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2011-12-05 | Chemical Composition Tuning of the Anomalous Hall Effect in Isoelectronic L10 FePd1-xPtx Alloy Films | The anomalous Hall effect (AHE) in L10 FePd1-xPtx alloy films is studied both
experimentally and theoretically. We find that the intrinsic contribution
(?sigma^int_AH) to the AHE can be significantly increased whereas the extrinsic
side-jump contribution (sigma^?sj_AH) can be continuously reduced from being
slightly larger than sigma^?int_AH in L10 FePd to being much smaller than
sigma^?int_AH in L10 FePt, by increasing the Pt composition x. We show that
this chemical composition tuning of the intrinsic contribution is afforded by
the stronger spin-orbit coupling strength on the Pd/Pt site when the lighter Pd
atoms are replaced by the heavier Pt atoms. Our results provide a means of
manipulating the competing AHE mechanisms in ferromagnetic alloys for fuller
understanding the AHE and also for technological applications of ferromagnetic
alloys. | 1112.0834v2 |
2012-01-03 | CHUF and `unfreezing' (or de-arrest) of kinetic arrest in magnetic shape memory alloys | CHUF (cooling and heating in unequal field) protocol allows establishing
phase coexistence through macroscopic measurements and enables distinguishing
the metastable and stable phases (amongst the coexisting phase fractions across
a first order magnetic transition) of a glass-like arrested state (GLAS). The
observation of kinetic arrest in magnetic shape memory alloys has become a very
active area of research. We highlight recent CHUF measurements on these
materials that show behaviour similar to that reported earlier in manganites
and other intermetallic alloys, emphasizing the generality. The martensitic
transition provides an opportunity to study various properties as the CHUF
protocol enables tuning of phase fractions, and controlled devitrification, of
GLAS. | 1201.0575v1 |
2012-01-13 | Towards predictive modelling of near-edge structures in electron energy loss spectra of AlN based ternary alloys | Although electron energy loss near edge structure analysis provides a tool
for experimentally probing unoccupied density of states, a detailed comparison
with simulations is necessary in order to understand the origin of individual
peaks. This paper presents a density functional theory based technique for
predicting the N K-edge for ternary (quasi-binary) nitrogen alloys by adopting
a core hole approach, a methodology that has been successful for binary nitride
compounds. It is demonstrated that using the spectra of binary compounds for
optimising the core hole charge ($0.35\,\mathrm{e}$ for cubic Ti$_{1-x}$Al$_x$N
and $0.45\,\mathrm{e}$ for wurtzite Al$_x$Ga$_{1-x}$N), the predicted spectra
evolutions of the ternary alloys agree well with the experiments. The spectral
features are subsequently discussed in terms of the electronic structure and
bonding of the alloys. | 1201.2842v1 |
2012-01-20 | Magnetic origin of chemical balance in alloyed Fe-Cr stainless steels: first-principles and Ising model study | Iron-chromium forms the basis of most of the stainless steel grades in the
markets. Recently new insights into the physical and chemical properties of
Fe-Cr based alloys have been obtained. Some of the new results are quite
unexpected and call for further investigations. The present study addresses the
magnetic contribution in the atomic driving forces behind the chemical
composition in Fe-Cr alloyed with Al, Ti, V, Mn, Co, Ni, and Mo. Using the ab
initio exact muffin-tin orbitals method and an Ising-type spin model, it is
found that the magnetic moment of the solute atom combined with the induced
changes in the magnetic moments of the host atoms form the main framework in
determining the mixing energy and chemical potentials of low-Cr Fe-Cr based
alloys. The results obtained in the present work are related to tuning of the
microstructure and corrosion protection of low-Cr steels. | 1201.4336v1 |
2012-01-29 | Statistical Derivation of Basic Equations of Diffusional Kinetics in Alloys with Application to the Description of Diffusion of Carbon in Austenite | Basic equations of diffusional kinetics in alloys are statistically derived
using the master equation approach. To describe diffusional transformations in
substitution alloys, we derive the "quasi-equilibrium" kinetic equation which
generalizes its earlier versions by taking into account possible "interaction
renormalization" effects. For the interstitial alloys Me-X, we derive the
explicit expression for the diffusivity D of an interstitial atom X which
notably differs from those used in previous phenomenological treatments. This
microscopic expression for D is applied to describe the diffusion of carbon in
austenite basing on some simple models of carbon-carbon interaction. The
results obtained enable us to make certain conclusions about the real form of
these interactions, and about the scale of the "transition state entropy" for
diffusion of carbon in austenite. | 1201.6056v1 |
2012-02-29 | Magneto-transport in a binary alloy ring | Magneto-transport properties are investigated in a binary alloy ring
subjected to an Aharonov-Bohm (AB) flux \phi within a single-band
non-interacting tight-binding framework. In the first part, we expose
analytically the behavior of persistent current in an isolated ordered binary
alloy ring as functions of electron concentration N_e and AB flux \phi. While,
in the second part of the article, we discuss electron transport properties
through a binary alloy ring attached to two semi-infinite one-dimensional
metallic electrodes. The effect of impurities is also analyzed. From our study
we propose that under suitable choices of the parameter values the system can
act as a p-type or an n-type semiconductor. | 1202.6519v3 |
2012-03-21 | Structural and electronic properties of Pb1-xCdxTe and Pb1-xMnxTe ternary alloys | A systematic theoretical study of two PbTe-based ternary alloys, Pb1-xCdxTe
and Pb1-xMnxTe, is reported. First, using ab initio methods we study the
stability of the crystal structure of CdTe - PbTe solid solutions, to predict
the composition for which rock-salt structure of PbTe changes into zinc-blende
structure of CdTe. The dependence of the lattice parameter on Cd (Mn) content x
in the mixed crystals is studied by the same methods. The obtained decrease of
the lattice constant with x agrees with what is observed in both alloys. The
band structures of PbTe-based ternary compounds are calculated within a
tight-binding approach. To describe correctly the constituent materials new
tight-binding parameterizations for PbTe and MnTe bulk crystals as well as a
tight-binding description of rock-salt CdTe are proposed. For both studied
ternary alloys, the calculated band gap in the L point increases with x, in
qualitative agreement with photoluminescence measurements in the infrared. The
results show also that in p-type Pb1-xCdxTe and Pb1-xMnxTe mixed crystals an
enhancement of thermoelectrical power can be expected. | 1203.4763v1 |
2012-04-17 | Orientation relationships between icosahedral clusters in hexagonal MgZn2 and monoclinic Mg4Zn7 phases in Mg-Zn(-Y) alloys | Intermetallic precipitates formed in heat-treated and aged Mg-Zn and Mg-Zn-Y
alloys have been investigated via electron microscopy. Coarse spheroidal
precipitates formed on deformation twin boundaries contained domains belonging
to either the MgZn2 hexagonal Laves phase or the monoclinic Mg4Zn7 phase. Both
phases are structurally related to the quasi-crystalline phase formed in
Mg-Zn-Y alloys, containing icosahedrally coordinated zinc atoms arranged as a
series of broad rhombohedral units. This rhombohedral arrangement was also
visible in intragranular precipitates where local regions with the structures
of hexagonal MgZn2 and Mg4Zn7 were found. The orientation adopted by the MgZn2
and Mg4Zn7 phases in twin-boundary and intragranular precipitates was such that
the icosahedral clusters were aligned similarly. These results highlight the
close structural similarities between the precipitates of the Mg-Zn-Y alloy
system. | 1204.3708v1 |
2012-08-21 | Theory of the electronic structure of dilute bismide and bismide-nitride alloys of GaAs: Tight-binding and k.p models | The addition of dilute concentrations of bismuth (Bi) into GaAs to form
GaBiAs alloys results in a large reduction of the band gap energy Eg
accompanied by a significant increase of the spin-orbit-splitting energy
(delta_SO), leading to an Eg < delta_SO regime for ~10% Bi composition which is
technologically relevant for the design of highly efficient photonic devices.
The quaternary alloy GaBiNAs offers further flexibility for band gap tuning,
because both nitrogen and bismuth can independently induce band gap reduction.
This work reports sp3s* tight binding and 14-band k.p models for the study of
the electronic structure of GaBiAs and GaBiNAs alloys. Our results are in good
agreement with the available experimental data. | 1208.4296v1 |
2012-10-11 | Contributions of point defects, chemical disorder, and thermal vibrations to electronic properties of Cd(1-x)Zn(x)Te alloys | We present a first principles study based on density functional theory of
thermodynamic and electronic properties of the most important intrinsic defects
in the semiconductor alloy Cd(1-x)Zn(x)Te with x<0.13. The alloy is represented
by a set of supercells with disorder on the Cd/Zn sublattice. Defect formation
energies as well as electronic and optical transition levels are analyzed as a
function of composition. We show that defect formation energies increase with
Zn content with the exception of the neutral Te vacancy. This behavior is
qualitatively similar to but quantitatively rather different from the effect of
volumetric strain on defect properties in pure CdTe. Finally, the relative
carrier scattering strengths of point defects, alloy disorder, and phonons are
obtained. It is demonstrated that for realistic defect concentrations carrier
mobilities are limited by phonon scattering for temperature above approximately
150 K. | 1210.3305v1 |
2012-10-31 | A Multi-Component Phase Field Crystal Model for Structural Transformations in Metal Alloys | We present a new phase field crystal model for structural transformations in
multi-component alloys. The formalism builds upon the two-point correlation
kernel developed in Greenwood et al. for describing structural transformations
in pure materials [Phys. Rev. Lett. 105, 045702 (2010)]. We introduce an
effective twopoint correlation function for multi-component alloys that uses
the local species concentrations to interpolate between different crystal
structures. A simplified version of the model is derived for the particular
case of threecomponent (ternary) alloys, and its equilibrium properties are
demonstrated. Dynamical equations of motion for the density and multiple
species concentration fields are derived, and the robustness of the model is
illustrated with examples of complex microstructure evolution in dendritic
solidification and solid-state precipitation. | 1211.0003v1 |
2012-11-21 | Grain boundary segregation in UFG alloys processed by severe plastic deformation | Grain boundary segregations were investigated by Atom Probe Tomography in an
Al-Mg alloy, a carbon steel and Armco\trademark Fe processed by severe plastic
deformation (SPD). In the non-deformed state, the GBs of the aluminium alloy
are Mg depleted, but after SPD some local enrichment up to 20 at.% was
detected. In the Fe-based alloys, large carbon concentrations were also
exhibited along GBs after SPD. These experimental observations are attributed
to the specific structure of GBs often described as "non-equilibrum" in ultra
fine grained materials processed by SPD. The grain boundary segregation
mechanisms are discussed and compared in the case of substitutional (Mg in fcc
Al) and interstitial (C in bcc Fe) solute atoms. | 1211.5036v1 |
2012-11-29 | Phase diagram, ferromagnetic martensitic transformation and magnetoresponsive properties of Fe-doped MnCoGe alloys | The crystal structure and magnetoresponsive properties of Fe-doped MnCoGe
alloys have been investigated using x-ray diffraction (XRD) and magnetic
measurements. By alloying the Fe-containing isostructure compounds into MnCoGe,
a magnetostructural transition from paramagnetic austenite to ferromagnetic
martensite with large magnetization difference can be realized in a temperature
window between the Curie temperatures of the austenite and martensite,
resulting in magnetic-field-induced martensitic transformations and large
magnetic-entropy changes. A structural and magnetic phase diagram of Fe-doped
MnCoGe alloys has been proposed. | 1211.6815v1 |
2013-01-08 | Deformation mechanisms in a TiNi shape memory alloy during cyclic loading | The deformation mechanisms governing the cyclic stress-strain behaviour of a
TiNi shape memory alloy were investigated in this work. To understand the
development of these mechanisms during cyclic loading, three low-cycle fatigue
tests were performed and stopped at different stages. The first test was
stopped after the first cycle; the second one was stopped after 40 cycles,
corresponding to the beginning of the stabilisation of the cyclic strain-stress
behaviour; and the last one was carried out to failure (3324 cycles). Submitted
to fatigue loading, the response of the TiNi shape memory alloy presents a
classical pseudoelastic response. Two deformation mechanisms, identified by TEM
observations, are highlighted, the first one by twins and the second by
dislocation slip and its interaction with precipitates. These two mechanisms
evolve without competition during cyclic loading. The nanomechanical properties
of the alloy were also examined, and the evolution of the microhardness or
indentation modulus was monitored. | 1301.1433v1 |
2013-01-21 | Role of critical spin fluctuations in ultrafast demagnetization of transition-metal rare-earth alloys | Ultrafast magnetization dynamics induced by femtosecond laser pulses have
been measured in ferrimagnetic Co0.8Gd0.2, Co.74Tb.26 and Co.86Tb.14 alloys.
