperfections are the
main flux line pinning centres in the Mo$_{100-x}$Re$_x$ alloys in the
intermediate fields, i.e., in the small bundle flux line pinning regime. In
this regime, the critical current density is also quite robust against
increasing magnetic field. In still higher fields, the critical current density
is affected by flux creep. In the low field regime, on the other hand, the
pinning of the flux lines seems to be influenced by the presence of two
superconducting energy gaps in the Mo$_{100-x}$Re$_x$ alloys. This modifies the
field dependence of critical current density, and also seems to contribute to
the asymmetry in the magnetic irreversibility exhibited by the isothermal field
dependence of magnetization | 1801.05093v1 |
2018-03-12 | Understanding the mechanical properties of reduced activation steels | Reduced activation ferritic/martensitic (RAFM) steels are structural
materials with potential application in Generation-IV fission and fusion
reactors. We use density-functional theory to scrutinize the micro-mechanical
properties of the main alloy phases of three RAFM steels based on the
body-centered cubic FeCrWVMn solid solution. We assess the lattice parameters
and elastic properties of ferromagnetic $\alpha$-Fe and Fe$_{91}$Cr$_{9}$,
which are the main building blocks of the RAFM steels, and present a detailed
analysis of the calculated alloying effects of V, Cr, Mn, and W on the
mechanical properties of Fe$_{91}$Cr$_{9}$. The composition dependence of the
elastic parameters is decomposed into electronic and volumetric contributions
and studied for alloying levels that cover the typical intervals in RAFM
steels. A linear superposition of the individual solute effects on the
properties of Fe$_{91}$Cr$_{9}$ is shown to provide an excellent approximation
for the \emph{ab initio} values obtained for the RAFM steels. The intrinsic
ductility is evaluated through Rice's phenomenological theory using the surface
and unstable stacking fault energies, and the predictions are contrasted with
those obtained by empirical criteria. Alloying with V or W is found to enhance
the ductility, whereas additional Cr or Mn turns the RAFM base alloys more
brittle. | 1803.04178v1 |
2018-05-10 | First-principles investigation on diffusion mechanism of alloying elements in dilute Zr alloys | Impurity diffusion in Zr is potentially important for many applications of Zr
alloys, and in particular for their use of nuclear reactor cladding. However,
significant uncertainty presently exists about which elements are vacancy vs.
interstitial diffusers, which can inhibit understanding and prediction of their
behavior under different temperature, irradiation, and alloying conditions.
Therefore, first-principles calculations based on density functional theory
(DFT) have been employed to predict the temperature-dependent dilute impurity
diffusion coefficients for 14 substitutional alloying elements in hexagonal
closed packed (HCP) Zr. Vacancy-mediated diffusion was modeled with the
eight-frequency model. Interstitial contributions to diffusion are estimated
from interstitial formation and select migration energies. Formation energies
for each impurity in nine high-symmetry interstitial sites were determined,
including significant effects of thermal expansion. The dominant diffusion
mechanism of each solute in HCP Zr was identified in terms of the calculated
vacancy-mediated activation energy, lower and upper bounds of interstitial
activation energy, and the formation entropy, suggesting a rough relation with
the metallic radii of solutes. It is predicted that Cr, Cu, V, Zn, Mo, W, Au,
Ag, Al, Nb, Ta and Ti all diffuse predominantly by an interstitial mechanism,
while Hf, Zr, and Sn are likely to be predominantly vacancy-mediated diffusers
at low temperature and interstitial diffusers at high temperature, although the
identification of mechanisms for these elements at high-temperature is quite
uncertain. | 1805.04128v1 |
2018-11-29 | Magnetocrystalline Anisotropy of Fe-based $L1_0$ Alloys: Validity of Approximate Methods to Treat the Spin-Orbit Interaction | First-principles calculations are used to gauge different levels of
approximation to calculate the magnetocrystalline anisotropy energies (MAE) of
five $L1_0$ FeMe alloys (Me=Co, Cu, Pd, Pt, Au). We find that a second-order
perturbation (2PT) treatment of the spin-orbit interaction (SOI) breaks down
for the alloys containing heavier ions, while it provides a very accurate
description of the MAE behaviour of FeCo, FeCu, and FePd. Moreover, the
robustness of the 2PT approximation is such that in these cases it accounts for
the MAE of highly-non-neutral alloys and, thus, it can be used to predict their
performance when dopants are present or when they are subject to applied gate
bias, which are typical conditions in working magnetoelectric devices. We also
observe that switching of the easy axis direction can be induced in some of
these alloys by addition or removal of, at least, one electron per cell. In all
cases, the details of the bandstructure are responsible for the finally
observed MAE value and, therefore, suggest a limited predicting power of models
based on the expected orbital moment values and bandwidths. Finally, we have
confirmed the importance of various calculation parameters to obtain converged
MAE values, in particular, those related to the accuracy of the Fermi level
determination. | 1811.12100v1 |
2019-11-12 | Semiconducting SiGeSn High-Entropy Alloy: A Density Functional Theory Study | High-entropy alloys (HEAs), which have been intensely studied due to their
excellent mechanical properties, generally refer to alloys with multiple
equimolar or nearly equimolar elements. According to this definition, Si-Ge-Sn
alloys with equal or comparable concentrations of the three Group IV elements
belong to the category of HEAs. As a result, the equimolar elements of Si-Ge-Sn
alloys likely cause their atomic structures to exhibit the same core effects of
metallic HEAs such as lattice distortion. Here we apply density functional
theory (DFT) calculations to show that the SiGeSn HEA indeed exhibits a large
local distortion effect. Unlike metallic HEAs, our Monte Carlo and DFT
calculations show that the SiGeSn HEA exhibits no chemical short-range order
due to the similar electronegativity of the constituent elements, thereby
increasing the configurational entropy of the SiGeSn HEA. Hybrid density
functional calculations show that the SiGeSn HEA remains semiconducting with a
band gap of 0.38 eV, promising for economical and compatible mid-infrared
optoelectronics applications. We then study the energetics of neutral single
Si, Ge, and Sn vacancies and (expectedly) find wide distributions of vacancy
formation energies, similar to those found in metallic HEAs. However, we also
find anomalously small lower bounds (e.g., 0.04 eV for a Si vacancy) in the
energy distributions, which arise from the bond reformation near the vacancy.
Such small vacancy formation energies and their associated bond reformations
retain the semiconducting behavior of the SiGeSn HEA, which may be a signature
feature of a semiconducting HEA that differentiates from metallic HEAs. | 1911.04677v1 |
2020-01-09 | Tuning of structural phase, magnetic spin order and electrical conductivity in mechanical alloyed material of alpha-Fe2O3 and alpha-Cr2O3 oxides | Alpha-Fe2O3 and alpha-Cr2O3 has been mechanical alloyed to prepare Fe1-xCrxO3
oxides for x = 0.2-0.8. Synchrotron X-ray diffraction and Raman spectra have
shown inhomogeneous structure of {\alpha}-Fe2O3 and {\alpha}-Cr2O3 phases in
as-alloyed samples. The as-alloyed samples have shown soft ferromagnetic
properties with signature of two Morin transitions. The heat treatment of
as-alloyed samples has homogenized structure and successfully incorporated the
Cr atoms into the lattice sites of Fe atoms in {\alpha}-Fe2O3. The magnetic and
electrical properties have been modified in the heat treated samples. For
example, canted antiferromagnetic order has been appeared as an effect of heat
treatment, irrespective of the Cr content in Fe1-xCrxO3. The magnetic field
induced spin flop transition has been observed at a critical magnetic field
that depends on Cr content in the system. The M\"ossbauer spectrum at room
temperature has been fitted with two sextets. The variation of M\"ossbauer
parameters suggest a distribution of magnetic spin order between Fe and Cr ions
in the rhombohedral structure of Fe1-xCrxO3. The electrical conductivity,
derived from current-voltage characteristics of the heat treated samples, has
been enhanced by increasing Cr content in alpha-Fe2O3 structure. The
experimental results have been explained based on the theoretical models
available in literature. | 2001.02831v1 |
2021-07-12 | Machine-learning potentials enable predictive $\textit{and}$ tractable high-throughput screening of random alloys | We present an automated procedure for computing stacking fault energies in
random alloys from large-scale simulations using moment tensor potentials
(MTPs) with the accuracy of density functional theory (DFT). To that end, we
develop an algorithm for training MTPs on random alloys. In the first step, our
algorithm constructs a set of ~10000 or more training candidate configurations
with 50-100 atoms that are representative for the atomic neighborhoods
occurring in the large-scale simulation. In the second step, we use active
learning to reduce this set to ~100 most distinct configurations - for which
DFT energies and forces are computed and on which the potential is ultimately
trained. We validate our algorithm for the MoNbTa medium-entropy alloy by
showing that the MTP reproduces the DFT $\frac{1}{4}[111]$ unstable stacking
fault energy over the entire compositional space up to a few percent.
Contrary to state-of-the-art methods, e.g., the coherent potential
approximation (CPA) or special quasi-random structures (SQSs), our algorithm
naturally accounts for relaxation, is not limited by DFT cell sizes, and opens
opportunities to efficiently investigate follow-up problems, such as chemical
ordering. In a broader sense, our algorithm can be easily modified to compute
related properties of random alloys, for instance, misfit volumes, or grain
boundary energies. Moreover, it forms the basis for an efficient construction
of MTPs to be used in large-scale simulations of multicomponent systems. | 2107.05620v2 |
2019-01-07 | Strain and Band-Gap Engineering in Ge-Sn Alloys via P Doping | Ge with a quasi-direct band gap can be realized by strain engineering,
alloying with Sn, or ultrahigh n-type doping. In this work, we use all three
approaches together to fabricate direct-band-gap Ge-Sn alloys. The heavily
doped n-type Ge-Sn is realized with CMOS-compatible nonequilibrium material
processing. P is used to form highly doped n-type Ge-Sn layers and to modify
the lattice parameter of P-doped Ge-Sn alloys. The strain engineering in
heavily-P-doped Ge-Sn films is confirmed by x-ray diffraction and micro Raman
spectroscopy. The change of the band gap in P-doped Ge-Sn alloy as a function
of P concentration is theoretically predicted by density functional theory and
experimentally verified by near-infrared spectroscopic ellipsometry. According
to the shift of the absorption edge, it is shown that for an electron
concentration greater than 1x10^20 cm-3 the band-gap renormalization is
partially compensated by the Burstein-Moss effect. These results indicate that
Ge-based materials have high potential for use in near-infrared optoelectronic
devices, fully compatible with CMOS technology. | 1901.01721v1 |
2019-10-12 | A phase-field approach for modeling equilibrium solute segregation at the interphase boundary in binary alloys | A number of experimental and theoretical findings in age hardening alloys
suggest that specific solute elements preferentially segregate to and reduce
the energy of the interphase boundary (IB). This segregation mechanism can
stabilize the precipitation microstructure against coarsening, allowing higher
operating temperatures in structural applications. Herein, we present a phase
field model of solute segregation to IBs that separate matrix and precipitate
phases in binary alloys. The proposed modeling framework is capable of
capturing bulk thermodynamics and interfacial free energies, while also
accounting for various mass transport mechanisms. Analytical equilibrium
solutions of one-dimensional systems are presented, and excess IB quantities
are evaluated independent of the Gibbs dividing surface convention. With the
aid of the parallel tangent construction, IB segregation isotherms are
established in terms of the alloy composition and the model parameters
describing the free energy functions. Under the regular solution approximation,
computational studies elucidating the dependence of the IB energy and
segregation levels on temperature and free energy model parameters are
presented. We show that the model is consistent with the Gibbs adsorption
equation; therefore, it is possible to compare the adsorption behavior
predicted by the model parameters with experiments and atomistic simulations.
Future work on extending the model to ternary alloys, and incorporating the
effect of elastic interactions on IB segregation is expected. | 1910.05606v2 |
2018-12-26 | Accurate high-resolution depth profiling of magnetron sputtered transition metal alloy films containing light species: A multi-method approach | We present an assessment of a multi-method approach based on ion beam
analysis to obtain high-resolution depth profiles of the total chemical
composition of complex alloy systems. As a model system we employ an alloy
based on several transition metals and containing light species. Samples have
been investigated by a number of different ion-beam based techniques, i.e.,
Rutherford Backscattering Spectrometry, Particle-Induced X-ray Emission,
Elastic Backscattering Spectrometry and Time-of-Flight/Energy Elastic Recoil
Detection Analysis. Sets of spectra obtained from these different techniques
were analyzed both independently and following an iterative and self-consistent
approach yielding a more accurate depth profile of the sample, including both
metallic heavy constituents (Cr, Fe and Ni) as well as the rather reactive
light species (C, O) in the alloy. A quantitative comparison in terms of
achievable precision and accuracy is made and the limitations of the single
method approach are discussed for the different techniques. The multi-method
approach is shown to yield significantly improved and accurate information on
stoichiometry, depth distribution, and thickness of the alloy with the
improvements being decisive for a detailed correlation of composition to the
material properties such as corrosion strength. The study also shows the
increased relative importance of experimental statistics for the achievable
accuracy in the multi-method approach. | 1812.10340v2 |
2019-02-07 | BInGaN alloys nearly lattice-matched to GaN for high-power high-efficiency visible LEDs | InGaN-based visible LEDs find commercial applications for solid-state
lighting and displays, but lattice mismatch limits the thickness of InGaN
quantum wells that can be grown on GaN with high crystalline quality. Since
narrower wells operate at a higher carrier density for a given current density,
they increase the fraction of carriers lost to Auger recombination and lower
the efficiency. The incorporation of boron, a smaller group-III element, into
InGaN alloys is a promising method to eliminate the lattice mismatch and
realize high-power, high-efficiency visible LEDs with thick active regions. In
this work we apply predictive calculations based on hybrid density functional
theory to investigate the thermodynamic, structural, and electronic properties
of BInGaN alloys. Our results show that BInGaN alloys with a B:In ratio of 2:3
are better lattice matched to GaN compared to InGaN and, for indium fractions
less than 0.2, nearly lattice matched. Deviations from Vegard's law appear as
bowing of the in-plane lattice constant with respect to composition. Our
thermodynamics calculations demonstrate that the solubility of boron is higher
in InGaN than in pure GaN. Varying the Ga mole fraction while keeping the B:In
ratio constant enables the adjustment of the (direct) gap in the 1.75-3.39 eV
range, which covers the entire visible spectrum. Holes are strongly localized
in non-bonded N 2p states caused by local bond planarization near boron atoms.