Using element sensitivity of X-ray magnetic circular dichroism at the Co L3, Tb
M5 and Gd M5 edges we evidence that the demagnetization dynamics is element
dependent. We show that a thermalization time as fast as 280 fs is observed for
the rare-earth in the alloy, when the laser excited state temperature is below
the compensation temperature. It is limited to 500 fs when the laser excited
state temperature is below the Curie temperature (Tc). We propose critical spin
fluctuations in the vicinity of TC as the mechanism which reduces the
demagnetization rates of the 4f electrons in transition-metal rare-earth alloys
whereas at any different temperature the limited demagnetization rates could be
avoided. | 1301.4885v3 |
2013-02-07 | Tight-binding branch-point energies and band offsets for cubic InN, GaN, AlN and AlGaN alloys | Starting with empirical tight-binding band structures, the branch-point (BP)
energies and resulting valence band offsets (VBOs) for the zincblende phase of
InN, GaN and AlN are calculated from their k-averaged midgap energy.
Furthermore, the directional dependence of the BPs of GaN and AlN is discussed
using the Green's function method of Tersoff. We then show how to obtain the
BPs for binary semiconductor alloys within a band-diagonal representation of
the coherent potential approximation (CPA) and apply this method to cubic AlGaN
alloys. The resulting band offsets show good agreement to available
experimental and theoretical data from the literature. Our results can be used
to determine the band alignment in isovalent heterostructures involving pure
cubic III-nitrides or AlGaN alloys for arbitrary concentrations. | 1302.1725v1 |
2013-02-10 | Measurement of orbital asymmetry and strain in Co90Fe10/Ni multilayers and alloys: Origins of perpendicular anisotropy | We use broadband ferromagnetic resonance spectroscopy and x-ray diffraction
to investigate the fundamental origin of perpendicular anisotropy in
Co90Fe10/Ni multilayers. By careful evaluation of the spectroscopic g-factor,
we determine the orbital moment along the out-of-plane and in-plane directions.
For the multilayers, we find a direct relationship between the orbital moment
asymmetry and the perpendicular anisotropy, consistent with the theory of Bruno
[P.Bruno, Phys. Rev. B, 39, 865 (1989)]. A systematic x-ray diffraction study
revealed the presence of a trigonal strain as high as 0.7 % in some samples.
However, we found no direct correlation between the strain and the anisotropy,
indicating that the anisotropy is not dominated by magnetoelastic effects. In
order to further study the interface structure on the anisotropy, we prepared a
set of equivalent alloy samples. The strain in the alloy samples was comparable
to that of the multilayer samples; however the orbital moment asymmetry in the
alloy samples showed a very different trend allowing us to isolate the effect
of the interfaces in the multilayers. | 1302.2366v1 |
2013-02-13 | Ideal strength of random alloys from first-principles theory | The all-electron exact muffin-tin orbitals method in combination with the
coherent-potential appproximation has been employed to investigate the ideal
tensile strengths of elemental V, Mo solids and V- and Mo-based random solid
solutions. The present ideal tensile strengths, calculated assuming isotropic
Poisson contraction, are 16.1, 26.7 and 37.6 GPa for bcc V in the [001], [111]
and [110] directions, respectively, and 26.7 GPa for bcc Mo in the [001]
direction, which are all in good agreement with the available theoretical data.
When a few percent Tc is introduced in Mo, it is found that the ideal strength
decreases in the [001] direction. For the V-based alloys, Cr increases and Ti
decreases the ideal tensile strength in all principal directions. Adding the
same concentration of Cr and Ti to V leads to ternary alloys with similar ideal
strength values as that of pure V. The alloying effects on the ideal strength
is explained using the electronic band structure. | 1302.3042v2 |
2013-02-23 | Atomistic origin of doping-enhanced rapid crystallization in Ag-doped Ge-Sb-Te alloys: a joint experimental and theoretical study | We have applied extended X-ray absorption fine structure (EXAFS) analyses and
ab-initio molecular dynamics (AIMD) simulations to study the atomic structure
of Ag-doped (up to 42%) Ge1Sb2Te4 alloys. Analysis of the models that are
consistent with the EXAFS experiment reveals that the Ge environment is
significantly modified by Ag doping whereas those of Sb and Te are barely
affected (except for high Ag concentrations), and suggests that Ag prefers
bonding with Te to Ge or Sb. Doping with Ag promotes the conversion of
tetrahedral Ge sites to octahedral Ge sites and enhances the speed of
crystallization of Ge-Sb-Te (GST) alloys as evidenced directly from the MD
simulations. Our study shed light on the atomistic mechanism of rapid
crystallization of GST alloys enhanced by Ag doping. | 1302.5757v3 |
2013-03-07 | Studies of homogeneous precipitation in very dilute iron-coper alloys using kinetic Monte Carlo simulations and statistical theory of nucleation | Kinetics of homogeneous nucleation and growth of copper precipitates under
electron irradiation of Fe_{1-x}Cu_x alloys at concentrations x from 0.06 at.%
to 0.4 at.% and temperatures T from 290 to 450C is studied using the kinetic
Monte Carlo (KMC) simulations and the statistical theory of nucleation (STN).
The conventional assumption about the similarity of mechanisms of precipitation
under electron irradiation and during thermal aging is adopted. The
earlier-developed ab initio model of interactions in Fe-Cu alloys is used for
both the KMC simulations and the STN calculations. Values of the nucleation
barrier F_c and the prefactor J_0 in the Zeldovich-Volmer relation for the
nucleation rate J are calculated for a number of concentrations and
temperatures. For the dilute alloys with x less then 0.2%, the STN and the KMC
results for the nucleation barrier F_c do virtually coincide with each other,
which seems to confirm a high reliability of the STN for this problem. The STN
calculations are also used to estimate the temperature dependencies of
concentrations which correspond to the homogeneous or the heterogeneous
precipitation limit and both dependencies are found to be rather sharp. | 1303.1753v1 |
2013-03-14 | Prediction of (TiO2)x(Cu2O)y Alloys for Photoelectrochemical Water Splitting | The formation of (TiO2)x(Cu2O)y solid-solutions are investigated using a
global optimization evolutionary algorithm. First-principles calculations based
on density functional theory are then used to gain insight into the electronic
properties of these alloys. We find that: (i) Ti and Cu in (TiO2)x(Cu2O)y
alloys have similar local environments as in bulk TiO2 and Cu2O except for
(TiO2)(Cu2O) which has some trigonal-planar Cu ions. (ii) The predicted optical
band gaps are around 2.1 eV (590 nm), thus having much better performance for
the absorption of visible light compared with both binary oxides. (iii)
(TiO2)2(Cu2O) has the lowest formation energy amongst all studied alloys and
the positions of its band edges are found to be suitable for solar-driven water
splitting applications. | 1303.3355v1 |
2013-06-13 | Investigation of the quaternary Fe2-xCoxMnSi alloys by structural, magnetic, resistivity and spin polarization measurements | Effects of the Co substitution have been observed on the structural, magnetic
and magneto-transport properties of Fe2-xCoxMnSi alloy. Curie temperature (TC)
and saturation magnetization (MS) of these alloys increased linearly with the
Co substitution. Competitive magnetic interaction between ferromagnetic (FM)
and anti-ferromagnetic (AFM) phases exists in Fe2-xCoxMnSi for x less than 0.2,
AFM phase is completely disappears for x greater than or equal to 0.2. The
value of Rhodes-Wohlfarth ratio pc/ps is greater than one for these alloys
which is the characteristics of iterant magnetism present in the system.
M\"ossbauer spectroscopic measurements have been done to investigate the atomic
disorder and local magnetic moment for some x values. Resistivity measurements
also confirm the stability of ferromagnetism with the concentration of Co and
also show a sign of half metallicity. Resistivity shows semiconducting
behaviour for x = 0.4 which is interesting in view of spin gapless
semiconductors. | 1306.3086v3 |
2013-08-31 | Wegner estimate and localization for alloy-type models with sign-changing exponentially decaying single-site potentials | We study Schr\"odinger operators on $L^2 (\RR^d)$ and $\ell^2(\ZZ^d)$ with a
random potential of alloy-type. The single-site potential is assumed to be
exponentially decaying but not necessarily of fixed sign. In the continuum
setting we require a generalized step-function shape. Wegner estimates are
bounds on the average number of eigenvalues in an energy interval of finite box
restrictions of these types of operators. In the described situation a Wegner
estimate which is polynomial in the volume of the box and linear in the size of
the energy interval holds. We apply the established Wegner estimate as an
ingredient for a localization proof via multiscale analysis. | 1309.0109v2 |
2014-01-16 | Plasticity of Zr-Nb-Ti-Ta-Hf high-entropy alloys | We have investigated the plastic deformation properties of non-equiatomic
single phase Zr-Nb-Ti-Ta-Hf high-entropy alloys from room temperature up to 300
{\deg}C. Uniaxial deformation tests at a constant strain rate of 10$^{-4}$
s$^{-1}$ were performed including incremental tests such as stress-relaxations,
strain-rate- and temperature changes in order to determine the thermodynamic
activation parameters of the deformation process. The microstructure of
deformed samples was characterized by transmission electron microscopy. The
strength of the investigated Zr-Nb-Ti-Ta-Hf phase is not as high as the values
frequently reported for high-entropy alloys in other systems. We find an
activation enthalpy of about 1 eV and a stress dependent activation volume
between 0.5 and 2 nm$^3$. The measurement of the activation parameters at
higher temperatures is affected by structural changes evolving in the material
during plastic deformation. | 1401.3997v1 |
2014-03-28 | Discrete alloy-type models: Regularity of distributions and recent results | We consider discrete random Schr\"odinger operators on $\ell^2
(\mathbb{Z}^d)$ with a potential of discrete alloy-type structure. That is, the
potential at lattice site $x \in \mathbb{Z}^d$ is given by a linear combination
of independent identically distributed random variables, possibly with
sign-changing coefficients. In a first part we show that the discrete
alloy-type model is not uniformly $\tau$-H\"older continuous, a frequently used
condition in the literature of Anderson-type models with general random
potentials. In a second part we review recent results on regularity properties
of spectral data and localization properties for the discrete alloy-type model. | 1403.7329v2 |
2014-05-06 | Formal Safety and Security Assessment of an Avionic Architecture with Alloy | We propose an approach based on Alloy to formally model and assess a system
architecture with respect to safety and security requirements. We illustrate
this approach by considering as a case study an avionic system developed by
Thales, which provides guidance to aircraft. We show how to define in Alloy a
metamodel of avionic architectures with a focus on failure propagations. We
then express the specific architecture of the case study in Alloy. Finally, we
express and check properties that refer to the robustness of the architecture
to failures and attacks. | 1405.1113v1 |
2014-05-09 | Spin Spirals in Surface Alloys on Ru(0001): A First-principles Study | We have used ab initio density functional theory to compute the magnetic
ground states of the surface alloy systems FeAu$_2$/Ru(0001) and
MnAu$_2$/Ru(0001). For both systems, we find that the lowest energy magnetic
configuration corresponds to a left-rotating spin spiral, in which the sense of
rotation is determined by the Dzyaloshinskii-Moriya interaction. These spirals
are lower in energy than the ferromagnetic configuration by 3-4 meV per nm$^2$.