Our results indicate that BInGaN alloys are promising for fabricating nitride
heterostructures with thick active regions for high-power, high-efficiency
LEDs. | 1902.02692v1 |
2020-04-09 | Magnetic Damping in Epitaxial Fe Alloyed with Vanadium and Aluminum | To develop low-moment, low-damping metallic ferromagnets for power-efficient
spintronic devices, it is crucial to understand how magnetic relaxation is
impacted by the addition of nonmagnetic elements. Here, we compare magnetic
relaxation in epitaxial Fe films alloyed with light nonmagnetic elements of V
and Al. FeV alloys exhibit lower intrinsic damping compared to pure Fe, reduced
by nearly a factor of 2, whereas damping in FeAl alloys increases with Al
content. Our experimental and computational results indicate that reducing the
density of states at the Fermi level, rather than the average atomic number,
has a more significant impact in lowering damping in Fe alloyed with light
elements. Moreover, FeV is confirmed to exhibit an intrinsic Gilbert damping
parameter of $\simeq$0.001, among the lowest ever reported for ferromagnetic
metals. | 2004.04840v3 |
2020-05-18 | Band Alignments of Emerging Wurtzite BAlN and BGaN Semiconductors | The wurtzite III-Nitrides family of semiconductors, which include the
compounds GaN, InN, and AlN, along with their derivative ternary alloys, is
highly priced for its wide range of bandgaps, lattice constant tunability, high
breakdown voltages, and thermal and chemical stability. The incorporation of
wurtzite BxAl1-xN and BxGa1-xN ternary alloys into this family introduces an
even larger range of bandgaps, lattice constants, and refractive indices, which
indicates their potential in the fields of optoelectronics and power devices.
An important parameter in the design of cutting edge devices is the band
alignment between the different alloys. In our work, the natural band offset
values between wz-BxAl1-xN and wz-BxGa1-xN alloys were investigated using ab
initio simulations. The Vienna Ab initio Simulation Package was used to perform
density functional theory calculations in order to obtain lattice parameters,
band gap energies, and relative electrostatic potential lineups. Through these
calculations, we were able to quantify the natural band offset values for the
materials of interest, and as such were able to identify some general
qualitative features associated with the different alloys we studied. As the
growth and fabrication of wz-BAlN and wz-BGaN crystals matures, we hope that
our results can provide a theoretical basis for design and analysis of
cutting-edge devices. | 2005.08407v1 |
2020-05-22 | Photo-degradation Protection in 2D In-Plane Heterostructures Revealed by Hyperspectral Nanoimaging: the Role of Nano-Interface 2D Alloys | Single-layer heterostructures exhibit striking quasiparticle properties and
many-body interaction effects that hold promise for a range of applications.
However, their properties can be altered by intrinsic and extrinsic defects,
thus diminishing their applicability. Therefore, it is of paramount importance
to identify defects and understand 2D materials' degradation over time using
advanced multimodal imaging techniques as well as stabilize degradation via
built-in interface protection. Here we implemented a liquid-phase precursor
approach to synthesize 2D in-plane MoS2-WS2 heterostructures exhibiting
nanoscale alloyed interfaces and map exotic interface effects during
photo-degradation using a novel combination of hyperspectral tip-enhanced
photoluminescence, Raman and near-field nanoscopy. Surprisingly, 2D alloyed
regions exhibit remarkable thermal and photo-degradation stability providing
protection against oxidation. Coupled with surface and interface strain, 2D
alloy regions create localized potential wells that concentrate excitonic
species via a charge carrier funneling effect. These results provide a clear
understanding of the importance of 2D alloys as systems able to withstand
degradation effects over time, and could be now used to stabilize
optoelectronic devices based on 2D materials. | 2005.11361v1 |
2020-05-28 | A Comparative Analysis of Inconel 718 Made by Additive Manufacturing and Suction Casting: Microstructure Evolution in Homogenization | Homogenization is one of the critical stages in the post-heat treatment of
additive manufacturing (AM) component to achieve uniform microstructure. During
homogenization, grain coarsening could be an issue to reserve strength, which
requires careful design of both time and temperature. Therefore, a proper
design of homogenization becomes particularly important for AM design, for
which work hardening is usually no longer an option. In this work, we
discovered an intriguing phenomenon during homogenization of suction-cast and
AM Inconel 718 superalloys. Through both short and long-term isothermal heat
treatments at 1180{\deg}C, we observed an abnormal grain growth in the
suction-cast alloy but continuous recrystallization in the alloy made by laser
powder bed fusion (LPBF). The grain size of AM samples keeps as small as 130
{\mu}m and is even slightly reduced after homogenization for 12 hours. The
homogeneity of Nb in the AM alloys is identified as the critical factor for NbC
formation, which further influences the recrystallization kinetics at
1180{\deg}C. Multi-type dislocation behaviors are studied to elucidate the
grain refinement observed in homogenized alloys after LPBF. This work provides
a new pathway on microstructure engineering of AM alloys for improved
mechanical performance superior to traditionally manufactured ones. | 2005.14089v2 |
2020-08-01 | Trends in elastic properties of Ti-Ta alloys from first-principles calculations | The martensitic start temperature ($M_{\text{s}}$) is a technologically
fundamental characteristic of high-temperature shape memory alloys. We have
recently shown [Phys. Rev. B 94, 224104 (2016)] that the two key features in
describing the composition dependence of $M_\text{s}$ are the $T=0$ K phase
stability and the difference in vibrational entropy which, within the Debye
model, is directly linked to the elastic properties. Here, we use density
functional theory together with special quasi-random structures to study the
elastic properties of disordered martensite and austenite Ti-Ta alloys as a
function of composition. We observe a softening in the tetragonal shear elastic
constant of the austenite phase at low Ta content and a \emph{non-linear}
behavior in the shear elastic constant of the martensite. A minimum of 12.5$\%$
Ta is required to stabilize the austenite phase at $T = 0$ K. Further, the
shear elastic constants and Young's modulus of martensite exhibit a maximum for
Ta concentrations close to 30$\%$. Phenomenological, elastic-constant-based
criteria suggest that the addition of Ta enhances the strength, but reduces the
ductile character of the alloys. In addition, the directional elastic
stiffness, calculated for both martensite and austenite, becomes more isotropic
with increasing Ta content. The reported trends in elastic properties as a
function of composition may serve as a guide in the design of alloys with
optimized properties in this interesting class of materials. | 2008.00165v1 |
2021-02-28 | Accommodation mechanisms in strain-transformable titanium alloys | A new $\beta$-metastable Ti-alloy is designed with the aim to obtain a TWIP
alloy but positioned at the limit between the TRIP/TWIP and the TWIP dominated
regime. The designed alloy exhibits a large ductility combined with an elevated
and stable work-hardening rate. Deformation occurring by formation and
multiplication of {332}<113> twins is evidenced and followed by in-situ
electron microscopy, and no primary stress induced martensite is observed.
Since microstructural investigations of the deformation mechanisms show a
highly heterogeneous deformation, the reason of the large ductility is then
investigated. The spatial strain distribution is characterized by micro-scale
digital image correlation, and the regions highly deformed are found to stand
at the crossover between twins, or at the intersection between deformation
twins and grain boundaries. Detailed electron back-scattered imaging in such
regions of interest finally allowed to evidence the formation of thin needles
of stress induced martensite. The latter is thus interpreted as an
accommodation mechanism, relaxing the local high strain fields, which ensures a
large and stable plastic deformation of this newly designed Ti-alloy. | 2103.00440v2 |
2021-03-26 | Dissecting functional degradation in NiTi shape-memory-alloys containing amorphous regions via atomistic simulations | Molecular dynamics simulations are performed to provide a detailed
understanding of the functional degradation of shape memory alloys at small
scale. The origin of the experimentally reported accumulation of plastic
deformation and the anomalous sudden increase of the residual strain under
cyclic mechanical loading are explained by detailed insights into the relevant
atomic scale processes. Our work reveals that the mechanical response of
shape-memory-alloy pillars under cyclic compression is significantly influenced
by the presence of an amorphous-like surface region as experimentally induced
by focused ion beam milling. The main factor responsible for the observed
degradation of superelasticity under cyclic loading is the accumulated plastic
deformation and the resultant retained martensite originating from a synergetic
contribution of the amorphous and crystalline shape-memory-alloy regions. We
show that the reported sudden diminishment of the stress plateaus and
hysteresis under cyclic loading is caused by the increased stability of the
martensite phase due to the presence of the amorphous phase. Based on the
identified mechanism responsible for the degradation, we validate reported
methods of recovering the superelasticity and propose a new method to prohibit
the synergetic contribution of the amorphous and crystalline regions, such as
to achieve a sustainable operation of shape memory alloys at small scale. | 2103.14319v2 |
2022-02-25 | Role of Zirconium Conversion Coating in Corrosion Performance of Aluminum Alloys: An Integrated First-Principles and Multiphysics Modeling Approach | A variety of chromate-free conversion coatings are being actively
investigated to improve the corrosion performance of light-weight alloys for
aerospace and defense applications. Advancing conversion coating, however,
requires an in-depth understanding of the underlying corrosion mechanisms in
order to rationally design sustainable coatings. Here, we present a multiscale
modeling approach to predict corrosion performance of metallic materials, with
a focus on localized corrosion of Cu-containing aluminum alloys coated with
ZrO2 layer. First-principles and transition-state theory are used to implement
the kinetics model, which includes electrolyte-metal interfacial reactions. The
modeling framework systematically characterizes and couples multiple
electrochemical and physical (e.g., transport) phenomena to explore
interrelationships between pit morphology, surface chemistry, and local
environment. This multiscale model can quantitatively link the corrosion rate
of ZrO2-coated aluminum alloys with the evolution of interfacial reactions
during immersion, which is very difficult to establish using in situ
experiments. We have evaluated the presented multiscale model using available
experimental data. The rate of corrosion and pit stability were quantitatively
assessed for various environmental parameters and applied potentials. Results
show that Zr-based conversion coating strongly enhances the corrosion
performance of aluminum alloys due to zirconium involvement in interfacial
kinetics. | 2202.12990v3 |
2022-03-16 | Determination of Al occupancy and local structure for \b{eta}-(AlxGa1-x)2O3 alloys across nearly full composition range from Rietveld analysis | Al occupancy and local structure (bond length and bond angles) for monoclinic
\b{eta}-(AlxGa1-x)2O3 alloys, with Al compositions (x) up to 90%, have been
determined from Rietveld refinement of x-ray diffraction data. Al atom
preferentially occupies octahedron (Oh) atomic site in comparison to
tetrahedron (Td) atomic site. However, sizable number of Td atomic sites i.e.
20% for Al composition of 5% remain occupied by Al atoms, which is found to
increase sharply with Al composition. The Oh atomic sites are not fully
occupied by Al atoms even for Al composition of 90%. The lattice parameters
(band gap) of \b{eta}-(AlxGa1-x)2O3 alloy decrease (increases) linearly with Al
composition, but a change in slope of variation of both lattice parameters and
band gap is observed at around Al composition of 50%. The lattice is found to
be distorted for Al compositions more than 50% as indicated by large change in
the bond angles. The lattice distortion is determined to be the origin for the
observed change in slope for the variation of both lattice parameters and band
gap for monoclinic \b{eta}-(AlxGa1-x)2O3 alloy system. Our results provide an
insight in to the local structure of \b{eta}-(AlxGa1-x)2O3 alloys, which are
required to have better understanding of their physical properties. | 2203.08494v1 |
2016-03-07 | Structural Phase Transition and Carrier Density Tuning in SnSexTe1-x Nanoplates for Topological Crystalline Insulators | For topological insulators and topological crystalline insulators (TCIs),
their exotic surface states are promising for fundamental condensed matter
physics research as well as future electronics such as low-dissipation
electronics and spintronics. However, the high bulk carrier density that often
dominates the transport property is the major materials challenge, critically
hindering our ability to study and manipulate the surface states. In this
manuscript, we demonstrate an alloying strategy, SnSexTe1-x, to effectively
reduce the bulk carrier density. As long as SnSexTe1-x remains in the cubic
crystal structure, it is predicted to be a TCI. We show systematic decrease of
the bulk carrier density with the increasing Se concentration, demonstrating
that the alloying principle works. In addition, we map out the phase diagram of
the cubic to the orthorhombic structural transition as a function of the Se
concentration. This was made possible by studying alloy nanoplates which remain
single-crystalline and is either in the cubic or the orthorhombic phase, in
contrast to bulk alloys that would exhibit polycrystalline grains. Lastly, we
investigate systematically the ferroelectric transition associated with the
structural transition from the cubic to the rhombohedral phase for SnSexTe1-x.
This is the first ferroelectric transition study of the alloy system
SnSexTe1-x. | 1603.02241v1 |
2016-03-11 | Quantum Critical Behavior in a Concentrated Ternary Solid Solution | Quantum critical behavior has been associated with some of the most exotic
emergent states of matter including high-temperature superconductivity. Much of
the research into quantum critical point (QCP) physics has been hampered by the
lack of model systems simple enough to be analyzed by theory. Here, we show
that the concentrated solid solution fcc alloys, including the so-called
high-entropy alloys, are ideal model systems to study the effects of chemical
disorder on emergent properties near a quantum critical region. The face
centered cubic (fcc) alloy NiCoCrx with x near 1 is found to be close to the Cr
concentration where the ferromagnetic transition temperature, Tc, goes to 0.
Near this composition these alloys exhibit a resistivity linear in temperature
to 2 K, a linear magnetoresistance, an excess -TlnT contribution to the low
temperature heat capacity and excess low temperature entropy. All of the low
temperature electrical, magnetic and thermodynamic properties of the alloys
with compositions near x near 1 are not typical of a Fermi liquid and suggest
strong magnetic fluctuations associated with a quantum critical region. The
limit of extreme chemical disorder in these simple fcc materials thus provides
a novel and unique platform to study quantum critical behavior in a highly
tunable system. | 1603.03781v1 |
2016-12-19 | Anomalous Hardening in Magnesium Driven by a Size-Dependent Transition in Deformation Modes | Magnesium (Mg) and its alloys hold great potential as an energy-saving
structural material for automative, aerospace applications. However, the use of
Mg alloys has been limited due to poor ductility and formability. Poor
mechanical properties of Mg alloys origin from the insufficient number of slip
systems, and deformation twinning plays an important role to accommodate
plastic deformation. Here, we report a comprehensive experimental and modeling
study to understand crystal size effect on the transformation in deformation
modes in twin oriented Mg single crystals. The experiments reveal two regimes
of size effects: (1) single twin propagation, where a typical "smaller the
stronger" behavior was dominant in pillars {\le} 18 {\mu}m in diameter, and (2)
twin-twin interaction, which results in anomalous strain hardening in pillars >
18 {\mu}m. Molecular dynamics simulations further indicate a transition from
twinning to dislocation mediated plasticity for crystal sizes below a few
hundred nanometers. Our results provide new understanding of the fundamental
deformation modes of twin oriented Mg from nano-scale to bulk, and insights to
design Mg alloys with superior mechanical properties through dimensional
refinement. This subsequently can materialize into more utilization of Mg
alloys as a structural material in technologically relevant applications. | 1612.06275v1 |
2018-04-11 | Clustering kinetics during natural ageing of Al-Cu based alloys with (Mg, Li) additions | Room temperature solute clustering in aluminium alloys, or natural ageing,
despite its industrial relevance, is still subject to debate, mostly due to its
experimentally challenging nature. To better understand the complex
multi-constituents' interactions at play, we have studied ternary and
quaternary subsystems based on the Al-Cu alloys, namely Al-Cu-Mg, Al-Cu-Li and
Al-Cu-Li-Mg. We used a recently introduced correlative technique using
small-angle neutrons and X-ray scattering (SANS and SAXS) to extract the
chemically resolved kinetics of room temperature clustering in these alloys,
which we completed with differential scanning calorimetry (DSC) and
micro-hardness measurements. The comparison of the clustering behaviours of
each subsystem allowed us to highlight the paramount role of Mg as a trigger
for diffusion and clustering. Indeed, while a strong natural ageing was
observed in the Al-Cu-Mg alloy, virtually none was shown for Al-Cu-Li. A very
slight addition of Mg (0.4%) to this system, however, drastically changed the
situation to a rapid formation of essentially Cu-rich hardening clusters, Mg
only joining them later in the reaction. This diffusion enabling effect of Mg
is discussed in terms of diffusion mechanism and complex interactions with the
quenched-in vacancies. | 1804.03899v2 |
2018-04-12 | Unusual composition dependence of transformation temperatures in Ti-Ta-X shape memory alloys | Ti-Ta-X (X = Al, Sn, Zr) compounds are emerging candidates as
high-temperature shape memory alloys (HTSMAs). The stability of the one-way
shape memory effect (1WE), the exploitable pseudoelastic (PE) strain intervals
as well as the transformation temperature in these alloys depend strongly on
composition, resulting in a trade-off between a stable shape memory effect and
a high transformation temperature. In this work, experimental measurements and
first-principles calculations are combined to rationalize the effect of
alloying a third component to Ti-Ta based HTSMAs. Most notably, an
$\textit{increase}$ in the transformation temperature with increasing Al
content is detected experimentally in Ti-Ta-Al for low Ta concentrations, in
contrast to the generally observed dependence of the transformation temperature
on composition in Ti-Ta-X. This inversion of trend is confirmed by the
$\textit{ab-initio}$ calculations. Furthermore, a simple analytical model based
on the $\textit{ab-initio}$ data is derived. The model can not only explain the
unusual composition dependence of the transformation temperature in Ti-Ta-Al,
but also provide a fast and elegant tool for a qualitative evaluation of other
ternary systems. This is exemplified by predicting the trend of the
transformation temperature of Ti-Ta-Sn and Ti-Ta-Zr alloys, yielding a
remarkable agreement with available experimental data. | 1804.04546v1 |
2019-12-22 | Ab initio modeling of the energy landscape for screw dislocations in body-centered cubic high-entropy alloys | In traditional body-centered cubic (bcc) metals, the core properties of screw
dislocations play a critical role in plastic deformation at low temperatures.
Recently, much attention has been focused on refractory high-entropy alloys
(RHEAs), which also possess bcc crystal structures. However, unlike
face-centered cubic high-entropy alloys (HEAs), there have been far fewer
investigations on bcc HEAs, specifically on the possible effects of chemical
short-range order (SRO) in these multiple principal element alloys on
dislocation mobility. Here, using density functional theory, we investigate the
distribution of dislocation core properties in MoNbTaW RHEAs alloys, and how
they are influenced by SRO. The average values of the core energies in the RHEA
are found to be larger than those in the corresponding pure constituent bcc
metals, and are relatively insensitive to the degree of SRO. However, the
presence of SRO is shown to have a large effect on narrowing the distribution
of dislocation core energies and decreasing the spatial heterogeneity of
dislocation core energies in the RHEA. It is argued that the consequences for
the mechanical behavior of HEAs is a change in the energy landscape of the
dislocations which would likely heterogeneously inhibit their motion. | 1912.10506v3 |
2020-02-04 | The Crystallography of Aluminum and its Alloys | This chapter begins with pure aluminium and a discussion of the form of the
crystal structure and different unit cells that can be used to describe the
crystal structure. Measurements of the face-centred cubic lattice parameter and
thermal expansion coefficient in pure aluminium are reviewed and
parametrisations given that allow the reader to evaluate them across the full
range of temperatures where aluminium is a solid. A new concept called the
vacancy triangle is introduced and demonstrated as an effective means for
determining vacancy concentrations near the melting point of aluminium. The
Debye-Waller factor, quantifying the thermal vibration of aluminium atoms in
pure aluminium, is reviewed and parametrised over the full range of
temperatures where aluminium is a solid. The nature of interatomic bonding and
the history of its characterisation in pure aluminium is reviewed with the
unequivocal conclusion that it is purely tetrahedral in nature. The
crystallography of aluminium alloys is then discussed in terms of all of the
concepts covered for pure aluminium, using prominent alloy examples. The
electron density domain theory of solid-state nucleation and precipitate growth
is introduced and discussed as a new means of rationalising phase
transformations in alloys from a crystallographic point of view. | 2002.01562v1 |
2020-02-11 | Modification of the charge and magnetic order of a low dimensional ferromagnet by molecule-surface bonding | The ability to design and control the spin and charge order of low
dimensional materials on the molecular scale offers an intriguing pathway
towards the miniaturization of spintronic technology towards the nanometer
scale. In this work, we focus on the adsorption induced modifications of the
magnetic and electronic properties of a low dimensional ferromagnetic surface
alloy after the adsorption of the prototypical organic molecule
perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). For this metal-organic
interface, we observe the formation of a localized $\sigma$-like bond between
the functional molecular groups and the surface alloy atoms. This strong
chemical bonding coincides with a lifting of the characteristic surface alloy
band structure and a reduction of the magnitude of the local magnetic moments
of the Dy atoms by 18%. We attribute both findings to a mixing of
spin-degenerate molecular states with spin-split states of the Dy-Ag surface
alloy via the sigma-like bonds between PTCDA and the Dy surface alloy atoms.
Our findings clearly demonstrate the potential of tailored molecule-surface
sigma-bonds to control not only the electronic but also the magnetic order of
low dimensional materials. | 2002.04296v2 |
2020-02-11 | A Multiscale Constitutive Model for Metal Forming of Dual Phase Titanium Alloys by Incorporating Inherent Deformation and Failure Mechanisms | Ductile metals undergo a considerable amount of plastic deformation before
failure. Void nucleation, growth and coalescence is the mechanism of failure in
such metals. {\alpha}/{\beta} titanium alloys are ductile in nature and are
widely used for their unique set of properties like specific strength, fracture
toughness, corrosion resistance and resistance to fatigue failures. Voids in
these alloys were reported to nucleate on the phase boundaries between {\alpha}
and {\beta} phase. Based on the findings of crystal plasticity finite element
method (CPFEM) based investigation of the void growth at the interface of
{\alpha} and {\beta} phases [1], [2], a void nucleation, growth, and
coalescence model has been formulated. An existing single-phase crystal
plasticity theory is extended to incorporate underlying physical mechanisms of
deformation and failure in dual phase titanium alloys. Effects of various
factors (stress triaxiality, Lode parameter, deformation state (equivalent
strain), and phase boundary inclination) on void nucleation, growth and
coalescence are used to formulate the constitutive model while their
interaction with a conventional crystal plasticity theory is established. An
extensive parametric assessment of the model is carried out to quantify and
understand the effects of the material parameters on the overall material
response. Performance of the proposed model is then assessed and verified by
comparing the results of the proposed model with the RVE study results.
Application of the constitutive model for utilisation in the design and
optimisation of the forming process of {\alpha}/{\beta} titanium alloy
components is also demonstrated using experimental data. | 2002.04459v1 |
2020-02-18 | Magnetic and all-optical switching properties of amorphous Tb$_x$Co$_{100-x}$ alloys | Amorphous Tb$_{x}$Co$_{100-x}$ magnetic alloys exhibit a list of intriguing
properties, such as perpendicular magnetic anisotropy, high magneto-optical
activity and magnetization switching using ultrashort optical pulses. Varying
the Tb:Co ratio in these alloys allows for tuning properties such as the
saturation magnetic moment, coercive field and the performance of the
light-induced magnetization switching. In this work, we investigate the
magnetic, optical and magneto-optical properties of various
Tb$_{x}$Co$_{100-x}$ thin film alloy compositions. We report on the effect the
choice of different seeding layers has on the structural and magnetic
properties of Tb$_{x}$Co$_{100-x}$ layers. We also demonstrate that for a range
of alloys, deposited on fused silica substrates, with Tb content of 24-30
at.$\%$, helicity dependent all-optical switching of magnetization can be
achieved, albeit in a multi-shot framework. We explain this property to arise
from the helicity-dependent laser induced magnetization on the Co sublattice
due to the inverse Faraday effect. Our study provides an insight into material
aspects for future potential hybrid magneto-plasmonic TbCo-based architectures. | 2002.07544v3 |
2020-07-01 | Bandgap Lowering in Mixed Alloys of Cs2Ag(SbxBi1-x)Br6 Double Perovskite Thin Films | Halide double perovskites have gained significant attention, owing to their
composition of low-toxicity elements, stability in air and long charge-carrier
lifetimes. However, most double perovskites, including Cs2AgBiBr6, have wide
bandgaps, which limit photo conversion efficiencies. The bandgap can be reduced
through hallowing with Sb3+, but Sb-rich alloys are difficult to synthesise due
to the high formation energy of Cs2AgSbBr6, which itself has a wide bandgap. We
develop a solution-based route to synthesis phase-pure Cs2Ag(SbxBi1-x)Br6 thin
films, with the mixing parameter x continuous varying over the entire
composition range. We reveal that the mixed alloys (x between 0.5 and 0.9)
demonstrate smaller bandgaps (as low as 2.08 eV) than the pure Sb- (2.18 eV)
and Bi-based (2.25 eV) compounds, with strong deviation from Vegard's law.