We also find that FeAu$_2$/Ru(0001) has a significantly high magnetic
anisotropy energy, of the order 1 meV per Fe atom. By comparing with the
corresponding freestanding alloy monolayers, we find that the presence of the
Ru substrate plays a significant role in determining the magnetic properties of
the surface alloy systems. | 1405.2152v1 |
2014-05-28 | Barocaloric and Magnetocaloric Effects in Fe49Rh51 | We report on calorimetry under applied hydrostatic pressure and magnetic
field at the antiferromagnetic (AFM)-ferromagnetic (FM) transition of
Fe$_{49}$Rh$_{51}$. Results demonstrate the existence of a giant barocaloric
effect in this alloy, a new functional property that adds to the magnetocaloric
and elastocaloric effects previously reported for this alloy. All caloric
effects originate from the AFM/FM transition which encompasses changes in
volume, magnetization and entropy. The strong sensitivity of the transition
temperatures to both hydrostatic pressure and magnetic field confers to this
alloy outstanding values for the barocaloric and magnetocaloric strengths
($|\Delta S|$/$\Delta p$ $\sim$ 12 J kg$^{-1}$ K $^{-1}$ kbar$^{-1}$ and
$|\Delta S|$/$\mu_0\Delta H$ $\sim$ 12 J kg$^{-1}$ K$^{-1}$ T$^{-1}$). Both
barocaloric and magnetocaloric effects have been found to be reproducible upon
pressure and magnetic field cycling. Such a good reproducibility and the large
caloric strengths make Fe-Rh alloys particularly appealing for solid-state
cooling technologies at weak external stimuli. | 1405.7156v1 |
2014-07-07 | Efficient determination of alloy ground-state structures | We propose an efficient approach to accurately finding the ground-state
structures in alloys based on the cluster expansion method. In this approach, a
small number of candidate ground-state structures are obtained without any
information of the energy. To generate the candidates, we employ the convex
hull constructed from the correlation functions of all possible structures by
using an efficient algorithm. This approach is applicable to not only simple
lattices but also complex lattices. Firstly, we evaluate the convex hulls for
binary alloys with four types of simple lattice. Then we discuss the structures
on the vertices. To examine the accuracy of this approach, we perform a set of
density functional theory calculations and the cluster expansion for Ag-Au
alloy and compare the formation energies of the vertex structures with those of
all possible structures. As applications, the ground-state structures of the
intermetallic compounds CuAu, CuAg, CuPd, AuAg, AuPd, AgPd, MoTa, MoW and TaW
are similarly evaluated. Finally, the energy distribution is obtained for
different cation arrangements in MgAl$_2$O$_4$ spinel, for which long-range
interactions are essential for the accurate description of its energetics. | 1407.1734v1 |
2014-07-08 | Robustness against Disorder of Relativistic Spectral Properties in Chalcogenide Alloys | In order to carefully address the interplay between substitutional disorder
and spin-orbit-coupling in IV-VI alloys, we propose a novel theoretical
approach that integrates the reliability of plane-wave based density-functional
theory beyond the local-density approximation with the Coherent Potential
Approximation. By applying the proposed method to ternary chalcogenide alloys,
we predict a substantial robustness of spectral features close to the Fermi
energy against substitutional disorder. Supplementing our first-principles
calculations with the analysis of the $k \cdot p$ model for rock-salt
chalcogenides, we show that the disorder self-energy is vanishingly small close
to the band gap, thus allowing for bulk Rashba-like spin splitting to be
observed in ferroelectric alloys, such as PbS$_x$Te$_{1-x}$, and protecting the
band-character inversion related to the topological transition in the recently
discovered Topological Crystalline Insulator Pb$_{1-x}$Sn$_x$Te. | 1407.2064v1 |
2014-09-08 | CD2Alloy: Class Diagrams Analysis Using Alloy Revisited | We present CD2Alloy, a novel, powerful translation of UML class diagrams
(CDs) to Alloy. Unlike existing translations, which are based on a shallow
embedding strategy, and are thus limited to checking consistency and generating
conforming object models of a single CD, and support a limited set of CD
language features, CD2Alloy uses a deeper embedding strategy. Rather than
mapping each CD construct to a semantically equivalent Alloy construct,
CD2Alloy defines (some) CD constructs as new concepts within Alloy. This
enables solving several analysis problems that involve more than one CD and
could not be solved by earlier works, and supporting an extended list of CD
language features. The ideas are implemented in a prototype Eclipse plug-in.
The work advances the state-of-the-art in CD analysis, and can also be viewed
as an interesting case study for the different possible translations of one
modeling language to another, their strengths and weaknesses. | 1409.2314v1 |
2014-09-22 | Three dimensional thermal-solute phase field simulation of binary alloy solidification | We employ adaptive mesh refinement, implicit time stepping, a nonlinear
multigrid solver and parallel computation, to solve a multi-scale, time
dependent, three dimensional, nonlinear set of coupled partial differential
equations for three scalar field variables. The mathematical model represents
the non-isothermal solidification of a metal alloy into a melt substantially
cooled below its freezing point at the microscale. Underlying physical
molecular forces are captured at this scale by a specification of the energy
field. The time rate of change of the temperature, alloy concentration and an
order parameter to govern the state of the material (liquid or solid) is
controlled by the diffusion parameters and variational derivatives of the
energy functional. The physical problem is important to material scientists for
the development of solid metal alloys and, hitherto, this fully coupled thermal
problem has not been simulated in three dimensions, due to its computationally
demanding nature. By bringing together state of the art numerical techniques
this problem is now shown here to be tractable at appropriate resolution with
relatively moderate computational resources. | 1409.6213v1 |
2014-10-01 | Coherent phonon study of (GeTe)$_{l}$(Sb$_{2}$Te$_{3}$)$_{m}$ interfacial phase change memory materials | The time-resolved reflectivity measurements were carried out on the
interfacial phase change memory (iPCM) materials
([(GeTe)$_{2}$(Sb$_{2}$Te$_{3}$)$_{4}$]$_{8}$ and
[(GeTe)$_{2}$(Sb$_{2}$Te$_{3}$)$_{1}$]$_{20}$) as well as conventional
Ge$_{2}$Sb$_{2}$Te$_{5}$ alloy at room temperature and above the RESET-SET
phase transition temperature. In the high-temperature phase, coherent phonons
were clearly observed in the iPCM samples while drastic attenuation of coherent
phonons was induced in the alloy. This difference strongly suggests the atomic
rearrangement during the phase transition in iPCMs is much smaller than that in
the alloy. These results are consistent with the unique phase transition model
in which a quasi-one-dimensional displacement of Ge atoms occurs for iPCMs and
a conventional amorphous-crystalline phase transition takes place for the
alloy. | 1410.0097v1 |
2014-10-27 | Ti$_{1-x}$Au$_x$ Alloys: Hard Biocompatible Metals and Their Possible Applications | The search for new hard materials is often challenging from both theoretical
and experimental points of view. Furthermore, using materials for biomedical
applications calls for alloys with high biocompatibility which are even more
sparse. The Ti$_{1-x}$Au$_x$ ($0.22 \leq x \leq 0.8$) exhibit extreme hardness
and strength values, elevated melting temperatures (compared to those of
constituent elements), reduced density compared to Au, high malleability, bulk
metallicity, high biocompatibility, low wear, reduced friction, potentially
high radio opacity, as well as osseointegration. All these properties render
the Ti$_{1-x}$Au$_x$ alloys particularly useful for orthopedic, dental, and
prosthetic applications, where they could be used as both permanent and
temporary components. Additionally, the ability of Ti$_{1-x}$Au$_x$ alloys to
adhere to ceramic parts could reduce the weight and cost of these components. | 1410.7308v1 |
2014-11-15 | Mixed-bond spin-1 Ising model with nonlinear interactions for the Fe-Mn alloys | In this letter, we apply the mixed-bond spin-1 Ising model to the study of
the magnetic properties of Fe-Mn alloys in the $\alpha$ phase by employing the
effective field theory (EFT). Here, we suggest a new approach to the
ferromagnetic coupling between nearest neighbours Fe-Fe that depends on the
ratio between the Mn-Mn coupling and Fe-Mn coupling and of second power of the
Mn concentration $q$ in contrast with linear dependence proposed in the other
papers. Also, we propose a new probability distribution for binary alloys with
mixed-bonds based on the distribution for ternary alloys and we obtain a very
good agreement for all considered values of $q$ in $T-q$ plane, in particular
for $q>0.11$. | 1411.4181v1 |
2014-11-19 | Structural ${γ\textrm{-}\varepsilon}$ phase transition in Fe-Mn alloys from CPA+DMFT approach | We present a computational scheme for total energy calculations of disordered
alloys with strong electronic correlations. It employs the coherent potential
approximation combined with the dynamical mean-field theory and allows one to
study the structural transformations. The material-specific Hamiltonians in the
Wannier function basis are obtained by density functional theory. The proposed
computational scheme is applied to study the ${\gamma\textrm{-}\varepsilon}$
structural transition in paramagnetic Fe-Mn alloys for Mn content from 10 to 20
at. %. The electronic correlations are found to play a crucial role in this
transition. The calculated transition temperature decreases with increasing Mn
content and is in a good agreement with experiment. We demonstrate that in
contrast to the ${\alpha\textrm{-}\gamma}$ transition in pure iron, the
${\gamma\textrm{-}\varepsilon}$ transition in Fe-Mn alloys is driven by a
combination of kinetic and Coulomb energies. The latter is found to be
responsible for the decrease of the ${\gamma\textrm{-}\varepsilon}$ transition
temperature with Mn content. | 1411.5356v3 |
2014-12-02 | The glass-forming ability of model metal-metalloid alloys | Bulk metallic glasses (BMGs) are amorphous alloys with desirable mechanical
properties and processing capabilities. To date, the design of new BMGs has
largely employed empirical rules and trial-and-error experimental approaches.
Ab initio computational methods are currently prohibitively slow to be
practically used in searching the vast space of possible atomic combinations
for bulk glass formers. Here, we perform molecular dynamics simulations of a
coarse-grained, anisotropic potential, which mimics interatomic covalent
bonding, to measure the critical cooling rates for metal-metalloid alloys as a
function of the atomic size ratio $\sigma_S/\sigma_L$ and number fraction $x_S$
of the metalloid species. We show that the regime in the space of
$\sigma_S/\sigma_L$ and $x_S$ where well-mixed, optimal glass formers occur for
patchy and LJ particle mixtures coincides with that for experimentally observed
metal-metalloid glass formers. Our simple computational model provides the
capability to perform combinatorial searches to identify novel glass-forming
alloys. | 1412.0766v1 |
2015-01-22 | Atomistic analysis of the impact of alloy and well-width fluctuations on the electronic and optical properties of InGaN/GaN quantum wells | We present an atomistic description of the electronic and optical properties
of $\text{In}_{0.25}\text{Ga}_{0.75}$N/GaN quantum wells. Our analysis accounts
for fluctuations of well width, local alloy composition, strain and built-in
field fluctuations as well as Coulomb effects. We find a strong hole and much
weaker electron wave function localization in InGaN random alloy quantum wells.
The presented calculations show that while the electron states are mainly
localized by well-width fluctuations, the holes states are already localized by
random alloy fluctuations. These localization effects affect significantly the
quantum well optical properties,leading to strong inhomogeneous broadening of
the lowest interband transition energy. Our results are compared with
experimental literature data. | 1501.05482v1 |
2015-04-01 | Multiscale modeling of ultrafast element-specific magnetization dynamics of ferromagnetic alloys | A hierarchical multiscale approach to model the magnetization dynamics of
ferromagnetic ran- dom alloys is presented. First-principles calculations of
the Heisenberg exchange integrals are linked to atomistic spin models based
upon the stochastic Landau-Lifshitz-Gilbert (LLG) equation to calculate
temperature-dependent parameters (e.g., effective exchange interactions,
damping param- eters). These parameters are subsequently used in the
Landau-Lifshitz-Bloch (LLB) model for multi-sublattice magnets to calculate
numerically and analytically the ultrafast demagnetization times. The developed
multiscale method is applied here to FeNi (permalloy) as well as to copper-
doped FeNi alloys. We find that after an ultrafast heat pulse the Ni sublattice
demagnetizes faster than the Fe sublattice for the here-studied FeNi-based
alloys. | 1504.00199v1 |
2015-04-05 | Pathway to oxide photovoltaics via band-structure engineering of SnO | The prospects of scaling current photovoltaic technologies to terawatt levels
remain uncertain. All-oxide photovoltaics could open rapidly scalable
manufacturing routes, if only oxide materials with suitable electronic and
optical properties were developed. A potential candidate material is tin
monoxide (SnO), which has exceptional doping and transport properties among
oxides, but suffers from a low adsorption coefficient due to its strongly
indirect band gap. Here, we address this shortcoming of SnO by band-structure
engineering through isovalent but heterostructural alloying with divalent
cations (Mg, Ca, Sr, Zn). Using first-principles calculations, we show that
suitable band gaps and optical properties close to that of direct-gap
semiconductors are achievable in such SnO based alloys. Due to the defect
tolerant electronic structure of SnO, the dispersive band-structure features
and comparatively small effective masses are preserved in the alloys. Initial
Sn1-xZnxO thin films deposited by sputtering exhibit crystal structure and
optical properties in accord with the theoretical predictions, which confirms
the feasibility of the alloying approach. Thus, the implications of this work
are important not only for terawatt scale photovoltaics, but also for other
large-scale energy technologies where defect-tolerant semiconductors with high
quality electronic properties are required. | 1504.01168v2 |
2015-05-21 | Compositional bowing of band energies and their deformation potentials in strained InGaAs ternary alloys: a first-principles study | Using first-principles calculations, we show that the conduction and valence
band energies and their deformation potentials exhibit a non-negligible
compositional bowing in strained ternary semiconductor alloys such as InGaAs.
The electronic structure of these compounds has been calculated within the
framework of local density approximation and hybrid functional approach for
large cubic supercells and special quasi-random structures, which represent two
kinds of model structures for random alloys. We find that the predicted bowing
effect for the band energy deformation potentials is rather insensitive to the
choice of the functional and alloy structural model. The direction of bowing is
determined by In cations that give a stronger contribution to the formation of
the In$_{x}$Ga$_{1-x}$As valence band states with $x\gtrsim 0.5$, compared to
Ga cations. | 1505.05659v2 |
2015-08-10 | Electronic structure and spin polarization of Fe$_{1-x}$Mn(Co,Ni)$_x$S$_2$ alloys from first principles | Alloying effects by T=Mn,Co,Ni-substitution on FeS$_2$ have been investigated
using density-functional calculations. The ferromagnetic alloys
Fe$_{1-x}$T$_x$S$_2$ have been investigated for concentrations
$x=\frac{1}{4},\frac{1}{2},\frac{3}{4}$ together with the ground states of the
pure compounds. The electronic structure is discussed with the goal to identify
candidates for half metals, which are of interest for spintronics applications.
We find interesting candidates at high concentration of the Mn-doped FeS$_2$
and at low concentrations for Ni-doped materials. For the Mn alloys we also
note the proximity to a low-spin to high spin transition. For Co-doped
materials we reproduce the well known finding of half metallicity over the
entire concentration range. | 1508.02191v2 |
2015-08-18 | Effects of temperature gradient on the interface microstructure and diffusion of diffusion couples: phase-field simulation | The temporal interface microstructure and diffusion in the diffusion couples
with the mutual interactions of temperature gradient, concentration difference
and initial aging time of the alloys were studied by phase-field simulation,
the diffusion couples are produced by the initial aged spinodal alloys with
different compositions. Temporal composition evolution and volume fraction of
the separated phase indicates the element diffusion direction through the
interface under the temperature gradient. The increased temperature gradient
induces a wide single-phase region at two sides of the interface. The uphill
diffusion proceeds through the interface, no matter the diffusion directions
are up or down to the temperature gradient. For an alloy with short initial
aging time, phase transformation accompanying the interdiffusion results in the
straight interface with the single-phase regions at both sides. Comparing with
the temperature gradient, composition difference of diffusion couple and
initial aging time of the alloy show greater effect on the diffusion and
interface microstructure. | 1508.04367v1 |
2015-09-15 | Temperature and composition dependence of the band gaps of Ga$_{1-x}$In$_x$N alloy: a first-principles study based on the virtual crystal approximation | We report on the structural, electronic and vibrational properties of the
Ga$_{1-x}$In$_x$N alloy using virtual crystal approximation (VCA) from
first-principles. A band gap bowing parameter of 3.85 eV is obtained with the
TB09 functional. Phonon density of states shifts to lower frequency as the In
content is increased. However, VCA ignores disorder effect and is therefore
unable to describe the broadening of the phonon spectra upon alloying. The role
of electron-phonon interaction in the temperature dependence of the band gap is
also studied for GaN, InN and their alloy Ga$_{1-x}$In$_x$N. The calculated
zero-point motion renormalization and the fitted Varshni parameter over the
entire composition range are discussed. | 1509.04735v1 |
2015-10-03 | Strong Suppression of the Spin Hall Effect in the Spin Glass State | We have measured spin Hall effects in spin glass metals, CuMnBi alloys, with
the spin absorption method in the lateral spin valve structure. Far above the
spin glass temperature Tg where the magnetic moments of Mn impurities are
randomly frozen, the spin Hall angle of CuMnBi ternary alloy is as large as
that of CuBi binary alloy. Surprisingly, however, it starts to decrease at
about 4Tg and becomes as little as 7 times smaller at 0.5Tg. A similar tendency
was also observed in anomalous Hall effects in the ternary alloys. We propose
an explanation in terms of a simple model considering the relative dynamics
between the localized moment and the conduction electron spin. | 1510.00808v2 |
2015-10-13 | Nonlocal torque operators in ab initio theory of the Gilbert damping in random ferromagnetic alloys | We present an ab initio theory of the Gilbert damping in substitutionally
disordered ferromagnetic alloys. The theory rests on introduced nonlocal
torques which replace traditional local torque operators in the well-known
torque-correlation formula and which can be formulated within the atomic-sphere
approximation. The formalism is sketched in a simple tight-binding model and
worked out in detail in the relativistic tight-binding linear muffin-tin
orbital (TB-LMTO) method and the coherent potential approximation (CPA). The
resulting nonlocal torques are represented by nonrandom, non-site-diagonal and
spin-independent matrices, which simplifies the configuration averaging. The
CPA-vertex corrections play a crucial role for the internal consistency of the
theory and for its exact equivalence to other first-principles approaches based
on the random local torques. This equivalence is also illustrated by the
calculated Gilbert damping parameters for binary NiFe and FeCo random alloys,
for pure iron with a model atomic-level disorder, and for stoichiometric FePt
alloys with a varying degree of L10 atomic long-range order. | 1510.03571v2 |
2015-10-26 | Misfit stabilized embedded nanoparticles in metallic alloys | Nanoscale inhomogeneities are typical for numerous metallic alloys and
crucially important for their practical applications. At the same time,
stabilization mechanisms of such a state are poorly understood. We present a
general overview of the problem, together with a more detailed discussion of
the prototype example, namely, Guinier-Preston zones in Al-based alloys. It is
shown that coherent strain due to a misfit between inclusion and host crystal
lattices plays a decisive role in the emergence of the inhomogeneous state. We
suggest a model explaining formation of ultrathin plates (with the thickness of
a few lattice constants) typical for Al-Cu alloys. Discreteness of the array of
misfit dislocations and long-ranged elastic interactions between them are the
key ingredients of the model. This opens a way to a general understanding of
the nature of (meta)stable embedded nanoparticles in practically important
systems. | 1510.07443v1 |
2015-10-28 | Scaling of alloy interfacial properties under compositional strain | Complex morphologies and microstructures that emerge during materials growth
and solidification are often determined by both equilibrium and kinetic
properties of the interface and their crystalline anisotropies. However limited
knowledge is available for the alloying and particularly the compositionally
generated elastic effects on these interface characteristics. Here we
systematically investigate such compositional effects on the interfacial
properties of an alloy model system based on the phase-field-crystal analysis,
including the solid-liquid interfacial free energy, kinetic coefficient, and
lattice pinning strength. Scaling relations for these interfacial quantities
over various ranges of material parameters are identified and predicted. Our
results indicate the important effects of couplings among mesoscopic and
microscopic length scales of alloy structure and concentration, and also the
influence of compressive and tensile interface stresses induced by composition
variations. The approach developed here provides an efficient way to
systematically identify these key material properties beyond the traditional
atomistic and continuum methods. | 1510.08180v2 |
2015-12-11 | Nanoscale Origins of the Damage Tolerance of the High-Entropy Alloy CrMnFeCoNi | Damage-tolerance can be an elusive characteristic of structural materials
requiring both high strength and ductility, properties that are often mutually
exclusive. High-entropy alloys are of interest in this regard. Specifically,
the single-phase CrMnFeCoNi alloy displays tensile strength levels of ~1 GPa,
excellent ductility (~60-70%) and exceptional fracture toughness (KJIc > 200
MPa/m). Here, through the use of in-situ straining in an aberration-corrected
transmission electron microscope, we report on the salient atomistic to
micro-scale mechanisms underlying the origin of these properties. We identify a
synergy of multiple deformation mechanisms, rarely achieved in metallic alloys,
which generates high strength, work hardening and ductility, including the easy
motion of Shockley partials, their interactions to form stacking-fault
parallelepipeds, and arrest at planar-slip bands of undissociated dislocations.
We further show that crack propagation is impeded by twinned, nano-scale
bridges that form between the near-tip crack faces and delay fracture by
shielding the crack tip. | 1512.03621v1 |
2015-12-18 | The effect of quenching from different temperatures on Bi 0.88 Sb 0.12 alloy | Structural, thermal, resistive and magnetic properties of melt quenched Bi
0.88 Sb 0.12 alloys are reported. The samples are heated at three different
temperatures, followed by rapid quenching in liquid nitrogen. Large temperature
difference between liquidus and solidus lines, led to microscopic
in-homogeneity in the alloy. The effect of quenching from different
temperatures in polycrystalline Bi 0.88 Sb 0.12 alloy has been studied. The
parameters such as strain, unit cell volume, and resistivity are found to
increase with temperature. Thermal variation of resistivity depicts non
monotonic temperature dependence. The total negative susceptibility increases
and band gap of semiconducting Bi 0.88 Sb 0.12 samples decreases with
increasing temperature. | 1512.05883v1 |
2015-12-23 | Preferential site occupancy of alloying elements in TiAl-based phases | First principles calculations are used to study the preferential occupation
of ternary alloying additions into the binary Ti-Al phases, namely
$\gamma$-TiAl, $\alpha_2$-Ti$_3$Al, $\beta_{\mathrm{o}}$-TiAl, and B19-TiAl.
While the early transition metals (TMs, group IVB, VB , and VIB elements)
prefer to substitute for Ti atoms in the $\gamma$-, $\alpha_2$-, and
B19-phases, they preferentially occupy Al sites in the
$\beta_{\mathrm{o}}$-TiAl. Si is in this context an anomaly, as it prefers to
sit on the Al sublattice for all four phases. B and C are shown to prefer
octahedral Ti-rich interstitial positions instead of substitutional
incorporation. The site preference energy is linked with the alloying-induced
changes of energy of formation, hence alloying-related (de)stabilisation of the
phases. We further show that the phase-stabilisation effect of early TMs on
$\beta_{\mathrm{o}}$-phase has a different origin depending on their valency.