Through in-depth computations, we propose that bandgap lowering arises from the
Type II band alignment between Cs2AgBiBr6 and Cs2AgSbBr6. The energy mismatch
between the Bi and Sb s and p atomic orbitals, coupled with their non-linear
mixing, results in the alloys adopting a smaller bandgap than the pure
compounds. Our work demonstrates an approach to achieve bandgap reduction and
highlights that bandgap bowing may be found in other double perovskite alloys
by pairing together materials forming a Type II band alignment. | 2007.00388v1 |
2020-07-02 | Interfacial giant tunnel magnetoresistance and bulk-induced large perpendicular magnetic anisotropy in (111)-oriented junctions with fcc ferromagnetic alloys: A first-principles study | We study the tunnel magnetoresistance (TMR) effect and magnetocrystalline
anisotropy in a series of magnetic tunnel junctions (MTJs) with $L1_1$-ordered
fcc ferromagnetic alloys and MgO barrier along the [111] direction. Considering
the (111)-oriented MTJs with different $L1_1$ alloys, we calculate their TMR
ratios and magnetocrystalline anisotropies on the basis of the first-principles
calculations. The analysis shows that the MTJs with Co-based alloys (CoNi,
CoPt, and CoPd) have high TMR ratios over 2000$\%$. These MTJs have
energetically favored Co-O interfaces where interfacial antibonding between Co
$d$ and O $p$ states is formed around the Fermi level. We find that the
resonant tunneling of the antibonding states, called the interface resonant
tunneling, is the origin of the obtained high TMR ratios. Our calculation of
the magnetocrystalline anisotropy shows that many $L1_1$ alloys have large
perpendicular magnetic anisotropy (PMA). In particular, CoPt has the largest
value of anisotropy energy $K_{\rm u} \approx 10\,{\rm MJ/m^3}$. We further
conduct a perturbation analysis of the PMA with respect to the spin-orbit
interaction and reveal that the large PMA in CoPt and CoNi mainly originates
from spin-conserving perturbation processes around the Fermi level. | 2007.01068v2 |
2020-09-10 | Atomic and electronic structure of two-dimensional Mo(1-x)WxS2 alloys | Alloying enables engineering of the electronic structure of semiconductors
for optoelectronic applications. Due to their similar lattice parameters, the
two-dimensional semiconducting transition metal dichalcogenides of the MoWSeS
group (MX2 where M= Mo or W and X=S or Se) can be grown as high-quality
materials with low defect concentrations. Here we investigate the atomic and
electronic structure of Mo(1-x)WxS2 alloys using a combination of
high-resolution experimental techniques and simulations. Analysis of the Mo and
W atomic positions in these alloys, grown by chemical vapour transport, shows
that they are randomly distributed, consistent with Monte Carlo simulations
that use interaction energies determined from first-principles calculations.
Electronic structure parameters are directly determined from angle resolved
photoemission spectroscopy measurements. These show that the spin-orbit
splitting at the valence band edge increases linearly with W content from MoS2
to WS2, in agreement with linear-scaling density functional theory (LS-DFT)
predictions. The spin-orbit splitting at the conduction band edge is predicted
to reduce to zero at intermediate compositions. Despite this,
polarisation-resolved photoluminescence spectra on monolayer Mo0.5W0.5S2 show
significant circular dichroism, indicating that spin-valley locking is
retained. These results demonstrate that alloying is an important tool for
controlling the electronic structure of MX2 for spintronic and valleytronic
applications. | 2009.04807v1 |
2020-10-27 | Surface segregation in multicomponent high entropy alloys: Atomistic simulations versus a multilayer analytical model | This paper compares two approaches for investigating the near-surface
composition profile that results from surface segregation in the so-called
Cantor alloy, an equi-molar alloy of CoCrFeMnNi. One approach consists of
atomistic computer simulations by a combination of Monte Carlo, molecular
dynamics and molecular statics techniques, and the other is a nearest neighbor
analytical calculation performed in the regular solution approximation with a
multilayer model, developed here for the first time for a N-component system
and tested for the 5-component Cantor alloy. This type of comparison is useful
because a typical computer simulation requires the use of ~100 parallel
processors for 2 to 3 hours, whereas a similar calculation by means of the
analytical model can be performed in a few seconds on a laptop machine. The
results obtained show qualitatively good agreement between the two approaches.
Thus, while the results of the computer simulations are presumably more
reliable, and provide an atomic scale picture, if massive computations are
required, for example, in order to optimize the composition of a multicomponent
alloy, then an initial screening of the composition space by the analytical
model could provide a highly useful means of narrowing the regions of interest,
in the same way that the CALPHAD method allows rapid investigation of phase
diagrams in complex multinary systems. | 2010.14141v1 |
2020-11-05 | Preparation of the AlTiNiCuCox system high-entropy alloys and structural analysis | This study aimed to explore and develop a new material with high
cost-effectiveness, excellent strength, light weight, high hardness, great wear
resistance, corrosion resistance, and favorable oxidation resistance.
Structural analysis suggested that, with the change in Co addition amount, the
surface morphology and structure of the alloy system changed. XRD analysis
indicated that, the alloy system was the FCC+BCC mixed structure. In addition,
metallographical demonstrated that, with the increase in Co content, the
dendritic crystal transformed from big block to dendritic structure, then to
snowflake, gradually to petal-like, and finally to petal shape. SEM-EDS
analysis revealed that, Cu element was enriched in interdendritic site, while
Ti, Ni, Al and Co elements were enriched in dendrite. Besides, TEM and TEM-EDS
analysis indicated that, there was nano-size precipitate of small particles in
the Cu-enriched block region, along with dislocation; further, there was twin
structure inside the dendrite, as well as the second phase with different
morphology, and the second phase showed coherency with the matrix. The above
analysis suggested that, the intercrystalline structure was the Cu-enriched
phase of FCC structure; the internal matrix of grain was the NiTi and TiCo
phases of BCC structure; and the second phases inside the grain were the
AlCu2Ti,AlNi2Ti,AlCo2Ti and CuNi phases of FCC structure. Taken together, the
AlTiNiCuCox system novel alloys have changed phase structures and phase types
of the alloy system. | 2011.02799v1 |
2020-12-16 | A tool to predict coercivity in magnetic materials | Magnetic coercivity is often viewed to be lower in alloys with negligible (or
zero) values of the anisotropy constant. However, this explains little about
the dramatic drop in coercivity in FeNi alloys at a non-zero anisotropy value.
Here, we develop a theoretical and computational tool to investigate the
fundamental interplay between material constants that govern coercivity in bulk
magnetic alloys. The two distinguishing features of our coercivity tool are
that: (a) we introduce a large localized disturbance, such as a spike-like
magnetic domain, that provides a nucleation barrier for magnetization reversal;
and (b) we account for magneto-elastic energy -- however small -- in addition
to the anisotropy and magnetostatic energy terms. We apply this coercivity tool
to show that the interactions between local instabilities and material
constants, such as anisotropy and magnetostriction constants, are key factors
that govern magnetic coercivity in bulk alloys. Using our model, we show that
coercivity is minimum at the permalloy composition (Fe-21.5Ni-78.5) at which
the alloy's anisotropy constant is not zero. We systematically vary the values
of the anisotropy and magnetostriction constants, around the permalloy
composition, and identify new combinations of material constants at which
coercivity is small. More broadly, our coercivity tool provides a theoretical
framework to potentially discover novel magnetic materials with low coercivity. | 2012.09320v1 |
2021-08-03 | On the martensitic transformation in Fe$_{x}$Mn$_{80-x}$Co$_{10}$Cr$_{10}$ high-entropy alloy | High-entropy alloys (HEAs), and even medium-entropy alloys (MEAs), are an
intriguing class of materials in that structure and property relations can be
controlled via alloying and chemical disorder over wide ranges in the
composition space. Employing density-functional theory combined with the
coherent-potential approximation to average over all chemical configurations,
we tune free energies between face-centered-cubic (fcc) and
hexagonal-close-packed (hcp) phases in Fe$_{x}$Mn$_{80-x}$Co$_{10}$Cr$_{10}$
systems.~Within Fe-Mn-based alloys, we show that the martensitic transformation
and chemical short-range order directly correlate with the fcc-hcp energy
difference and stacking-fault energies, which are in quantitative agreement
with recent experiments on a $x$=40~at.\% polycrystalline HEA/MEA. Our
predictions are further confirmed by single-crystal measurements on
a$x$=40at.\% using transmission-electron microscopy, selective-area
diffraction, and electron-backscattered-diffraction mapping. The results herein
offer an understanding of transformation-induced/twinning-induced plasticity
(TRIP/TWIP) in this class of HEAs and a design guide for controlling the
physics behind the TRIP effect at the electronic level. | 2108.01636v1 |
2021-08-15 | Characterization of Fe_3 O_4/Au-Ag@MoS_2 nanoparticles for brain cancer treatment using magneto plasmonic approach | This study investigates the treatment of brain cancer by the magnetic
hyperthermia approach and nanoparticles including Fe_3 O_4 core with gold,
silver alloy shell, and MoS_2 coating. Optical properties of these
nanoparticles within the tumor, including the extinction coefficient and
surface plasmon peak (SPR) as a function of size, structure, different
compositions, and thickness are also investigated using the effective medium
theory. Moreover, the impact of temperature distribution was assessed through
the analytical modeling of alternating current (AC) magnetic field. The results
of this study indicated that nanoparticles with a compound of Fe_3 O_4 -
Au_0.25 Ag_0.75@MoS_2 and a thickness of 3 nm of gold-silver alloy and 3 layers
of MoS_2 have the best coefficient of extinction and SPR in the biological
window. The gold-silver alloy improved the extinction coefficient and, at the
same time, prevented the accumulation of magnetic nanoparticles. Since the
gold-silver alloy alone cannot function within the range of biological windows,
MoS_2 was used, which increased the extinction efficiency at higher
wavelengths. Examination of the temperature distribution in the tumor for the
proposed alloy compound indicated that after a short time from the start of
irradiation, the tumor temperature reaches 45 C degree. Also, the temperature
distribution within the tumor tissue reached its maximum value at the center of
the tumor and decreased dramatically as getting away from the center. The use
of magnetic hyperthermia enabled localized delivery of therapeutic dose to
malignant brain tumors; hence, exhibiting superior performance/efficiency over
the photothermal method. | 2108.06715v2 |
2021-08-24 | Identifying the Dirac point composition in Bi1-xSbx alloys using the temperature dependence of quantum oscillations | The thermal chiral anomaly is a new mechanism for thermal transport that
occurs in Weyl semimetals (WSM). It is attributed to the generation and
annihilation of energy at Weyl points of opposite chirality. The effect was
observed in the Bi1-xSbx alloy system, at x=11% and 15%, which are topological
insulators at zero field and driven into an ideal WSM phase by an external
field. Given that the experimental uncertainty on x is of the order of 1%, any
systematic study of the effect over a wider range of x requires precise
knowledge of the transition composition xc at which the electronic bands at the
L-point in these alloys have Dirac-like dispersions. At x>xc, the L-point bands
are inverted and become topologically non-trivial. In the presence of a
magnetic field along the trigonal direction, these alloys become WSMs. This
paper describes how the temperature dependence of the frequency of the
Shubnikov-de Haas oscillations F(x,T) at temperatures of the order of the
cyclotron energy can be used to find xc and characterize the topology of the
electronic Fermi surface. Alloys with topologically trivial bands have
dF(x,T)/dT>0, those with Dirac/Weyl fermions display dF(x,T)/dT<0. | 2108.10917v1 |
2021-09-09 | Investigation of the phase occurrence and H sorption properties in the Y33.33Ni66.67-xAlx (0 <= x <= 33.33) system | The Y33.33Ni66.67-xAlx system has been investigated in the region 0 <= x <=
33.3. The alloys were synthesized by induction melting. Phase occurrence and
structural properties were studied by X-Ray powder Diffraction (XRD). The Al
solubility in each phase has been investigated by XRD and Electron Probe
Micro-Analysis (EPMA). The hydrogenation properties were characterized by
pressure-composition isotherm measurements and kinetic curves at 473 K. For x =
0, the binary Y33.33Ni66.67 alloy crystallizes in the cubic superstructure with
F4-3m space group and ordered Y vacancies. For 1.67 <= x <= 8.33, the alloys
contain mainly Y(Ni, Al)2 and Y(Ni, Al)3 pseudobinary phases; while for 16.67
<= x <= 33.33 they are mainly formed by ternary line compounds Y3Ni6Al2,
Y2Ni2Al and YNiAl. Contrary to the binary Y33.33Ni66.67, Y(Ni, Al)2
pseudo-binary compounds crystalize in C15 phase (space group Fd-3m ) with
disordered Y vacancies. The solubility limit of Al in the C15 YNi2-yAly phase
is y = 0.11 (i.e., x = 3.67). The Y(Ni, Al)3 phase changes from rhombohedral
(PuNi3-type, R-3m) to hexagonal (CeNi3-type, P63/mmc) structure for x
increasing from 5.00 to 6.67. Upon hydrogenation, the disproportion of C15
Y(Ni, Al)2 and losses of crystallinity of YNi and Y2Ni2Al are the main reasons
causing capacity decay of Y33.33Ni66.67-xAlx (0 <= x <= 33.33) alloys upon
cycling. | 2109.04182v1 |
2021-09-14 | Creep properties and deformation mechanisms of single-crystalline $γ^\prime$-strengthened superalloys in dependence of the Co/Ni ratio | Co-base superalloys are considered as promising high temperature materials
besides the well-established Ni-base superalloys. However, Ni appears to be an
indispensable alloying element also in Co-base superalloys. To address the
influence of the base elements on the deformation behavior, high-temperature
compressive creep experiments were performed on a single crystal alloy series
that was designed to exhibit a varying Co/Ni ratio and a constant Al, W and Cr
content. Creep tests were performed at 900 {\deg}C and 250 MPa and the
resulting microstructures and defect configurations were characterized via
electron microscopy. The minimum creep rates differ by more than one order of
magnitude with changing Co/Ni ratio. An intermediate CoNi-base alloy exhibits
the overall highest creep strength. Several strengthening contributions like
solid solution strengthening of the $\gamma$ phase, effective diffusion
coefficients or stacking fault energies were quantified. Precipitate shearing
mechanisms differ significantly when the base element content is varied. While
the Ni-rich superalloys exhibit SISF and SESF shearing, the Co-rich alloys
develop extended APBs when the $\gamma^\prime$ phase is cut. This is mainly
attributed to a difference in planar fault energies, caused by a changing
segregation behavior. As result, it is assumed that the shearing resistivity
and the occurring deformation mechanisms in the $\gamma^\prime$ phase are
crucial for the creep properties of the investigated alloy series. | 2109.06767v1 |
2021-11-02 | Multiscale simulations of uni-polar hole transport in (In,Ga)N quantum well systems | Understanding the impact of the alloy micro-structure on carrier transport
becomes important when designing III-nitride-based LED structures. In this
work, we study the impact of alloy fluctuations on the hole carrier transport
in (In,Ga)N single and multi-quantum well systems. To disentangle hole
transport from electron transport and carrier recombination processes, we focus
our attention on uni-polar (p-i-p) systems. The calculations employ our
recently established multi-scale simulation framework that connects atomistic
tight-binding theory with a macroscale drift-diffusion model. In addition to
alloy fluctuations, we pay special attention to the impact of quantum
corrections on hole transport. Our calculations indicate that results from a
virtual crystal approximation present an upper limit for the hole transport in
a p-i-p structure in terms of the current-voltage characteristics. Thus we find
that alloy fluctuations can have a detrimental effect on hole transport in
(In,Ga)N quantum well systems, in contrast to uni-polar electron transport.