Finally, an extensive comparison of our predictions with available theoretical
and experimental data (which is, however, limited mostly to the $\gamma$-phase)
shows a consistent picture. | 1512.07601v2 |
2016-01-21 | Multiscale model of global inner-core anisotropy induced by hcp-alloy plasticity | $\bullet$ Multiscale model of inner-core anisotropy produced by hcp alloy
deformation$\bullet$ 5 to 20% single-crystal elastic anisotropy and plastic
deformation by pyramidal slip $\bullet$ Low-degree inner-core formation model
with faster crystallization at the equatorThe Earth's solid inner-core exhibits
a global seismic anisotropy of several percents. It results from a coherent
alignment of anisotropic Fe-alloy crystals through the inner-core history that
can be sampled by present-day seismic observations. By combining
self-consistent polycrystal plasticity, inner-core formation models,
Monte-Carlo search for elastic moduli, and simulations of seismic measurements,
we introduce a multiscale model that can reproduce a global seismic anisotropy
of several percents aligned with the Earth's rotation axis. Conditions for a
successful model are an hexagonal-close-packed structure for the inner-core
Fe-alloy, plastic deformation by pyramidal \textless{}c+a\textgreater{} slip,
and large-scale flow induced by a low-degree inner-core formation model. For
global anisotropies ranging between 1 and 3%, the elastic anisotropy in the
single crystal ranges from 5 to 20% with larger velocities along the c-axis. | 1601.05674v1 |
2016-02-08 | A Generalized Ising Model for studying Alloy Evolution under Irradiation and its use in Kinetic Monte Carlo Simulations | We derive an Ising Hamiltonian for kinetic simulations involving interstitial
and vacancy defects in binary alloys. Our model, which we term `ABVI',
incorporates solute transport by both interstitial defects and vacancies into a
mathematically-consistent framework , and thus represents a generalization to
the widely-used ABV model for alloy evolution simulations. The Hamiltonian
captures the three possible interstitial configurations in a binary alloy: A-A,
A-B, and B-B, which makes it particularly useful for irradiation damage
simulations. All the constants of the Hamiltonian are expressed in terms of
bond energies that can be computed using first-principles calculations. We
implement our ABVI model in kinetic Monte Carlo simulations and perform a
verification exercise by comparing our results to published irradiation damage
simulations in simple binary systems with Frenkel pair defect production and
several microstructural scenarios, with matching agreement found. | 1602.02470v1 |
2016-04-29 | Hydrostatic pressure tuned magneto-structural transition and occurrence of pressure induced exchange bias effect in Mn$_{0.85}$Fe$_{0.15}$NiGe alloy | Magnetic and magneto-functional behavior of a Fe-doped MnNiGe alloy with
nominal composition Mn$_{0.85}$Fe$_{0.15}$NiGe have been investigated in
ambient as well as in high pressure condition. The alloy undergoes first order
martensitic phase transition (MPT) around 200 K and also shows large
conventional magnetocaloric effect (MCE) ($\Delta S$ $\sim$ -21 J/kg-K for
magnetic field ($H$) changing from 0-50 kOe) around the transition in ambient
condition. Application of external hydrostatic pressure ($P$) results a shift
in MPT towards the lower temperature and a clear decrease in the saturation
moment of the alloy at 5 K. The peak value of MCE is also found to decrease
with increasing external $P$ ($\sim$ 18 J/kg-K decrease in $\Delta S$ has been
observed for $P$ = 12.5 kbar). The most interesting observation is the
occurance of exchange bias effect (EBE) on application of external $P$. The
competing ferromagnetic and antiferromagnetic interaction in presence of
external $P$ plays the pivotal role towards the observation of $P$ induced EBE. | 1604.08761v1 |
2016-04-30 | Effects of aluminum on hydrogen solubility and diffusion in deformed Fe-Mn alloys | We discuss hydrogen diffusion and solubility in aluminum alloyed Fe-Mn
alloys. The systems of interest are subjected to tetragonal and isotropic
deformations. Based on ab initio modelling, we calculate solution energies,
then employ Oriani's theory which reflects the influence of Al alloying via
trap site diffusion. This local equilibrium model is complemented by
qualitative considerations of Einstein diffusion. Therefore, we apply the
climbing image nudged elastic band method to compute the minimum energy paths
and energy barriers for hydrogen diffusion. Both for diffusivity and solubility
of hydrogen, we find that the influence of the substitutional Al atom has both
local chemical and nonlocal volumetric contributions. | 1605.00084v2 |
2016-05-18 | Shear Melting and High Temperature Embrittlement: Theory and Application to Machining Titanium | We show that alloying with rare earth metals (REMs) can dramatically improve
the machineability of a range of titanium alloys, even though the REM is not
incorporated in the alloy matrix. The mechanism for this is that under cutting,
shear bands are formed within which the nano-precipitates of REM are shear
mixed. This lowers the melting point such that the mechanism of deformation
changes from dislocation mechanism to localised amorphisation and shear
softening. The material then fractures along the thin, amorphous shear-band.
Outside the shear band, the REM remains as precipitates. The new alloys have
similar mechanical properties and biocompatibility to conventional materials. | 1605.05514v1 |
2016-06-29 | Improved model for the thermal conductivity of binary metallic systems | We extended and corrected Mott's two-band model for the
composition-dependence of thermal and electrical conductivity in binary metal
alloys based on high-throughput time-domain thermoreflectance (TDTR)
measurements on diffusion multiples and scatterer-density calculations from
first principles. Examining PdAg, PtRh, AuAg, AuCu, PdCu, PdPt, and NiRh binary
alloys, we found that the nature of the two dominant scatterer-bands considered
in the Mott model changes with the alloys, and should be interpreted as a
combination of the dominant element-specific s- and/or d-orbitals. Using
calculated orbital and element-resolved density-of-states values calculated
with density functional theory as input, we determined the correct orbital mix
that dominates electron scattering for all examined alloys and find excellent
agreement between fitted models and experiments. The proposed description of
the composition dependence of the resistivity can be readily implemented into
the CALPHAD framework. | 1606.09287v2 |
2017-01-11 | Morphology and mechanical properties of nanocrystalline Cu/Ag alloy | Hybrid Monte Carlo (MC)/molecular dynamics (MD) simulations are conducted to
study the microstructures of nanocrystalline (nc) Cu/Ag alloys with various Ag
concentrations. When the Ag concentration is below 50 Ag atoms/nm!, an increase
in Ag concentration leads to a gradual growth of monolayer grain boundary (GB)
complexions into nanolayer complexions. Above the concentration of 50 Ag
atoms/nm!, wetting layers with a bulk crystalline phase are observed. The
effects of Ag on mechanical properties and deformation mechanisms of nc Cu/Ag
alloys are investigated in MD simulations of uniaxial tension. GB sliding
resistance is found to first increase and then decrease with an increase in Ag
concentration. Surprisingly, we also find that the dislocation density
decreases monotonically with an increase in Ag concentration, which suggests
that the grain interiors are softened by the introduction of Ag dopants at GBs.
In addition, there is a critical Ag concentration that maximizes flow stress of
nc Cu/Ag alloys. The flow stress, GB sliding resistance, and the intragranular
dislocation densities become less sensitive to Ag dopants when the grain
diameter increases from 5nm to 40nm. | 1701.03013v1 |
2017-07-04 | Short-Range-Order for fcc-based binary alloys Revisited from Microscopic Geometry | Short-range order (SRO) in disordered alloys is typically interpreted as
competition between chemical effect of negative (or positive) energy gain by
mixing constituent elements and geometric effects comes from difference in
effective atomic radius. Although we have a number of theoretical approaches to
quantitatively estimate SRO at given temperatures, it is still unclear to
systematically understand trends in SRO for binary alloys in terms of geometric
character, e.g., effective atomic radius for constituents. Since chemical
effect plays significant role on SRO, it has been believed that purely
geometric character cannot quantitatively explain the SRO trends. Despite these
considerations, based on the density functional theory (DFT) calculations on
fcc-based 28 equiatomic binary alloys, we find that while convensional
Goldschmidt or DFT-based atomic radius for constituents have no significant
correlation with SRO, atomic radius for specially selected structure,
constructed purely from information about underlying lattice, can successfully
capture the magnitude of SRO. These facts strongly indicate that purely
geometric information of the system plays central role to determine
characteristic disordered structure. | 1707.00901v1 |
2018-01-09 | Micro(point)contact spectroscopy of dilute magnetic (Kondo) alloys CuMn and CuFe | The method of microcontact spectroscopy is used to study alloys with magnetic
impurities CuMn and CuFe in the range of concentrations 0.01-1 at.%. Minima or
maxima (so-called zero-bias anomalies) were observed in the microcontact
spectra of these alloys at voltages ~ 1 mV. The Lande g-factor for the Mn
impurity in a Cu matrix was determined from the splitting of the minimum of the
zero-bias anomaly in a magnetic field. The quantitative calculations carried
out agree well with theory and permit determining both the important geometric
contact parameter <K> i.e., the averaged geometric form factor, and the
characteristics of the alloy itself, for example, J/E_F, i.e., the ratio of the
magnitude of the exchange interaction energy between an electron and the
magnetic impurity to the Fermi energy, from the microcontact spectra. | 1801.02972v1 |
2018-01-16 | Vortex-glass transformation within the surface superconducting state of $β$-phase Mo$_{1-x}$Re$_x$ alloys | We have performed an experimental study on the temperature dependence of
electrical resistivity $\rho$($T$) and heat capacity $C$($T$) of the
Mo$_{1-x}$Re$_x$ $(x = 0.20, 0.25)$ alloy superconductors in different magnetic
fields. In the presence of applied magnetic field, the electrical resistivity
of these alloys go to zero at a temperature well above the bulk superconducting
transition temperature obtained with the help of heat capacity measurements in
the same magnetic field. Our study indicates the presence of surface
superconducting state in these alloys, where the flux lines are pinned in the
surface sheath of the superconductor. The configuration of the flux-lines (2d
pancake-like) in the surface sheath is understood in the realm of the flux-spot
model. Experimental evidence in support of the surface mixed-state state or
"Kulik vortex-state" and the occurrence of a vortex-liquid to vortex-glass
transition is presented. | 1801.05080v1 |
2018-01-25 | Sensitivity of ferromagnetic resonance in PdCo alloyed films to hydrogen gas | In this work we studied the ferromagnetic resonance response of Co(x)Pd(1-x)
alloy samples with different cobalt contents (x = 0.65, 0.39, 0.24 & 0.14). We
found significant differences in the response of the samples to the presence of
hydrogen gas in the samples' environment. Two particular films (with x=0.39 and
0.24) demonstrated behaviour which is promising for application in hydrogen gas
sensing. Using the Co(39)Pd(61) alloy thin film, we were able to measure
hydrogen gas concentration in a very broad range (from <0.1% to 100%) at a
fixed value of the external magnetic field. Finally, we demonstrate that the
Co(24)Pd(76) alloy thin film is sensitive to ultra-low hydrogen gas
concentrations - from 10 to 10000 ppm. | 1801.08324v1 |
2018-03-10 | Alloy engineering of topological semimetal phase transition in MgTa$_{2-x}$Nb$_x$N$_3$ | Dirac, triple-point and Weyl fermions represent three topological semimetal
phases, characterized with a descending degree of band degeneracy, which have
been realized separately in specific crystalline materials with different
lattice symmetries. Here we demonstrate an alloy engineering approach to
realize all three types of fermions in one single material system of
MgTa$_{2-x}$Nb$_x$N$_3$. Based on symmetry analysis and first-principles
calculations, we map out a phase diagram of topological order in the parameter
space of alloy concentration and crystalline symmetry, where the intrinsic
MgTa$_2$N$_3$ with the highest symmetry hosts the Dirac semimetal phase which
transforms into the triple-point and then the Weyl semimetal phase with the
increasing Nb concentration that lowers the crystalline symmetries. Therefore,
alloy engineering affords a unique approach for experimental investigation of
topological transitions of semimetallic phases manifesting different fermionic
behaviors. | 1803.03773v1 |
2018-03-28 | High field paramagnetic Meissner effect in Mo$_{100-x}$Re$_x$ alloy superconductors | We have performed an experimental study on the temperature and field
dependence of magnetization of a series of superconducting Mo$_{100-x}$Re$_x$
alloys. Our studies reveal the presence of high field paramagnetic effect
(HFPME) in these low temperature superconductors. The results of our studies
indicate that the HFPME in the Mo$_{100-x}$Re$_x$ alloys is related to the
inhomogeneous distribution of strong and weak flux-line pinning centers, and
the flux compression resulting due to the same while cooling down the samples
in the presence of high magnetic fields. The results are complemented by the
studies on the temperature dependence of the electrical resistivity and heat
capacity of these alloys in different constant magnetic fields. We compare our
findings with the studies reported in literature on both low $T_C$ and high
$T_C$ superconductors. | 1803.10845v1 |
2018-09-06 | Atomistic modeling of interfacial segregation and structural transitions in ternary alloys | Grain boundary engineering via dopant segregation can dramatically change the
properties of a material. For metallic systems, most current studies concerning
interfacial segregation and subsequent transitions of grain boundary structure
are limited to binary alloys, yet many important alloy systems contain more
than one type of dopant. In this work, hybrid Monte Carlo/molecular dynamics
simulations are performed to investigate the behavior of dopants at interfaces
in two model ternary alloy systems: Cu-Zr-Ag and Al-Zr-Cu. Trends in boundary
segregation are studied, as well as the propensity for the grain boundary
structure to become disordered at high temperature and doping concentration.