However, our studies also reveal that the magnitude by which the random alloy
results deviate from virtual crystal approximation data depends on several
factors, e.g. how quantum corrections are treated in the transport
calculations. | 2111.01644v1 |
2022-04-06 | First principles and Monte Carlo studies of adsorption and desorption properties from Pd$\rm_{1-x}$Ag$\rm_{x}$ surface alloy | The FCC structure of Pd$\rm_{1-x}$Ag$\rm_{x}$ ($\rm{x}=$ 0.25, 0.50, 0.75)
alloys is considered as a fuel cell component in this study. We have looked
into its qualities as a component of a fuel cell to see whether it could be
potentially used as an alternative replacement of the Pt catalyst. We used
Density Functional Theory (DFT) to study H and CO interaction with the surface,
and Kinetic Monte Carlo~(KMC) to study H and CO desorption from the surface.
The bulk modulus and equilibrium crystal structures of Pd$\rm_{1-x}$Ag$\rm_{x}$
alloys were computed using the GPAW code within plane wave basis set $\&$ a PBE
exchange correlation functional treatment. The best values of a lattice
constant for the system are obtained by total energy calculations versus
lattice cell volumes as fitted to the stabilized jellium model. Surface
energies, cohesive energies, and\ binding energy of Pd$\rm_{1-x}$Ag$\rm_{x}$
alloys were computed to analyze the stability properties of structures. Band
structure calculations reveal the electronic and optical properties of these
alloys. The density of states~(DOS) and projected density of states~(PDOS) show
the availability of the eigenstates for occupation. The desorption process is
studied within the Arrhenius type desorption rate $\&$ a temperature
programming. The effects of lateral interactions between adsorbed molecules on
first order desorption (molecular adsorption) $\&$ second order desorption were
taken into account. Adsorption energies of H and CO on Pd$\rm_3$Ag~(111) as
calculated using DFT is used in the process. The outcomes show good qualitative
agreement with literature. | 2204.02812v1 |
2022-06-21 | Laser induced ultrafast Gd 4f spin dynamics in Co100-xGdx alloys by means of time-resolved XMCD | We have studied the laser induced ultrafast quenching of Gd 4f magnetic order
in ferrimagnetic Co100-xGdx alloys to highlight the role of the inter-atomic
exchange coupling. We have taken advantage of the ultrashort soft X-ray pulses
deliver by the femtoslicing beamline at the BESSY II synchrotron radiation
source at the Helmholtz-Zentrum Berlin to perform element- and time-resolved
X-ray Magnetic Circular Dichroism spectroscopy.Our results show that the laser
induced quenching of Gd 4f magnetic order occurs on very different time-scales
for the Co72Gd28, the Co77Gd23 and the Co79Gd21 alloys. Most of the magnetic
moment losses occur within the first picosecond (ps) while the electron
distribution is strongly out of equilibrium. After the equilibration of the
electrons and lattice temperatures (t > 1 ps), the magnetic losses occur on
slower rates that depend on the alloy composition: increasing the Co
composition speeds up the demagnetization of Gd 4f sublattice. The strength of
the inter-atomic exchange coupling which depends on composition, determines the
efficiency of the angular momentum flow from the Gd 4f spin towards the
lattice. Our results are in qualitative agreements with the predictions of the
microscopic three temperatures model for ferrimagnetic alloys. | 2206.10422v1 |
2022-07-14 | A Phase-Field Study on the Effects of Nanoparticles on Solidification and Grain Growth | Nanoparticle reinforced alloys offer the potential of high strength, high
temperature alloys. While promising, during rapid solidification processes,
alloys suffer from nanoparticle clustering, which can discount any strength
benefit. An open-source phase-field model is developed using PRISMS-PF to
explore the impact of nanoparticles and clustering on alloy solidification.
Heterogenous nucleation and grain boundary pinning are explicitly included, and
a wide range of nanoparticle area fractions and nucleation rates are modeled.
At low area fractions less than 0.05, particle clustering increases grain size
between 15-45% compared to a random distribution. Our quantitative analyses
inform a modified Zener grain size relationship that not only depends on
nanoparticle size and area fraction, but also on the nucleation rate. Grain
size first drastically decreases before plateauing at higher nucleation rates.
Our simulations reveal a strong preference of nanoparticles pinning grain
boundaries. Pinning fraction increases rapidly with nucleation rate before
saturating between 0.85-0.90. Across the range of area fractions and nucleation
rates considered, the random and clustered grain sizes each collapse to a
simple analytical expression that depends only on nanoparticle radius and
pinning fraction. Comparisons against experimental data reveal the expressions
deduced from our analyses fit reported grain sizes better than classic Zener
analysis. A simple model of strength and cost tradeoffs indicates nanoparticles
can be a cost-effective way to improve alloy strength. | 2207.07153v1 |
2022-07-28 | Inner relaxations in equiatomic single-phase high-entropy cantor alloy | The superior properties of high-entropy multi-functional materials are
strongly connected with their atomic heterogeneity through many different local
atomic interactions. The detailed element-specific studies on a local scale can
provide insight into the primary arrangements of atoms in multicomponent
systems and benefit to unravel the role of individual components in certain
macroscopic properties of complex compounds. Herein, multi-edge X-ray
absorption spectroscopy combined with reverse Monte Carlo simulations was used
to explore a homogeneity of the local crystallographic ordering and specific
structure relaxations of each constituent in the equiatomic single-phase
face-centered cubic CrMnFeCoNi high-entropy alloy at room temperature. Within
the considered fitting approach, all five elements of the alloy were found to
be distributed at the nodes of the fcc lattice without any signatures of the
additional phases at the atomic scale and exhibit very close statistically
averaged interatomic distances (2.54-2.55 \r{A}) with their nearest-neighbors.
Enlarged structural displacements were found solely for Cr atoms. The
macroscopic magnetic properties probed by conventional magnetometry demonstrate
no opening of the hysteresis loops at 5 K and illustrate a complex character of
the long-range magnetic order after field-assisted cooling in $\pm$5 T. The
observed magnetic behavior is assigned to effects related to structural
relaxations of Cr. Besides, the advantages and limitations of the reverse Monte
Carlo approach to studies of multicomponent systems like high-entropy alloys
are highlighted. | 2207.14063v1 |
2022-09-17 | Superfunctional high-entropy alloys and ceramics by severe plastic deformation | High-entropy alloys and ceramics containing at least five principal elements
have recently received high attention for various mechanical and functional
applications. The application of severe plastic deformation (SPD), particularly
the high-pressure torsion (HPT) method, combined with the CALPHAD and
first-principles calculations resulted in the development of numerous
superfunctional high-entropy materials with superior properties compared to the
normal functions of engineering materials. This article reviews the recent
advances in the application of SPD to developing superfunctional high-entropy
materials. These superfunctional properties include (i) ultrahigh hardness
levels comparable to the hardness of ceramics in high-entropy alloys, (ii) high
yield strength and good hydrogen embrittlement resistance in high-entropy
alloys; (iii) high strength, low elastic modulus, and high biocompatibility in
high-entropy alloys, (iv) fast and reversible hydrogen storage in high-entropy
hydrides, (v) photovoltaic performance and photocurrent generation on
high-entropy semiconductors, (vi) photocatalytic oxygen and hydrogen production
from water splitting on high-entropy oxides and oxynitrides, and (vii) CO2
photoreduction on high-entropy ceramics. These findings introduce SPD as not
only a processing tool to improve the properties of existing high-entropy
materials but also as a synthesis tool to produce novel high-entropy materials
with superior properties compared with conventional engineering materials. | 2209.08291v3 |
2022-10-02 | Data-mining of In-Situ TEM Experiments: on the Dynamics of Dislocations in CoCrFeMnNi Alloys | High entropy alloys are a class of materials with many significant
improvements in terms of mechanical properties as compared to ``classical''
alloys. The corresponding structure-property relations are not yet entirely
clear, but it is commonly believed that the good mechanical performance is
strongly related to dislocation interactions with the complex energy landscape
formed due to alloying. Although in-situ Transmission Electron Microscopy (TEM)
allows high-resolution studies of the structure and dynamics of moving
dislocations and makes the local obstacle/energy ``landscape'' directly visible
in the geometry of dislocations; such observation, however, are merely
qualitative, and detailed three-dimensional analyses of the interaction between
dislocations and the energy landscape is still missing. In this work, we
utilized dislocations as ``probes'' for the local energy maxima which play the
role of pinning points for the dislocation movement. To this end, we developed
a unique data-mining approach that can perform coarse-grained spatio-temporal
analysis, making ensemble averaging of a considerable number of snapshots
possible. We investigate the effect of pinning points on the dislocation
gliding behavior of CoCrFeMnNi alloy during in-situ TEM straining and find that
(i) the pinning point strength changes when dislocations glide through and (ii)
the pinning point moves along the direction close to the Burgers vector
direction. Our data-mining method can be applied to dislocation motion in
general, making it a useful tool for dislocation research. | 2210.00478v1 |
2022-11-05 | Accurate prediction of chemical short-range order and its effect on thermodynamic, structural, and electronic properties of disordered alloys: exemplified in Cu$_{3}$Au | Electronic-structure methods based on density-functional theory (DFT) were
used to directly quantify the effect of chemical short-range order (SRO) on
thermodynamic, structural, and electronic properties of archetypal
face-centered-cubic (fcc) Cu$_{3}$Au alloy. We show that SRO can be tuned to
alter bonding and lattice dynamics (i.e., phonons) and detail how these
properties are changed with SRO. Thermodynamically favorable SRO improves phase
stability of Cu$_{3}$Au from -0.0343 eV-atom$^{-1}$ to -0.0682 eV-atom$^{-1}$.