For Al-Zr-Cu, we find that the two solutes prefer to occupy different sites at
the grain boundary, leading to a synergistic doping effect. Alternatively, for
Cu-Zr-Ag, there is site competition because the preferred segregation sites are
the same. Finally, we find that thicker amorphous intergranular films can be
formed in ternary systems by controlling the concentration ratio of different
solute elements. | 1809.02218v2 |
2018-11-27 | Atomistic Insights Into Cluster Strengthening in Aluminum Alloys | In certain naturally aged aluminum alloys, significant strengthening can be
obtained due to the decomposition of a super-saturated solid solution into
clusters. The origins of such strengthening remain unclear due to the challenge
of differentiating solute cluster strengthening from solid solution or
precipitate strengthening. To shed light on the origin of cluster strengthening
in aluminum alloys, the interaction between the smallest possible type of
clusters (i.e. dimers) and moving dislocations in a model Al-Mg alloy is
studied using atomistic simulations. Additionally, theoretical models for both
the parelastic and dielastic interactions between clusters and dislocations is
used to identify which factor among order strengthening, elastic interaction,
and change of stacking fault energy controls cluster strengthening. The
comparison of the results from these models to that of the atomistic
simulations show that in the case of Mg dimers, the strength of the strongest
ones are dominated by the dielastic contribution through the change of stacking
fault energy. | 1811.10774v1 |
2018-11-27 | Improving the Visualization of Alloy Instances | Alloy is a lightweight formal specification language, supported by an IDE,
which has proven well-suited for reasoning about software design in early
development stages. The IDE provides a visualizer that produces graphical
representations of analysis results, which is essential for the proper
validation of the model. Alloy is a rich language but inherently static, so
behavior needs to be explicitly encoded and reasoned about. Even though this is
a common scenario, the visualizer presents limitations when dealing with such
models. The main contribution of this paper is a principled approach to
generate instance visualizations, which improves the current Alloy Visualizer,
focusing on the representation of behavior. | 1811.10817v1 |
2019-11-27 | Study of RF Sputtered Antimony Alloyed Bismuth Vanadium Oxide (Sb:BiVO4) Thin Films for Enhanced Photoelectrochemical (PEC) performance from Bandgap Modulation to Thickness Optimization | Monoclinic scheelite bismuth vanadate (BiVO4) is a promising photoanode for
water splitting yet the PEC performance is limited due to its relatively higher
(2.4 eV) band gap. Here, we successfully decreased its the band gap to 1.72 eV
by controlled antimony alloying. Low bandgap antimony alloyed bismuth vanadium
oxide (Sb:BiVO4) thin film was prepared by RF sputtering of high purity
homemade target, fabricated by solid-state reaction using a mixture of Sb2O3,
Bi2O3, and V2O5 powders with desired stoichiometric ratios. Several growth
parameters, powder crystallography, post-deposition effects, and surface
treatments, thickness dependence, effect of electrolytes on photocorrosion were
studied along with its optical and electrochemical characterization. We
discovered that Sb:BiVO4 is a direct band gap material in the visible light
range (1.72 eV) and a valence band position suitable for driving water
oxidation reaction under illumination. Furthermore, hole diffusion length is
increased with antimony alloying and achieved optimum thickness of 400 nm for
higher photocurrent. The controllably prepared Sb:BiVO4 particles are having
the sizes of 10-15 nm in room temperature deposition and can be grown up to 0.5
microns under air annealing. | 1911.12191v1 |
2021-07-07 | Ultra strong and ductile eutectic high entropy alloy fabricated by selective laser melting | With important application prospects, eutectic high entropy alloys have
received extensive attention for their excellent strength and ductility in a
large temperature range. The excellent casting characteristics of eutectic high
entropy alloys make it possible to achieve well manufacturability of selective
laser melting. For the first time, we have achieved crack-free eutectic high
entropy alloy fabricated by selective laser melting, which has excellent
mechanical properties in a wide temperature range of -196 degrees Celsius~760
degrees Celsius due to ultra-fine eutectic lamellar spacing of 150 ~ 200nm and
lamellar colony of 2 ~ 6 {\mu}m. Specifically, the room temperature tensile
strength exceeds 1400MPa and the elongation is more than 20%, significantly
improved compared with those manufactured by other techniques with lower
cooling rate. | 2107.03202v1 |
2021-07-12 | Laser Ablation of Al-Ni Alloys and Al-Ni Layer Systems simulated with Molecular Dynamics and the Two-Temperature Model | Laser ablation of Al-Ni alloys and Al films on Ni substrates has been studied
by molecular dynamics simulations (MD). The MD method was combined with a
two-temperature model to describe the interaction between the laser beam, the
electrons and the atoms. The challenge for alloys and mixtures is to find the
electronic parameters: electron heat conductivity, electron heat capacity and
electron-phonon coupling parameter. The challenge for layered systems is to run
simulations of an inhomogeneous system which requires modification of the
simulation code. Ablation and laser-induced melting was studied for several
Al-Ni compounds. At low fluences above the threshold ordinary ablation behavior
occurred while at high fluences the ablation mechanism changed in Al$_3$Ni and
AlNi$_3$ from phase explosion to vaporization. Al films of various thicknesses
on a Ni substrate have also been simulated. Above threshold, 8 nm Al films are
ablated as a whole while 24 nm Al films are only partially removed. Below
threshold, alloying with a mixture gradient has been observed in the thin layer
system. | 2107.05415v1 |
2017-04-11 | CoFeAlB alloy with low damping and low magnetization for spin transfer torque switching | We investigate the effect of Al doping on the magnetic properties of the
alloy CoFeB. Comparative measurements of the saturation magnetization, the
Gilbert damping parameter $\alpha$ and the exchange constant as a function of
the annealing temperature for CoFeB and CoFeAlB thin films are presented. Our
results reveal a strong reduction of the magnetization for CoFeAlB in
comparison to CoFeB. If the prepared CoFeAlB films are amorphous, the damping
parameter $\alpha$ is unaffected by the Al doping in comparison to the CoFeB
alloy. In contrast, in the case of a crystalline CoFeAlB film, $\alpha$ is
found to be reduced. Furthermore, the x-ray characterization and the evolution
of the exchange constant with the annealing temperature indicate a similar
crystallization process in both alloys. The data proves the suitability of
CoFeAlB for spin torque switching properties where a reduction of the switching
current in comparison with CoFeB is expected. | 1704.03326v1 |
2017-05-04 | Short-range order in high entropy alloys:Theoretical formulation and application to Mo-Nb-Ta-V-W system | In high-entropy alloys (HEAs), the local chemical fluctuations from
disordered solute solution state into segregation, precipitation and ordering
configurations are complex due to the large number of elements. In this work,
the cluster expansion (CE) Hamiltonian for multi-component alloy systems is
developed in order to investigate the dependence of chemical ordering of HEAs
as a function of temperature dependence due to derivation of configuration
entropy from the ideal solute solution. Analytic expressions for Warren-Cowley
short-range order (SRO) parameters are derived for a five component alloy
system. The theoretical formulation is used to investigate the evolution of the
ten different SRO parameters in the MoNbTaVW and the sub-quaternary systems
obtained by MonteCarlo simulations within the combined CE and first-principles
formalism. | 1705.01844v1 |
2017-05-15 | The bi-layered precipitate phase zeta in the Al-Ag alloy system | The Al-Ag system is thought to be a well-understood model system used to
study diffusional phase transformations in alloys. Here we report the existence
of a new precipitate phase, zeta, in this classical system using scanning
transmission electron microscopy (STEM). The zeta phase has a modulated
structure composed of alternating bilayers enriched in Al or Ag. Our in situ
annealing experiments reveal that the zeta phase is an intermediate precipitate
phase between GP zones epsilon and gamma prime. First-principles calculations
show that z is a local energy minimum state formed during Ag clustering in Al.
The layered structure of zeta is analogous to the well-known Ag segregation at
the precipitate-matrix interfaces when Ag is micro- alloyed in various
aluminium alloys. | 1705.05175v1 |
2018-07-23 | Fault Localization for Declarative Models in Alloy | Fault localization is a popular research topic and many techniques have been
proposed to locate faults in imperative code, e.g. C and Java. In this paper,
we focus on the problem of fault localization for declarative models in Alloy
-- a first order relational logic with transitive closure. We introduce
AlloyFL, the first set of fault localization techniques for faulty Alloy models
which leverages multiple test formulas. AlloyFL is also the first set of fault
localization techniques at the AST node granularity. We implements in AlloyFL
both spectrum-based and mutation-based fault localization techniques, as well
as techniques that are based on Alloy's built-in unsat core. We introduce new
metrics to measure the accuracy of AlloyFL and systematically evaluate AlloyFL
on 38 real faulty models and 9000 mutant models. The results show that the
mutation-based fault localization techniques are significantly more accurate
than other types of techniques. | 1807.08707v1 |
2019-04-09 | Band structure of Ge$_{1-x}$Sn$_{x}$alloy: a full-zone 30-band $k$$\cdot$$p$ model | A full-zone 30-band $k$$\cdot$$p$ model is developed as an efficient and
reliable tool to compute electronic band structure in Ge$_{1-x}$Sn$_{x}$ alloy.
The model was first used to reproduce the electronic band structures in Ge and
$\alpha$-Sn obtained with empirical tight binding and \textit{ab initio}
methods. Input parameters for the 30-band $k$$\cdot$$p$ model are carefully
calibrated against prior empirical predications and experimental data.