We use DFT-based linear-response theory to predict SRO and its electronic
origin, and accurately estimate the transition temperature, ordering
instability (L1$_2$), and Warren-Cowley SRO parameters, observed in
experiments. The accurate prediction of real-space SRO gives an edge over
computationally and resource intensive approaches such as Monte-Carlo methods
or experiments, which will enable large-scale molecular dynamic simulations by
providing supercells with optimized SRO. We also analyze phonon dispersion and
estimate the vibrational entropy changes in Cu$_{3}$Au (from 9k$_{B}$ at 300 K
to 6$k_{B}$ at 100 K). We establish from SRO analysis that exclusion of
chemical interactions may lead to a skewed view of true properties in
chemically complex alloys. The first-principles methods described here are
applicable to any arbitrary complex solid-solution alloys, including
multi-principal-element alloys, so hold promise for designing technologically
useful materials. | 2211.02985v3 |
2022-11-17 | Perspectives on Novel Refractory Amorphous High-Entropy Alloys in Extreme Environments | Two new refractory amorphous high-entropy alloys (RAHEAs) within the
W--Ta--Cr--V and W--Ta--Cr--V--Hf systems were herein synthesized using
magnetron-sputtering and tested under high-temperature annealing and displacing
irradiation using \textit{in situ} Transmission Electron Microscopy. While the
WTaCrV RAHEA was found to be unstable under such tests, additions of Hf in this
system composing a new quinary WTaCrVHf RAHEA was found to be a route to
achieve stability both under annealing and irradiation. A new effect of
nanoprecipitate reassembling observed to take place within the WTaCrVHf RAHEA
under irradiation indicates that a duplex microstructure composed of an
amorphous matrix with crystalline nanometer-sized precipitates enhances the
radiation response of the system. It is demonstrated that tunable chemical
complexity arises as a new alloy design strategy to foster the use of novel
RAHEAs within extreme environments. New perspectives for the alloy design and
application of chemically-complex amorphous metallic alloys in extreme
environments are presented with focus on their thermodynamic phase stability
when subjected to high-temperature annealing and displacing irradiation. | 2211.09853v1 |
2022-11-28 | A ductility metric for refractory-based multi-principal-element alloys | We propose a quantum-mechanical dimensionless metric, the local$-$lattice
distortion (LLD), as a reliable predictor of ductility in refractory
multi-principal-element alloys (RMPEAs). The LLD metric is based on
electronegativity differences in localized chemical environments and combines
atomic$-$scale displacements due to local lattice distortions with a weighted
average of valence$-$electron count. To evaluate the effectiveness of this
metric, we examined body$-$centered cubic (bcc) refractory alloys that exhibit
ductile$-$to$-$brittle behavior. Our findings demonstrate that local$-$charge
behavior can be tuned via composition to enhance ductility in RMPEAs. With
finite$-$sized cell effects eliminated, the LLD metric accurately predicted the
ductility of arbitrary alloys based on tensile$-$elongation experiments. To
validate further, we qualitatively evaluated the ductility of two refractory
RMPEAs, i.e., NbTaMoW and Mo$_{72}$W$_{13}Ta$_{10}Ti$_{2.5}Zr$_{2.5}, through
the observation of crack formation under indentation, again showing excellent
agreement with LLD predictions. A comparative study of three refractory alloys
provides further insights into the electronic-structure origin of ductility in
refractory RMPEAs. This proposed metric enables rapid and accurate assessment
of ductility behavior in the vast RMPEA composition space. | 2211.15797v2 |
2023-01-07 | Impact of Severe Plastic Deformation on Kinetics and Thermodynamics of Hydrogen Storage in Magnesium and Its Alloys | Magnesium and its alloys are the most investigated materials for solid-state
hydrogen storage in the form of metal hydrides, but there are still unresolved
problems with the kinetics and thermodynamics of hydrogenation and
dehydrogenation of this group of materials. Severe plastic deformation (SPD)
methods, such as equal-channel angular pressing (ECAP), high-pressure torsion
(HPT), intensive rolling and fast forging, have been widely used to enhance the
activation, air resistance, and hydrogenation/dehydrogenation kinetics of
Mg-based hydrogen storage materials by introducing ultrafine/nanoscale grains
and crystal lattice defects. These severely deformed materials, particularly in
the presence of alloying additives or second-phase nanoparticles, can show not
only fast hydrogen absorption/desorption kinetics but also good cycling
stability. It was shown that some materials that are apparently inert to
hydrogen can absorb hydrogen after SPD processing. Moreover, the SPD methods
were effectively used for hydrogen binding-energy engineering and synthesizing
new magnesium alloys with low thermodynamic stability for reversible
low/room-temperature hydrogen storage, such as nanoglasses, high-entropy
alloys, and metastable phases including the high-pressure {\gamma}-MgH2
polymorph. This article reviews recent advances in the development of Mg-based
hydrogen storage materials by SPD processing and discusses their potential in
future applications. | 2301.05009v1 |
2023-02-01 | Magnetochemical coupling effects on thermodynamics, point-defect formation and diffusion in Fe-Ni alloys: a theoretical study | This thesis is a theoretical study of thermodynamic, point-defect formation
and diffusion properties in Fe-Ni alloys with a focus on the magnetochemical
effects. The results are derived from density functional theory (DFT)
calculations and Monte Carlo (MC) simulations using a DFT-parametrized
effective interaction model (EIM) with explicit atomic and spin variables. The
first part of this work is focused on thermodynamics. We compute via DFT
energetic, magnetic and vibrational properties and the bcc-fcc phase diagram,
revealing the relative importance between magnetic and vibrational entropies.
Combining MC simulations with the EIM, we obtain an fcc phase diagram across
the Curie points. We also discuss Mn and Cr effects on phase stability. The
second part of the work is dedicated to point-defect properties. We develop MC
schemes to compute vacancy formation free energy in alloys. We show that
vacancy formation in fcc Fe and Ni exhibits features that are well distinct
from those in bcc Fe. The results in fcc Fe-Ni alloys reveal that magnetic
disorder tends to increase vacancy formation free energy, while chemical
disorder shows an opposite effect. We also study magnetic effects on the
properties of self-interstitials in fcc Fe and Ni. The final part of the work
is devoted to vacancy-mediated diffusion. We evaluate diffusion properties over
the whole concentration range, probe into the magnetochemical effects on
diffusion. This work fully takes into account the impacts of transversal and
longitudinal spin fluctuations and the magnetochemical interplay. It provides
an accurate and consistent prediction of thermodynamic, defect formation and
diffusion properties in the Fe-Ni system, and contributes to a better
understanding of effects of magnetism in austenitic steels. The applied
approach is also transferable to the investigation of other magnetic alloys. | 2302.00186v1 |
2023-02-07 | On the occurrence of buoyancy-induced oscillatory growth instability in directional solidification of alloys | Recent solidification experiments identified an oscillatory growth
instability during directional solidification of Ni-based superalloy CMSX4
under a given range of cooling rates. From a modeling perspective, the
quantitative simulation of dendritic growth under convective conditions remains
challenging, due to the multiple length scales involved. Using the dendritic
needle network (DNN) model, coupled with an efficient Navier-Stokes solver, we
reproduced the buoyancy-induced growth oscillations observed in CMSX4
directional solidification. These previous results have shown that, for a given
alloy and temperature gradient, oscillations occur in a narrow range of cooling
rates (or pulling velocity, $V_p$) and that the selected primary dendrite arm
spacing ($\Lambda$) plays a crucial role in the activation of the flow leading
to oscillations. Here, we show that the oscillatory behavior may be generalized
to other binary alloys within an appropriate range of $(V_p,\Lambda)$ by
reproducing it for an Al-4at.%Cu alloy. We perform a mapping of oscillatory
states as a function of $V_p$ and $\Lambda$, and identify the regions of
occurrence of different behaviors (e.g., sustained or damped oscillations) and
their effect on the oscillation characteristics. Our results suggest a minimum
of $V_p$ for the occurrence of oscillations and confirm the correlation between
the oscillation type (namely: damped, sustained, or noisy) with the ratio of
average fluid velocity $\overline V$ over $V_p$. We describe the different
observed growth regimes and highlight similarities and contrasts with our
previous results for a CMSX4 alloy. | 2302.03427v1 |
2023-03-01 | Interplay between magnetism and short-range order in medium- and high-entropy alloys: CrCoNi, CrFeCoNi, and CrMnFeCoNi | The impact of magnetism on predicted atomic short-range order in three
medium- and high-entropy alloys is studied using a first-principles,
all-electron, Landau-type linear response theory, coupled with lattice-based
atomistic modelling. We perform two sets of linear-response calculations: one
in which the paramagnetic state is modelled within the disordered local moment
picture, and one in which systems are modelled in a magnetically ordered state,
which is ferrimagnetic for the alloys considered in this work. We show that the
treatment of magnetism can have significant impact both on the predicted
temperature of atomic ordering and also the nature of atomic order itself. In
CrCoNi, we find that the nature of atomic order changes from being
$\mathrm{L}1_2$-like when modelled in the paramagnetic state to MoPt$_2$-like
when modelled assuming the system has magnetically ordered. In CrFeCoNi, atomic
correlations between Fe and the other elements present are dramatically
strengthened when we switch from treating the system as magnetically disordered
to magnetically ordered. Our results show it is necessary to consider the
magnetic state when modelling multicomponent alloys containing mid- to
late-$3d$ elements. Further, we suggest that there may be high-entropy alloy
compositions containing $3d$ transition metals that will exhibit specific
atomic short-range order when thermally treated in an applied magnetic field.
This has the potential to provide a route for tuning physical and mechanical
properties in this class of materials. | 2303.00641v2 |
2023-03-06 | High thermoelectric performance in metallic NiAu alloys | Thermoelectric (TE) materials seamlessly convert thermal into electrical
energy and vice versa, making them promising for applications such as power
generation or cooling. Although historically the TE effect was first discovered
in metals, state-of-the-art research mainly focuses on doped semiconductors
with large figure of merit, $zT$, that determines the conversion efficiency of
TE devices. While metallic alloys have superior functional properties, such as
high ductility and mechanical strength, they have mostly been discarded from
investigation in the past due to their small Seebeck effect. Here, we realize
unprecedented TE performance in metals by tuning the energy-dependent
electronic scattering. Based on our theoretical predictions, we identify binary
NiAu alloys as promising candidate materials and experimentally discover
colossal power factors up to 34 mWm$^{-1}$K$^{-2}$ (on average 30
mWm$^{-1}$K$^{-2}$ from 300 to 1100 K), which is more than twice larger than in
any known bulk material above room temperature. This system reaches a $zT$ up
to 0.5, setting a new world record value for metals. NiAu alloys are not only
orders of magnitude more conductive than heavily doped semiconductors, but also
have large Seebeck coefficients originating from an inherently different
physical mechanism: within the Au s band conduction electrons are highly mobile
while holes are scattered into more localized Ni d states, yielding a strongly
energy-dependent carrier mobility. Our work challenges the common belief that
good metals are bad thermoelectrics and presents an auspicious paradigm for
achieving high TE performance in metallic alloys through engineering
electron-hole selective s-d scattering. | 2303.03062v1 |
2023-05-19 | Multi-component low and high entropy metallic coatings synthesized by pulsed magnetron sputtering | This paper presents the findings of the synthesis of multicomponent (Al, W,
Ni, Ti, Nb) alloy coatings from mosaic targets. For the study, a pulsed
magnetron sputtering method was employed under different plasma generation
conditions: modulation frequency (10 Hz and 1000 Hz), and power (600 W and 1000
W). The processes achieved two types of alloy coatings, high entropy and
classical alloys. After the deposition processes, scanning electron microscopy,
X-ray diffraction, and energy-dispersive X-ray spectroscopy techniques were
employed to find the morphology, thickness, and chemical and phase compositions
of the coatings. Nanohardness and its related parameters, namely H3.Er2, H.E,
and 1.Er2H ratios, were measured. An annealing treatment was performed to
estimate the stability range for the selected coatings. The results indicated
the formation of as-deposited coatings exhibiting an amorphous structure as a
single-phase solid solution. The process parameters had an influence on the
resulting morphology-a dense and homogenous as well as a columnar morphology,
was obtained. The study compared the properties of high-entropy alloy (HEA)
coatings and classical alloy coatings concerning their structure and chemical
and phase composition. It was found that the change of frequency modulation and
the post-annealing process contributed to the increase in the hardness of the
material in the case of HEA coatings. | 2305.11466v1 |
2023-05-24 | Machine Learning Prediction of Critical Cooling Rate for Metallic Glasses From Expanded Datasets and Elemental Features | We use a random forest model to predict the critical cooling rate (RC) for
glass formation of various alloys from features of their constituent elements.
The random forest model was trained on a database that integrates multiple
sources of direct and indirect RC data for metallic glasses to expand the
directly measured RC database of less than 100 values to a training set of over
2,000 values. The model error on 5-fold cross validation is 0.66 orders of
magnitude in K/s. The error on leave out one group cross validation on alloy
system groups is 0.59 log units in K/s when the target alloy constituents
appear more than 500 times in training data. Using this model, we make
predictions for the set of compositions with melt-spun glasses in the database,
and for the full set of quaternary alloys that have constituents which appear
more than 500 times in training data. These predictions identify a number of
potential new bulk metallic glass (BMG) systems for future study, but the model
is most useful for identification of alloy systems likely to contain good glass
formers, rather than detailed discovery of bulk glass composition regions
within known glassy systems. | 2305.15390v1 |
2023-07-04 | Metallurgy, superconductivity, and hardness of a new high-entropy alloy superconductor Ti-Hf-Nb-Ta-Re | We explored quinary body-centered cubic (bcc) high-entropy alloy (HEA)
superconductors with valence electron concentrations (VECs) ranging from 4.6 to
5.0, a domain that has received limited attention in prior research. Our search
has led to the discovery of new bcc Ti-Hf-Nb-Ta-Re superconducting alloys,
which exhibit an interesting phenomenon of phase segregation into two bcc
phases with slightly different chemical compositions, as the VEC increases. The
enthalpy of the formation of each binary compound explains the phase
segregation. All the alloys investigated were categorized as type-II
superconductors, with superconducting critical temperatures ($T_\mathrm{c}$)
ranging from 3.25 K to 4.38 K. We measured the Vickers microhardness, which
positively correlated with the Debye temperature, and compared it with the
hardness values of other bcc HEA superconductors. Our results indicate that
$T_\mathrm{c}$ systematically decreases with an increase in hardness beyond a
threshold of approximately 350 HV. Additionally, we plotted $T_\mathrm{c}$ vs.