Important material properties such as effective mass for electrons and holes,
Luttinger parameters, and density of states are obtained for Ge$_{1-x}$Sn$_{x}$
alloy with the composition range $0<x<0.3$. The 30-band $k$$\cdot$$p$ model
that requires far less computing resources is a necessary capability for
optimization of sophisticated devices made from Ge$_{1-x}$Sn$_{x}$ alloy with a
large parameter space to explore. | 1904.04415v1 |
2019-04-15 | Mid-infrared (3-8 μm) $\text{Ge}_{1-y}\text{Sn}_y$ alloys (0.15 < $y$ < 0.30): synthesis, structural, and optical properties | $\text{Ge}_{1-y}\text{Sn}_y$ alloys with compositions in the 0.15 < $y$ <
0.30 range have been grown directly on Si substrates using a chemical vapor
deposition approach that allows for growth temperatures as high as 290
$^{\circ}$C. The films show structural properties that are consistent with
results from earlier materials with much lower Sn concentrations. These include
the lattice parameter and the Ge-Ge Raman frequency, which are found to depend
linearly on composition. The simplicity of the structures, directly grown on
Si, makes it possible to carry out detailed optical studies. Sharp absorption
edges are found, reaching 8 $\mu$m near $y$ =0.3. The compositional dependence
of edge energies shows a cubic deviation from the standard quadratic alloy
expression. The cubic term may dramatically impact the ability of the alloys to
cover the long-wavelength (8-12 $\mu$m) mid-IR atmospheric window. | 1904.07201v2 |
2019-10-29 | Icosahedral quasicrystal, 1/1 and 2/1 approximants in Zn-based ternary alloys containing Au and Yb/Tb | In a narrow composition range centered at Zn74.5Au10.5Yb15.0, Tsai-type
icosahedral quasicrystal is formed in alloys quenched from 880 C. This
quasicrystal belongs to the primitive type with the 6-dimensional lattice
parameter a6D=7.378 A. The quasicrystal was not formed in the slowly cooled
specimen, and was considered a metastable phase. The stable phase is a 2/1
approximant of the lattice parameter a2/1=23.271 A. This approximant forms
exclusively in Zn76.0Au9.0Yb15.0 alloy annealed at 530 C. In addition,
Zn70.5Au15.5Tb14.0 alloy annealed at 505 C forms a Tsai-type 1/1 approximant
(a1/1=14.343 A). These new Zn-based phases observed in this study correspond to
the quasicrystal-related phases in binary Cd-lanthanoid systems, and show the
possibility of isostructural substitution of Cd by Zn/Au. | 1910.13078v1 |
2012-09-25 | Automatic Unbounded Verification of Alloy Specifications with Prover9 | Alloy is an increasingly popular lightweight specification language based on
relational logic. Alloy models can be automatically verified within a bounded
scope using off-the-shelf SAT solvers. Since false assertions can usually be
disproved using small counter-examples, this approach suffices for most
applications. Unfortunately, it can sometimes lead to a false sense of
security, and in critical applications a more traditional unbounded proof may
be required. The automatic theorem prover Prover9 has been shown to be
particularly effective for proving theorems of relation algebras [7], a
quantifier-free (or point-free) axiomatization of a fragment of relational
logic. In this paper we propose a translation from Alloy specifications to fork
algebras (an extension of relation algebras with the same expressive power as
relational logic) which enables their unbounded verification in Prover9. This
translation covers not only logic assertions, but also the structural aspects
(namely type declarations), and was successfully implemented and applied to
several examples. | 1209.5773v1 |
2014-06-01 | The application of radiation diffuse scattering to the calculation of phase diagrams of F.C.C. substitutional alloys | By using quantitative information about the radiation diffuse-scattering
intensity of the disordered f.c.c. substitutional alloy the Fourier component
of mixing energies of atoms may be estimated. We have to use the measurement
data of the diffuse-scattering intensities at the corresponding
reciprocal-space points k of the disordered phase and then determine the
parameter w(k). The statistical thermodynamics of the nonideal solid solution
is determined by these energy parameters {w(k)}. Therefore, one can obtain the
configuration free energy of an alloy, F = U - TS (U - internal energy, S -
entropy), and then determine its fundamental thermodynamic characteristics,
including not only its phase diagram, but also the concentration-dependent
order-disorder transformation temperature, temperature and concentration
long-range order parameter dependences, chemical activity, heat capacity etc.
Some thermodynamic properties are calculated within the framework of the
statistical-thermodynamic approach for f.c.c.-Ni-Fe alloy. The
diffuse-scattering intensity values are taken from data in the literature. | 1406.0161v1 |
2018-02-19 | Borderline Magnetism: How Does Adding Magnesium to Paramagnetic CeCo$_3$ Make a 450 K Ferromagnet with Large Magnetic Anisotropy? | A recent experimental study (Phys. Rev. Appl. 9, 024023, 2018) on
paramagnetic CeCo$_3$ finds that Magnesium alloying induces a ferromagnetic
transition with intrinsic properties large enough for permanent magnet
applications. Here we explain these surprising results \textit{via} a first
principles study of the electronic structure and magnetism of Magnesium-alloyed
CeCo$_3$. We find the origin of this Magnesium-induced ferromagnetic transition
to be Stoner physics - the substantial increase in the Fermi-level
density-of-states $N(E_F)$ with Mg alloying. Our calculations suggest that both
Ce and Co atoms are important for generating large magnetic anisotropy
suggesting the viability of Co-3$d$, and Ce-4$f$ interaction for the generation
of magnetic anisotropy in magnetic materials. These results offer a new route
to the discovery of ferromagnetic materials and provide fundamental insight
into the magnetic properties of these alloys | 1802.06747v2 |
2018-10-31 | Voltage effects on the stability of Pd ensembles in Pd-Au/Au(111) surface alloys | The catalytic performance of multimetallic electrodes is often attributed to
a beneficial combination of ligand, strain, and ensemble effects. Understanding
the influence of the electrochemical environment on the stability of the alloy
surface structure is thus a crucial component to the design of highly active
and durable electrocatalysts. In this work, we study the effects of an applied
voltage to electrocatalytic Pd-Au/Au(111) surface alloys in contact with a
model continuum electrolyte. Using planewave density functional theory,
two-dimensional cluster expansions are parameterized and used to simulate
dilute Pd-Au surface alloys under electrochemical conditions via Metropolis
Monte Carlo within an extended canonical ensemble. While Pd monomers are stable
at all potentials considered, different extents of surface electrification are
observed to promote the formation of Pd dimers and trimers, as well as clusters
of Pd monomers. We find that the relative proportion of monomer, dimer, and
trimer surface fractions is in good agreement with in situ scanning tunneling
microscopy measurements. The further development and refinement of the
approaches described herein may serve as a useful aid in the development of
next-generation electrocatalysts. | 1812.11018v1 |
2019-02-01 | Metallic glasses for biodegradable implants | Metallic glasses are excellent candidates for biomedical implant applications
due to their inherent strength and corrosion resistance. Use of metallic
glasses in structural applications is limited, however, because bulk dimensions
are challenging to achieve. Glass-forming ability (GFA) varies strongly with
alloy composition and becomes more difficult to predict as the number of
chemical species in a system increases. Here we present a theoretical model -
implemented in the AFLOW framework - for predicting GFA based on the
competition between crystalline phases, and apply it to biologically relevant
binary and ternary systems. Elastic properties are estimated based on the rule
of mixtures for alloy systems that are predicted to be bulk glass-formers.
Focusing on Ca- and Mg-based systems for use in biodegradable orthopedic
support applications, we suggest alloys in the AgCaMg and AgMgZn families for
further study; and alloys based on the compositions: Ag$_{0.33}$Mg$_{0.67}$,
Cu$_{0.5}$Mg$_{0.5}$, Cu$_{0.37}$Mg$_{0.63}$ and
Cu$_{0.25}$Mg$_{0.5}$Zn$_{0.25}$. | 1902.00485v1 |
2019-02-25 | Anomalous local distortion in BCC refractory high-entropy alloys | Whereas exceptional mechanical and radiation performances have been found in
the emergent medium- and high-entropy alloys (MEAs and HEAs), the importance of
their complex atomic environment, reflecting diversity in atomic size and
chemistry, to defect transport has been largely unexplored at the atomic level.
Here we adopt a local structure approach based on the atomic pair distribution
function measurements in combination with density functional theory
calculations to investigate a series of body-centered cubic (BCC) MEAs and
HEAs. Our results demonstrate that all alloys exhibit local lattice distortions
(LLD) to some extent, but an anomalous LLD, merging of the first and second
atomic shells, occurs only in the Zr- and/or Hf-containing MEAs and HEAs. In
addition, through the ab-initio simulations we show that charge transfer among
the elements profoundly reduce the size mismatch effect. The observed
competitive coexistence between LLD and charge transfer not only demonstrates
the importance of the electronic effects on the local environments in MEAs and
HEAs, but also provides new perspectives to in-depth understanding of the
complicated defect transport in these alloys. | 1902.09279v1 |
2019-06-17 | Phase selection and microstructure of slowly solidified Al-Cu-Fe alloys | The search for effective methods to fabricate bulk single-phase
quasicrystalline Al-Cu-Fe alloys is currently an important task. Crucial to
solving this problem is to understand mechanisms of phase formation in this
system. Here we study crystallization sequence during solidification as well as
the conditions of solid phase formation in slowly solidified Al-Cu-Fe alloys in
a wide range of compositions. Concentration dependencies of undercoolability
were also constructed by differential thermal analysis method. These
experimental results are compared with data on chemical short-range order in
the liquid state determined from \textit{ab initio} molecular dynamic
simulations. We observe that main features of interatomic interaction in the
Al-Cu-Fe alloys are similar for both liquid and solid states and they change in
the vicinity of i-phase composition. In the concentration region, where the
i-phase forms from the melt, both the undercoolability and the crystallization
character depend on the temperature of the melts before cooling. | 1906.06941v1 |
2020-04-01 | Ab initio study of band gap properties in novel metastable BC8/ST12 Si$_x$Ge$_{1-x}$ alloys | The cubic $Ia\bar{3}$ (BC8) and tetragonal $P4_32_12$ (ST12) high pressure
modifications of Si and Ge are attractive candidates for applications in
optoelectronic, thermoelectric or plasmonic devices. Si$_x$Ge$_{1-x}$ alloys in
BC8/ST12 modifications could help overcome the indirect and narrow band gaps of
the pure phases and enable tailoring for specific use-cases. Such alloys have
experimentally been found to be stable at ambient conditions after release from
high pressure synthesis, however their fundamental properties are not known. In
this work, we employ {\it ab initio} calculations based on density functional
theory (DFT) to investigate the electronic properties of these compounds as a
function of composition $x$. We obtain the effective band structures of
intermediate alloys by constructing special quasi random structures (SQS) and
unfolding their band structure to the corresponding primitive cell.
Furthermore, we show that the indirect band gap of the ST12 Ge end-member can
be tuned to become direct at $x_\text{Si} \approx 0.16$. Finally, our
investigations also demonstrate that the BC8 modification, on the other hand,
is insensitive to compositional changes and is a narrow direct band gap
semiconductor only in the case of pure Si. | 2004.00461v1 |
2020-04-05 | Bulk nanostructured AlCoCrFeMnNi chemically complex alloy synthesized by laser-powder bed fusion process | We report the synthesis of a bulk nanostructured alloy using the laser-powder
bed fusion process. The equiatomic AlCoCrFeMnNi chemically complex alloy forms
a nanoscale modulated structure, which is homogeneously distributed in the
as-built condition. The nanostructure consisted of Al & Ni-rich ordered (B2)
and Cr & Fe-rich disordered (A2) BCC phases. The two phases form an
interconnected phase-network with coherent interface boundaries.
Atom-probe-tomography and aberration-corrected scanning transmission electron
microscopy analysis of the spatial distribution of the modulated structure
suggests the occurrence of nano-scale spinodal decomposition. These results
introduce a direct synthesis of bulk nanostructured alloys with promising
geometric flexibility. | 2004.02309v1 |
2020-04-08 | Ti-alloying of BaZrS3 chalcogenide perovskite for photovoltaics | BaZrS3, a prototypical chalcogenide perovskite, has been shown to possess a
direct band gap, an exceptionally strong near band edge light absorption, and
good carrier transport. Coupled with its great stability, non-toxicity with
earth abundant elements, it is thus a promising candidate for thin film solar
cells. However, its reported band gap in the range of 1.7-1.8 eV is larger than
the optimal value required to reach the Shockley-Queisser limit of a single
junction solar cell. Here we report the synthesis of Ba(Zr1-xTix)S3 perovskite
compounds with a reduced band gap. It is found that Ti alloying is extremely
effective in band gap reduction of BaZrS3: a mere 4 at% alloying decreases the
band gap from 1.78 to 1.51 eV, resulting in a theoretical maximum power
conversion efficiency of 32%. Higher Ti-alloying concentration is found to
destabilize the distorted chalcogenide perovskite phase. | 2004.04261v1 |
2020-04-18 | Seeking for low thermal conductivity atomic configurations in $\rm{Si_{0.5}Ge_{0.5}}$ alloys with Bayesian Optimization | The emergence of data-driven science has opened up new avenues for
understanding the thermophysical properties of materials. For decades, alloys
are known to possess very low thermal conductivity, but the extreme thermal
conductivity can be achieved by alloying has never been identified. In this
work, we combine the Bayesian optimization with a high throughput thermal
conductivity calculation to search for the lowest thermal conductivity atomic
configuration of $\rm{Si_{0.5}Ge_{0.5}}$ alloy. It is found layered structures
are most beneficial for reducing the thermal conductivity among all atomic
configurations, which is attributed to the strong branch-folding effect.