VEC for representative quinary bcc HEAs. The plot revealed the asymmetric VEC
dependence. The correlation between the hardness and $T_\mathrm{c}$, as well as
the asymmetric dependence of $T_\mathrm{c}$ on VEC can be attributed to the
simultaneous effects of the electronic density of states at the Fermi level and
electron-phonon coupling under the uncertainty principle, especially in the
higher VEC region. | 2307.01958v1 |
2023-08-23 | Plastic deformation mechanisms during nanoindentation of W, Mo, V body-centered cubic single crystals and their corresponding W-Mo, W-V equiatomic random solid solutions | Deformation plasticity mechanisms in alloys and compounds may unveil the
material capacity towards optimal mechanical properties. We conduct a series of
molecular dynamics (MD) simulations to investigate plasticity mechanisms due to
nanoindentation in pure tungsten, molybdenum and vanadium body-centered cubic
single crystals, as well as the also body-centered cubic, equiatomic, random
solid solutions (RSS) of tungsten--molybdenum and tungsten--vanadium alloys.
Our analysis focuses on a thorough, side-by-side comparison of dynamic
deformation processes, defect nucleation, and evolution, along with
corresponding stress--strain curves. We also check the surface morphology of
indented samples through atomic shear strain mapping. As expected, the presence
of Mo and V atoms in W matrices introduces lattice strain and distortion,
increasing material resistance to deformation and slowing down dislocation
mobility of dislocation loops with a Burgers vector of 1/2 $\langle 111
\rangle$. Our side-by-side comparison displays a remarkable suppression of the
plastic zone size in equiatomic W--V RSS, but not in equiatomic W--Mo RSS
alloys, displaying a clear prediction for optimal hardening response equiatomic
W--V RSS alloys. If the small-depth nanoindentation plastic response is
indicative of overall mechanical performance, it is possible to conceive a
novel MD-based pathway towards material design for mechanical applications in
complex, multi-component alloys. | 2308.12206v1 |
2023-09-18 | Incorporation of random alloy GaBi$_{x}$As$_{1-x}$ barriers in InAs quantum dot molecules: alloy strain and orbital effects towards enhanced tunneling | Self-assembled InAs quantum dots (QDs), which have long hole-spin coherence
times and are amenable to optical control schemes, have long been explored as
building blocks for qubit architectures. One such design consists of vertically
stacking two QDs to create a quantum dot molecule (QDM). The two dots can be
resonantly tuned to form "molecule-like" coupled hole states from the
hybridization of hole states otherwise localized in each respective dot.
Furthermore, spin-mixing of the hybridized states in dots offset along their
stacking direction enables qubit rotation to be driven optically, allowing for
an all-optical qubit control scheme. Increasing the magnitude of this spin
mixing is important for optical quantum control protocols. To enhance the
tunnel coupling and spin-mixing across the dots, we introduce Bi in the GaAs
inter-dot barrier. Previously, we showed how to model InAs/GaBiAs in an
atomistic tight-binding formalism, and how the dot energy levels are affected
by the alloy. In this paper, we discuss the lowering of the tunnel barrier,
which results in a three fold increase of hole tunnel coupling strength in the
presence of a 7% alloy. Additionally, we show how an asymmetric strain between
the two dots caused by the alloy shifts the resonance. Finally, we discuss
device geometries for which the introduction of Bi is most advantageous. | 2309.10115v4 |
2023-11-02 | Quantifying chemical short-range order in metallic alloys | Metallic alloys often form phases - known as solid solutions - in which
chemical elements are spread out on the same crystal lattice in an almost
random manner. The tendency of certain chemical motifs to be more common than
others is known as chemical short-range order (SRO) and it has received
substantial consideration in alloys with multiple chemical elements present in
large concentrations due to their extreme configurational complexity (e.g.,
high-entropy alloys). Short-range order renders solid solutions "slightly less
random than completely random", which is a physically intuitive picture, but
not easily quantifiable due to the sheer number of possible chemical motifs and
their subtle spatial distribution on the lattice. Here we present a multiscale
method to predict and quantify the SRO state of an alloy with atomic
resolution, incorporating machine learning techniques to bridge the gap between
electronic-structure calculations and the characteristic length scale of SRO.
The result is an approach capable of predicting SRO length scale in agreement
with experimental measurements while comprehensively correlating SRO with
fundamental quantities such as local lattice distortions. This work advances
the quantitative understanding of solid-solution phases, paving the way for SRO
rigorous incorporation into predictive mechanical and thermodynamic models. | 2311.01545v2 |
2023-10-07 | A holistic review on fatigue properties of additively manufactured metals | Additive manufacturing (AM) technology is undergoing rapid development and
emerging as an advanced technique that can fabricate complex near-net shaped
and light-weight metallic parts with acceptable strength and fatigue
performance. A number of studies have indicated that the strength or other
mechanical properties of AM metals are comparable or even superior to that of
conventionally manufactured metals, but the fatigue performance is still a
thorny problem that may hinder the replacement of currently used metallic
components by AM counterparts when the cyclic loading and thus fatigue failure
dominates. This article reviews the state-of-art published data on the fatigue
properties of AM metals, principally including $S$--$N$ data and fatigue crack
growth data. The AM techniques utilized to generate samples in this review
include powder bed fusion (e.g., EBM, SLM, DMLS) and directed energy deposition
(e.g., LENS, WAAM). Further, the fatigue properties of AM metallic materials
that involve titanium alloys, aluminum alloys, stainless steel, nickel-based
alloys, magnesium alloys, and high entropy alloys, are systematically
overviewed. In addition, summary figures or tables for the published data on
fatigue properties are presented for the above metals, the AM techniques, and
the influencing factors (manufacturing parameters, e.g., built orientation,
processing parameter, and post-processing). The effects of build direction,
particle, geometry, manufacturing parameters, post-processing, and
heat-treatment on fatigue properties, when available, are provided and
discussed. The fatigue performance and main factors affecting the fatigue
behavior of AM metals are finally compared and critically analyzed, thus
potentially providing valuable guidance for improving the fatigue performance
of AM metals. | 2311.07046v1 |
2024-01-05 | Hard-sphere model of the B2 to B19' phase transformation, and its application to predict the B19' structure in NiTi alloys and the B19 structures in other binary alloys | The pseudoelastic and pseudoplastic properties of NiTi alloys result from the
closeness of the structures between the B2 cubic austenite and the B19'
monoclinic martensite, and the facility to transform one into each other. Until
now, the paths followed by the atoms during the B2 to B19' transformation were
imagined as independent shears and shuffles. Here, we propose a simplified
hard-sphere atomistic model of phase transformation decomposed into three
distinct types of atomic movements. The model's inputs are the Ti and Ni atomic
or ionic diameters and the monoclinic angle beta. The outputs are the lattice
parameters of the B19' phase and the atomic positions. The results are
remarkably close to those reported in the literature from X-ray diffraction
experiments or DFT simulations. The hard-sphere model explains the change of
enthalpy by the formation of short Ti-Ti bonds in B19'. It is also shown that
the value of the monoclinic angle beta close to 97.9 degrees corresponds to the
highest molar volume among all the possible hard-sphere monoclinic B19'
structures; which suggests that it could be a consequence of a maximization of
the vibrational entropy. The hard-sphere model was applied in the special case
of absence of monoclinicity to predict the B19 structure. The calculations do
not agree well with the B19 structure reported in NiTi alloys; they are however
in excellent agreement with the B19 structures reported in other binary alloys,
such as in AuTi, PdTi, and AuCd. | 2401.02871v1 |
2024-01-29 | Competition between phase ordering and phase segregation in the Ti$_x$NbMoTaW and Ti$_x$VNbMoTaW refractory high-entropy alloys | Refractory high-entropy alloys are under consideration for applications where
materials are subjected to high temperatures and levels of radiation, such as
in the fusion power sector. However, at present, their scope is limited because
they are highly brittle at room temperature. One suggested route to mitigate
this issue is by alloying with Ti. In this theoretical study, using a
computationally efficient linear-response theory based on density functional
theory calculations of the electronic structure of the disordered alloys, we
study the nature of atomic short-range order in these multi-component
materials, as well as assessing their overall phase stability. Our analysis
enables direct inference of phase transitions in addition to the extraction of
an atomistic, pairwise model of the internal energy of an alloy suitable for
study via, e.g. Monte Carlo simulations. Once Ti is added into either the
NbMoTaW or VNbMoTaW system, we find that there is competition between chemical
phase ordering and segregation. These results shed light on observed chemical
inhomogeneity in experimental samples, as well as providing fundamental insight
into the physics of these complex systems. | 2401.16243v2 |
2024-02-27 | Linking Order to Strength in Metals | The metallurgy and materials communities have long known and exploited
fundamental links between chemical and structural ordering in metallic solids
and their mechanical properties. The highest reported strength achievable
through the combination of multiple metals (alloying) has rapidly climbed and
given rise to new classifications of materials with extraordinary properties.
Metallic glasses and high-entropy alloys are two limiting examples of how
tailored order can be used to manipulate mechanical behavior. Here, we show
that the complex electronic-structure mechanisms governing the peak strength of
alloys and pure metals can be reduced to a few physically-meaningful parameters
based on their atomic arrangements and used (with no fitting parameters) to
predict the maximum strength of any metallic solid, regardless of degree of
structural or chemical ordering. Predictions of maximum strength based on the
activation energy for a stress-driven phase transition to an amorphous state is
shown to accurately describe the breakdown in Hall-Petch behavior at the
smallest crystallite sizes for pure metals, intermetallic compounds, metallic
glasses, and high-entropy alloys. This activation energy is also shown to be
directly proportional to interstitial (electronic) charge density, which is a
good predictor of ductility, stiffness (moduli), and phase stability in
high-entropy alloys, and in solid metals generally. The proposed framework
suggests the possibility of coupling ordering and intrinsic strength to
mechanisms like dislocation nucleation, hydrogen embrittlement, and transport
properties. It additionally opens the prospect for greatly accelerated
structural materials design and development to address materials challenges
limiting more sustainable and efficient use of energy. | 2402.17728v1 |
2024-04-11 | Tuning Magnetic and Optical Properties in MnxZn1-xPS3 Single Crystals by the Alloying Composition | The exploration of two-dimensional (2D) antiferromagnetic (AFM) materials has
shown great promise and interest in tuning the magnetic and electronic
properties as well as studying magneto-optical effects. The current work
investigates the control of magneto-optical interactions in alloyed MnxZn1-xPS3
lamellar semiconductor single crystals, with the Mn/Zn ratio regulating the
coupling strength. Magnetic susceptibility results show a retention of AFM
order followed by a decrease in N\'eel temperatures down to ~ 40% Mn
concentration, below which a paramagnetic behavior is observed. Absorption
measurements reveal an increase in bandgap energy with higher Zn(II)
concentration, and the presence of Mn(II) d-d transition below the absorption
edge. DFT+U approach qualitatively explained the origin and the position of the
experimentally observed mid band-gap states in pure MnPS3, and corresponding
peaks visible in the alloyed systems MnxZn1-xPS3. Accordingly, emission at 1.3
eV in all alloyed compounds results from recombination from a 4T1g Mn(II)
excited state to a hybrid p-d state at the valence band. Most significant,
temperature-dependent photoluminescence (PL) intensity trends demonstrate
strong magneto-optical coupling in compositions with x > 0.65. This study
underscores the potential of tailored alloy compositions as a means to control
magnetic and optical properties in 2D materials, paving the way for advances in
spin-based technologies. | 2404.07643v1 |
2019-11-22 | Radiation Damage Studies on Titanium Alloys as High Intensity Proton Accelerator Beam Window Materials | A high-strength dual alpha+beta phase titanium alloy Ti-6Al-4V is utilized as
a material for beam windows in several accelerator target facilities. However,
relatively little is known about how material properties of this alloy are
affected by high-intensity proton beam irradiation. With plans to upgrade
neutrino facilities at J-PARC and Fermilab to over 1 MW beam power, the
radiation damage in the window material will reach a few displacements per atom
(dpa) per year, significantly above the ~0.3 dpa level of existing data. The
RaDIATE collaboration has conducted a high intensity proton beam irradiation of
various target and window material specimens at BLIP facility, including a
variety of titanium alloys. Post-Irradiation Examination of the specimens in
the 1st capsule, irradiated at up to 0.25 dpa, is in progress. Tensile tests in
a hot cell at PNNL exhibited a clear signature of radiation hardening and loss
of ductility for Ti-6Al-4V, while Ti-3Al-2.5V, with less beta phase, exhibited
less severe hardening. Microstructural investigations will follow to study the
cause of the difference in tensile behavior between these alloys. High-cycle
fatigue (HCF) performance is critical to the lifetime estimation of beam
windows exposed to a periodic thermal stress from a pulsed proton beam. The 1st
HCF data on irradiated titanium alloys are to be obtained by a conventional
bend fatigue test at Fermilab and by an ultrasonic mesoscale fatigue test at
Culham Laboratory. Specimens in the 2nd capsule, irradiated at up to ~1 dpa,
cover typical titanium alloy grades, including possible radiation-resistant
candidates. These systematic studies on the effects of radiation damage of
titanium alloys are intended to enable us to predict realistic lifetimes of
current beam windows made of Ti-6Al-4V and to extend the lifetime by choosing a
more radiation and thermal shock tolerant alloy. | 1911.10198v1 |
1998-03-04 | Non-mean-field theories of short range order and diffuse scattering anomalies in disordered alloys | Local, or short-range, order in disordered alloys is an important and
exciting phenomenon which is quantified in electron, X-ray and neutron
scattering experiments. It is discussed in many excellent reviews and books, as
well as in the multitude of original research papers.