Furthermore, the roles of interface roughness and layer thicknesses in
producing the lowest thermal conductivity are investigated. Through another
comprehensive search using Bayesian optimization, the layered structure with
smooth interfaces and optimized layer thickness arrangement is identified as
the optimal structure with the lowest thermal conductivity. | 2004.08654v1 |
2020-05-26 | Localization of electronic states resulting from electronic topological transitions in the Mo$_{1-x}$Re$_x$ alloys: A photoemission study | We present the results of resonant photoemission spectroscopy experiments on
the Mo$_{1-x}$Re$_{x}$ alloy compositions spanning over two electronic
topological transitions (ETT) at the critical concentrations $x_{C1}$ = 0.05
and $x_{C2}$ = 0.11. The photoelectrons show an additional resonance ($R3$) in
the constant initial state (CIS) spectra of the alloys along with two
resonances ($R1$ and $R2$) which are similar to those observed in molybdenum.
All the resonances show Fano-like line shapes. The asymmetry parameter $q$ of
the resonances $R1$ and $R3$ of the alloys is observed to be large and
negative. Our analysis suggests that the origin of large negative q is
associated with phonon assisted inter band scattering between the Mo-like
states and the narrow band that appeared due to the ETT. | 2005.12835v1 |
2020-05-26 | Machine learning formation enthalpies of intermetallics | Developing fast and accurate methods to discover intermetallic compounds is
relevant for alloy design. While density-functional-theory (DFT)-based methods
have accelerated design of binary and ternary alloys by providing rapid access
to the energy and properties of the stable intermetallics, they are not
amenable for rapidly screening the vast combinatorial space of multi-principal
element alloys (MPEAs). Here, a machine-learning model is presented for
predicting the formation enthalpy of binary intermetallics and used to identify
new ones. The model uses easily accessible elemental properties as descriptors
and has a mean absolute error (MAE) of 0.025 eV/atom in predicting the
formation enthalpy of stable binary intermetallics reported in the Materials
Project database. The model further predicts stable intermetallics to form in
112 binary alloy systems that do not have any stable intermetallics reported in
the Materials Project database. DFT calculations confirm one such stable
intermetallic identified by the model, NbV2 to be on the convex hull. The model
trained with binary intermetallics can also predict ternary intermetallics with
similar accuracy as DFT, which suggests that it could be extended to identify
compositionally complex intermetallics that may form in MPEAs. | 2005.13046v1 |
2020-05-28 | Robustness of helical magnetic structure under external pressure in Fe-doped MnNiGe alloy | We have investigated the detailed magnetic structure of a Fe-doped MnNiGe
alloy of nominal composition MnNi$_{0.75}$Fe$_{0.25}$Ge by ambient and high
pressure (6 kbar) neutron powder diffraction study. The alloy undergoes a
martensitic phase transition between 230 K and 275 K. The low-temperature
martensite phase orders antiferromagnetically with helical modulation of
Mn-spins. At 1.5 K, the incommensurate propagation vector $\bf k$ is found to
be (0.1790(1),0,0), and it remains almost unchanged with temperature under
ambient condition. The Application of external pressure ($P$) reduces the
martensitic transition temperature and results in a significant change in the
incommensurate magnetic propagation vector. At 10 K, with 6 kbar of external
pressure, the incommensurate magnetic structure of the alloy stabilizes with
$\bf k$ = (0.1569(1),0,0). Interestingly, the strong temperature dependence of
$\bf k$ in the presence of external $P$ has also been observed. | 2005.13909v1 |
2020-05-28 | A new high-throughput method using additive manufacturing for alloy design and heat treatment optimization | Many alloys made by Additive Manufacturing (AM) require careful design of
post-heat treatment as an indispensable step of microstructure engineering to
further enhance the performance. We developed a high-throughput approach by
fabricating a long-bar sample heat-treated under a monitored gradient
temperature zone for phase transformation study to accelerate the post-heat
treatment development of AM alloys. This approach has been proven efficient in
determining the aging temperature with peak hardness. We observed that the
precipitation strengthening is predominant for the studied superalloy by laser
powder bed fusion, and the grain size variation is insensitive on temperature
between 605 and 825 Celcius. This new approach can be applied to post-heat
treatment optimization of other materials made by AM, and further assist new
alloy development. | 2005.14092v2 |
2020-08-14 | Atomistic study of grain-boundary segregation and grain-boundary diffusion in Al-Mg alloys | Mg grain boundary (GB) segregation and GB diffusion can impact the processing
and properties of Al-Mg alloys. Yet, Mg GB diffusion in Al has not been
measured experimentally or predicted by simulations. We apply atomistic
computer simulations to predict the amount and the free energy of Mg GB
segregation, and the impact of segregation on GB diffusion of both alloy
components. At low temperatures, Mg atoms segregated to a tilt GB form clusters
with highly anisotropic shapes. Mg diffuses in Al GBs slower than Al itself,
and both components diffuse slowly in comparison with Al GB self-diffusion.
Thus, Mg segregation significantly reduces the rate of mass transport along GBs
in Al-Mg alloys. The reduced atomic mobility can be responsible for the
improved stability of the microstructure at elevated temperatures. | 2008.06145v2 |
2020-08-24 | Enhancement of Spin-charge Conversion in Dilute Magnetic Alloys by Kondo Screening | We derive a kinetic theory capable of dealing both with large spin-orbit
coupling and Kondo screening in dilute magnetic alloys. We obtain the collision
integral non-perturbatively and uncover a contribution proportional to the
momentum derivative of the impurity scattering S-matrix. The latter yields an
important correction to the spin diffusion and spin-charge conversion
coefficients, and fully captures the so-called side-jump process without
resorting to the Born approximation (which fails for resonant scattering), or
to otherwise heuristic derivations. We apply our kinetic theory to a quantum
impurity model with strong spin-orbit, which captures the most important
features of Kondo-screened Cerium impurities in alloys such as
La$_{1-x}$Cu$_6$. We find 1) a large zero-temperature spin Hall conductivity
that depends solely on the Fermi wave number and 2) a transverse spin diffusion
mechanism that modifies the standard Fick's diffusion law. Our predictions can
be readily verified by standard spin-transport measurements in metal alloys
with Kondo impurities. | 2008.10185v2 |
2021-03-23 | Dark-bright excitons mixing in alloyed InGaAs self-assembled quantum dots | Quantum dots are arguably one of the best platforms for optically accessible
spin based qubits. The paramount demand of extended qubit storage time can be
met by using quantum-dot-confined dark exciton: a longlived electron-hole pair
with parallel spins. Despite its name the dark exciton reveals weak
luminescence that can be directly measured. The origins of this optical
activity remain largely unexplored. In this work, using the atomistic
tight-binding method combined with configuration-interaction approach, we
demonstrate that atomic-scale randomness strongly affects oscillator strength
of dark excitons confined in self-assembled cylindrical InGaAs quantum dots
with no need for faceting or shape-elongation. We show that this process is
mediated by two mechanisms: mixing dark and bright configurations by exchange
interaction, and equally important appearance of non-vanishing optical
transition matrix elements that otherwise correspond to nominally forbidden
transitions in a non-alloyed case. The alloy randomness has essential impact on
both bright and dark exciton states, including their energy, emission
intensity, and polarization angle. We conclude that, due to the atomic-scale
alloy randomness, finding dots with desired dark exciton properties may require
exploration of a large ensemble, similarly to how dots with low bright exciton
splitting are selected for entanglement generation. | 2103.12356v1 |
2021-03-27 | Structure-Composition-Property Relationships in Antiperovskite Nitrides: Guiding a Rational Alloy Design | The alloy strategy through A- or X-site is a common method for experimental
preparation of high-performance and stable lead-based perovskite solar cells.
As one of the important candidates for lead-free and stable photovoltaic
absorber, the inorganic antiperovskite family has recently been reported to
exhibit excellent optoelectronic properties. However, the current reports on
the design of antiperovskite alloys are rare. In this work, we investigated the
previously overlooked electronic property (e.g., conduction band convergence),
static dielectric constant, and exciton binding energy in inorganic
antiperovskite nitrides by first-principles calculations. Then, we reveal a
linear relationship between tolerance factor and various physical quantities.
Guided by the established structure-composition-property relationship in six
antiperovskite nitrides X3NA (X2+ = Mg2+, Ca2+, Sr2+; A3- = P3-, As3-, Sb3-,
Bi3-), for the first time, we design a promising antiperovskite alloy
Mg3NAs0.5Bi0.5 with the quasi-direct band gap of 1.402 eV. Finally, we make a
comprehensive comparison between antiperovskite nitrides and conventional
halide perovskites for pointing out the future direction for device
applications. | 2103.14944v2 |
2022-03-01 | Towards quantitative inference of nanoscale defects in irradiated metals and alloys | Quantifying the population of nanoscale defects that are formed in metals and
alloys exposed to extreme radiation environments remains a pressing challenge
in materials science. These defects both fundamentally alter material
properties and seed long-timescale performance degradation, which often limits
the lifespan of engineering systems. Unlike ceramic and semiconducting
materials, these defects in metals and alloys are not spectroscopically active,
forcing characterization to rely on indirect measurements from which the
distribution of nanoscale defects may be inferred. In this mini-review,
different experimental methodologies which have been employed for defect
inference are highlighted to capture the current state of the art. Future
directions in this area are proposed, which, by combining data streams from
multiple and complementary characterization methods in concert with multi-scale
modeling and simulation, will enable the ultimate goal of quantifying the full
spectrum of defects in irradiated metals and alloys. | 2203.00760v2 |
2022-03-12 | Convolutional neural networks enable high-fidelity prediction of path-dependent diffusion barrier spectra in multi-principal element alloys | The emergent multi-principal element alloys (MPEAs) provide a vast
compositional space to search for novel materials for technological advances.
However, how to efficiently identify optimal compositions from such a large
design space for targeted properties is a grand challenge in material science.
Here we developed a convolutional neural network (CNN) model that can
accurately and efficiently predict path-dependent vacancy migration energy
barriers, which are critical to diffusion behaviors and many high-temperature
properties, of MPEAs at any compositions and with different chemical
short-range orders within a given alloy system. The success of the CNN model
makes it promising for developing a database of diffusion barriers for
different MPEA systems, which would accelerate alloy screening for the
discovery of new compositions with desirable properties. Besides, the length
scale of local configurations relevant to migration energy barriers is
uncovered, and the implications of this success to other aspects of materials
science are discussed. | 2203.06503v3 |
2022-03-16 | Simple machine-learned interatomic potentials for complex alloys | Developing data-driven machine-learning interatomic potentials for materials
containing many elements becomes increasingly challenging due to the vast
configuration space that must be sampled by the training data. We study the
learning rates and achievable accuracy of machine-learning interatomic
potentials for many-element alloys with different combinations of descriptors
for the local atomic environments. We show that for a five-element alloy
system, potentials using simple low-dimensional descriptors can reach
meV/atom-accuracy with modestly sized training datasets, significantly
outperforming the high-dimensional SOAP descriptor in data efficiency,
accuracy, and speed. In particular, we develop a computationally fast
machine-learned and tabulated Gaussian approximation potential (tabGAP) for
Mo-Nb-Ta-V-W alloys with a combination of two-body, three-body, and a new
simple scalar many-body density descriptor based on the embedded atom method. | 2203.08458v2 |
2022-03-19 | Multiscale simulation of powder-bed fusion processing of metallic alloys | We present a computational framework for the simulations of powder-bed fusion
of metallic alloys, which combines: (1) CalPhaD calculations of
temperature-dependent alloy properties and phase diagrams, (2) macroscale
finite element (FE) thermal simulations of the material addition and fusion,
and (3) microscopic phase-field (PF) simulations of solidification in the melt
pool. The methodology is applied to simulate the selective laser melting (SLM)
of an Inconel 718 alloy using realistic processing parameters. We discuss the
effect of temperature-dependent properties and the importance of accounting for
different properties between the powder bed and the dense material in the
macroscale thermal simulations. Using a two-dimensional longitudinal slice of
the thermal field calculated via FE simulations, we perform an
appropriately-converged PF solidification simulation at the scale of the entire
melt pool, resulting in a calculation with over one billion grid points, yet
performed on a single cluster node with eight graphics processing units (GPUs).
These microscale simulations provide new insight into the grain texture
selection via polycrystalline growth competition under realistic SLM
conditions, with a level of detail down to individual dendrites. | 2203.10370v1 |
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