This relatively short review of the subject does not attempt to discuss all
aspects of the problem of local correlations in alloys. In particular, we will
not touch such issues as multiatom (cluster) interactions, static displacements
and vibrations of alloy atoms, partially ordered, multicomponent or amorphous
alloys. As a result, we will concentrate on the Hamiltonian traditional for the
considered problem, that of the Ising model on a rigid ideal lattice with pair,
but otherwise arbitrary (i.e., of any range) interatomic interactions.
The central object of the paper is the pair correlation function of the
corresponding dynamical variables of the model, the occupation numbers or spin
variables, the Fourier transform of which is proportional to the intensity of
diffuse scattering caused by atomic short-range order. The main aim is to show
that the expression for this quantity has certain internal structure analogous,
e.g., to that of the averaged Green's function used in the electronic theory of
disordered alloys. This structure is independent of the approximation used for
the quantitative description of correlations. As will be seen, this structure
alone, without further specification of a particular theory of short-range
order, allows us to see new possibilities in diffuse scattering, some of which
have recently been observed experimentally. | 9803052v1 |
2008-11-14 | Electronic transport in ferromagnetic alloys and the Slater-Pauling Curve | Experimental measurements of the residual resistivity $\rho(x)$ of the binary
alloy system Fe$_{1-x}$Cr$_x$ have shown an anomalous concentration dependence
which deviates significantly from Nordheim's rule. In the low ($x < 10%$) Cr
concentration regime the resistivity has been found to increase linearly with
$x$ until $\approx$ 10% Cr where the resistivity reaches a plateau persisting
to $\approx$ 20% Cr. In this paper we present $ab$-$initio$ calculations of
$\rho(x)$ which explain this anomalous behavior and which are based on the
Korringa-Kohn-Rostoker (KKR) method in conjunction with the Kubo-Greenwood
formalism. Furthermore we are able to show that the effects of short-range
ordering or clustering have little effect via our use of the nonlocal
coherent-potential approximation (NL-CPA). For the interpretation of the
results we study the alloys' electronic structure by calculating the Bloch
spectral function particularly in the vicinity of the Fermi energy. From the
analysis of our results we infer that a similar behavior of the resistivity
should also be obtained for iron-rich Fe$_{1-x}$V$_x$ alloys - an inference
confirmed by further explicit resistivity calculations. Both of these alloy
systems belong to the same branch of the famous Slater-Pauling plot and we
postulate that other alloy systems from this branch should show a similar
behavior. Our calculations show that the appearance of the plateau in the
resistivity can be attributed to the dominant contribution of minority spin
electrons to the conductivity which is nearly unaffected by increase in Cr/V
concentration $x$ and we remark that this minority spin electron feature is
also responsible for the simple linear variation of the average moment in the
Slater-Pauling plot for these materials. | 0811.2303v1 |
2012-07-19 | The effect of size and distribution of rod-shaped β' precipitates on the strength and ductility of a Mg-Zn alloy | We report on a quantitative investigation into the effect of size and
distribution of rod-shaped \beta' precipitates on strength and ductility of a
Mg-Zn alloy. Despite precipitation strengthening being crucial for the
practical application of magnesium alloys this study represents the first
systematic examination of the effect of controlled deformation on the
precipitate size distribution and the resulting strength and ductility of a
magnesium alloy. Pre-ageing deformation was used to obtain various
distributions of rod-shaped \beta' precipitates through heterogeneous
nucleation. Alloys were extruded to obtain a texture so as to avoid formation
of twins and thus to ensure that dislocations were the primary nucleation site.
Pre-ageing strain refined precipitate length and diameter, with average length
reduced from 440 nm to 60 nm and diameter from 14 nm to 9 nm. Interparticle
spacings were measured from micrographs and indicated some inhomogeneity in the
precipitate distribution. The yield stress of the alloy increased from 273 MPa
to 309 MPa. The yield stress increased linearly as a function of reciprocal
interparticle spacing, but at a lower rate than predicted for Orowan
strengthening. Pre-ageing deformation also resulted in a significant loss of
ductility (from 17% to 6% elongation). Both true strain at failure and uniform
elongation showed a linear relationship with particle spacing, in agreement
with models for the accumulation of dislocations around non-deforming
obstacles. Samples subjected to 3% pre-ageing deformation showed a
substantially increased ageing response compared to non-deformed material;
however, additional deformation (to 5% strain) resulted in only modest changes
in precipitate distribution and mechanical properties. | 1207.4544v1 |
2014-10-20 | Modeling the Elastic Energy of Alloys: Potential Pitfalls of Continuum Treatments | Some issues that arise when modeling elastic energy for binary alloys are
discussed within the context of a Keating model and density functional
calculations. The Keating model is based on atomistic modeling of elastic
interactions in binary alloy using harmonic springs with species dependent
equilibrium lengths. It is demonstrated that the continuum limit for the strain
field are the usual equations of linear elasticity for alloys and that they
correctly capture the coarse-grained displacement field. In addition, it is
established that Euler-Lagrange equation of the continuum limit of the elastic
energy will yield the same strain field equation. However, a direct calculation
of the elastic energy of the atomistic model reveals that the continuum
expression for the elastic energy is both qualitatively and quantitatively
incorrect. This is because it does not take atomistic scale compositional
non-uniformity into account. Importantly, we also shows that finely mixed
alloys tend to have more elastic energy than segregated systems, which is the
opposite of predictions by some continuum theories. It is also shown that for
strained thin films the traditionally used effective misfit for alloys
systematically underestimate the strain energy. In some models, this drawback
is handled by including an elastic contribution to the enthalpy of mixing which
is characterized in terms of the continuum concentration. The direct
calculation of the atomistic model reveals that this approach suffers serious
difficulties. It is demonstrated that elastic contribution to the enthalpy of
mixing is non-isotropic and scale dependent. It also shown that such effects
are present in density-functional theory calculations for the Si/Ge and Ag/Pt
systems. This work demonstrates that it is critical to include the microscopic
arrangements in any elastic model to achieve even qualitatively correct
behavior. | 1410.5472v2 |
2019-01-04 | Alloy theory with atomic resolution for Rashba or topological systems | Interest in substitutional disordered alloys has recently reemerged with
focus on the symmetry-sensitive properties in the alloy such as topological
insulation and Rashba effect. A substitutional random alloy manifests a
distribution of local environments, creating a polymorphous network. While the
macroscopic average (monomorphous) structure may have the original high
symmetry of the constituent compounds, many observable physical properties are
sensitive to local symmetry, and are hence $<P(S_i)>$ rather than
$P(S_0)$=$P(<S_i>)$. The fundamental difference between polymorphous $<P(S_i)>$
and monomorphous $P(S_0)$ led to the often-diverging results and the missing
the atomic-scale resolution needed to discern symmetry-related physics. A
natural approach capturing the polymorphous aspect is supercell model, which
however suffers the difficulty of band folding ('spaghetti bands'), rendering
the E vs k dispersion needed in topology and Rashba physics and seen in
experiments, practically inaccessible. A solution that retains the polymorphous
nature but restores the E vs k relation is to unfold the supercell bands. This
yields alloy Effective Band Structure (EBS), providing a 3D picture of spectral
density consisting of E- and k-dependent spectral weight with coherent and
incoherent features, all created naturally by the polymorphous distribution of
many local environments. We illustrate this EBS approach for CdTe-HgTe,
PbSe-SnSe and PbS-PbTe alloys. We found properties that are critical for e.g.
topological phase transition and Rashba splitting but totally absent in
conventional monomorphous approaches, including (1) co-existing, wavevector-
and energy-dependent coherent band splitting and incoherent band broadening,
(2) coherent-incoherent transition along different k space directions, and (3)
Rashba-like band splitting having both coherent and incoherent features. | 1901.01289v2 |
2019-02-06 | Efficient method for calculating Raman spectra of solids with impurities and alloys and its application to two-dimensional transition metal dichalcogenides | Raman spectroscopy is a widely used, powerful, and nondestructive tool for
studying the vibrational properties of bulk and low-dimensional materials.
Raman spectra can be simulated using first-principles methods, but due to the
high computational cost calculations are usually limited only to fairly small
unit cells, which makes it difficult to carry out simulations for alloys and
defects. Here, we develop an efficient method for simulating Raman spectra of
alloys, benchmark it against full density-functional theory calculations, and
apply it to several alloys of two-dimensional transition metal dichalcogenides.
In this method, the Raman tensor for the supercell mode is constructed by
summing up the Raman tensors of the pristine system weighted by the projections
of the supercell vibrational modes to those of the pristine system. This
approach is not limited to 2D materials and should be applicable to any
crystalline solids with defects and impurities. To efficiently evaluate
vibrational modes of very large supercells, we adopt mass approximation,
although it is limited to chemically and structurally similar atomic
substitutions. To benchmark our method, we first apply it to
Mo$_x$W$_{(1-x)}$S$_2$ monolayer in the H-phase, where several experimental
reports are available for comparison. Second, we consider
Mo$_x$W$_{(1-x)}$Te$_2$ in the T'-phase, which has been proposed to be 2D
topological insulator, but where experimental results for the monolayer alloy
are still missing. We show that the projection scheme also provides a powerful
tool for analyzing the origin of the alloy Raman-active modes in terms of the
parent system eigenmodes. Finally, we examine the trends in characteristic
Raman signatures for dilute concentrations of impurities in MoS$_2$. | 1902.02143v1 |
2020-04-24 | Tensile behavior of dual-phase titanium alloys under high-intensity proton beam exposure: radiation-induced omega phase transformation in Ti-6Al-4V | A high-intensity proton beam exposure with 181 MeV energy has been conducted
at Brookhaven Linac Isotope Producer facility on various material specimens for
accelerator targetry applications, including titanium alloys as a beam window
material. The radiation damage level of the analyzed capsule was 0.25 dpa at
beam center region with an irradiation temperature around 120 degree C. Tensile
tests showed increased hardness and a large decrease in ductility for the dual
alpha+beta-phase Ti-6Al-4V Grade-5 and Grade-23 extra low interstitial alloys,
with the near alpha-phase Ti-3Al-2.5V Grade-9 alloy still exhibiting uniform
elongation of a few % after irradiation. Transmission Electron Microscope
analyses on Ti-6Al-4V indicated clear evidence of a high-density of defect
clusters with size less than 2 nm in each alpha-phase grain. The beta-phase
grains did not contain any visible defects such as loops or black dots, while
the diffraction patterns clearly indicated omega-phase precipitation in an
advanced formation stage. The radiation-induced omega-phase transformation in
the beta-phase could lead to greater loss of ductility in Ti-6Al-4V alloys in
comparison with Ti-3Al-2.5V alloy with less beta-phase. | 2004.11562v2 |
2021-03-28 | Radiation-tolerant high-entropy alloys via interstitial-solute-induced chemical heterogeneities | High-entropy alloys (HEAs) composed of multiple principal elements have been
shown to offer improved radiation resistance over their elemental or
dilute-solution counterparts. Using NiCoFeCrMn HEA as a model, here we
introduce carbon and nitrogen interstitial alloying elements to impart chemical
heterogeneities in the form of the local chemical order (LCO) and associated
compositional variations. Density functional theory simulations predict
chemical short-range order (CSRO) (nearest neighbors and the next couple of
atomic shells) surrounding C and N, due to the chemical affinity of C with (Co,
Fe) and N with (Cr, Mn). Atomic-resolution chemical mapping of the elemental
distribution confirms marked compositional variations well beyond statistical
fluctuations. Ni+ irradiation experiments at elevated temperatures demonstrate
a remarkable reduction in void swelling by at least one order of magnitude
compared to the base HEA without C and N alloying. The underlying mechanism is
that the interstitial-solute-induced chemical heterogeneities roughen the
lattice as well as the energy landscape, impeding the movements of, and
constraining the path lanes for, the normally fast-moving self-interstitials
and their clusters. The irradiation-produced interstitials and vacancies
therefore recombine more readily, delaying void formation. Our findings thus
open a promising avenue towards highly radiation-tolerant alloys. | 2103.15134v1 |
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