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2019-07-31 | The Effect of Chemical Disorder on Defect Formation and Migration in Disordered MAX Phases | MAX phases have attracted increased attention due to their unique combination
of ceramic and metallic properties. Point-defects are known to play a vital
role in the structural, electronic and transport properties of alloys in
general and this system in particular. As some MAX phases have been shown to be
stable in non-stoichiometric compositions, it is likely that such alloying
effects will affect the behavior of lattice point defects. This problem,
however, remains relatively unexplored. In this work, we investigate the
alloying effect on the structural-stability, energy-stability,
electronic-structure, and diffusion barrier of point defects in MAX phase
alloys within a first-principles density functional theory framework. The
vacancy (V$_{M}$, V$_{A}$, V$_{X}$) and antisite (M-A; M-X) defects are
considered with M and A site disorder in (Zr-M)$_{2}$(AA${'}$)C, where
M=Cr,Nb,Ti and AA${'}$=Al, Al-Sn, Pb-Bi. Our calculations suggest that the
chemical disorder helps lower the V$_{A}$ formation energies compared to
V$_{M}$ and V$_{X}$. The V$_{A}$ diffusion barrier is also significantly
reduced for M-site disorder compared to their ordered counterpart. This is very
important finding because reduced barrier height will ease the Al diffusion at
high-operating temperatures, which will help the formation of passivating oxide
layer (i.e., Al$_{2}$O$_{3}$ in aluminum-based MAX phases) and will slow down
or stop the material degradation. We believe that our study will provide a
fundamental understanding and an approach to tailor the key properties that can
lead to the discovery of new MAX phases. | 1908.00038v3 |
2019-08-02 | Tunable Electronic Structure in Gallium Chalcogenide van der Waals Compounds | Transition metal monochalcogenides comprise a class of two-dimensional
materials with electronic band gaps that are highly sensitive to material
thickness and chemical composition. Here, we explore the tunability of the
electronic excitation spectrum in GaSe using angle-resolved photoemission
spectroscopy. The electronic structure of the material is modified by
$\textit{in-situ}$ potassium deposition as well as by forming
GaS$_{x}$Se$_{1-x}$ alloy compounds. We find that potassium decouples the
top-most tetra-layer of the GaSe unit cell, leading to a substantial change of
the dispersion around the valence band maximum (VBM). The observed band
dispersion of a single tetralayer is consistent with a transition from the
direct gap character of the bulk to the indirect gap character expected for
monolayer GaSe. Upon alloying with sulfur, we observe a phase transition from
AB to $\text{AA}^{\prime}$ stacking. Alloying also results in a rigid energy
shift of the VBM towards higher binding energies which correlates with a blue
shift in the luminescence. The increase of the band gap upon sulfur alloying
does not appear to change the dispersion or character of the VBM appreciably,
implying that it is possible to engineer the gap of these materials while
maintaining their salient electronic properties. | 1908.01003v2 |
2019-08-05 | Prediction of a wide variety of linear complexions in face centered cubic alloys | Linear complexions are defect states that have been recently discovered along
dislocations in body centered cubic Fe-based alloys. In this work, we use
atomistic simulations to extend this concept and explore segregation-driven
structural transitions at dislocations in face centered cubic alloys. We
identify a variety of stable, nanoscale-size structural and chemical states,
which are confined near dislocations and can be classified as linear
complexions. Depending on the alloy system and thermodynamic conditions, such
new states can preserve, partially modify, or completely replace the original
defects they were born at. By considering different temperatures and
compositions, we construct linear complexion diagrams that are similar to bulk
phase diagrams, defining the important conditions for complexion formation
while also specifying an expected complexion size and type. Several notable new
complexion types were predicted here: (1) nanoparticle arrays comprised of L12
phases in Ni-Fe, Ni-Al, and Al-Zr, (2) replacement of stacking faults with
layered complexions comprised of (111) planes from the Cu5Zr intermetallic
phase in Cu-Zr, (3) platelet arrays comprised of two-dimensional
Guinier-Preston zones in Al-Cu, and finally (4) coexistence of multiple linear
complexions containing both Guinier-Preston zones and L12 phases in ternary
Al-Cu-Zr. All of these new complexion states are expected to alter material
properties and affect the stability of the dislocations themselves, offering a
unique opportunity for future materials design. | 1908.01849v2 |
2019-09-07 | Lithiation-delithiation cycles of amorphous Si nanowires investigated by molecular dynamics simulations | The atomistic mechanisms during lithiation and delithiation of amorphous Si
nanowires ($a$-SiNW) have been investigated over cycles by molecular dynamics
simulations. First, the Modified Embedded Atom Method (MEAM) potential from Cui
et al. [J. Power Sources. 2012, (207) 150] has been further optimized on static
(Li$_x$Si alloy phases and point defect energies) and on dynamic properties (Li
diffusion) to reproduce the lithiation of small crystalline Si nanowires
calculated at the {\it ab initio} level. The lithiation of $a$-SiNW reveals a
two-phase process of lithiation with a larger diffusion interface compared to
crystalline Si lithiation. Compressive axial stresses are observed in the
amorphous Si$_x$Li alloy outer shell. They are easily released thanks to the
soft glassy behavior of the amorphous alloy. Conversely, in crystalline SiNW,
the larger stress in the narrow crystalline lithiated interface is hardly
released and requires a phase transformation to amorphous to operate, which
delays the lithiation. The history of the charge-discharge cycles as well as
the temperature appear as driving forces for phase transformation from
amorphous Li$_x$Si alloy to the more stable crystalline phase counterpart. Our
work suggest that a full delithiation could heal the SiNWs to improve the life
cycles of Li-ion batteries with Si anode. | 1909.03268v1 |
2019-12-07 | Effect of chemical disorder on the electronic stopping of solid solution alloys | The electronic stopping power of nickel-based equiatomic solid solutions
alloys NiCr, NiFe and NiCo for protons and alpha projectiles is investigated in
detail using real-time time-dependent density functional theory over a wide
range of velocities. Recently developed numerical electronic structure methods
are used to probe fundamental aspects of electron-ion coupling
non-perturbatively and in a fully atomistic context, capturing the effect of
the atomic scale disorder. The effects of particular electronic band structures
and density of states reflect in the low velocity limit behavior. We compare
our results for the alloys with those of a pure nickel target to understand how
alloying affects the electronic stopping. We discover that NiCo and NiFe have
similar stopping behavior as Ni while NiCr has an asymptotic stopping power
that is more than a factor of two larger than its counterparts for velocities
below 0.1 a.u.. We show that the low-velocity limit of electronic stopping
power can be manipulated by controlling the broadening of the d-band through
the chemical disorder. In this regime, the Bragg's additive rule for the
stopping of composite materials also fails for NiCr. | 1912.03399v2 |
2019-12-18 | Fast and stable deep-learning predictions of material properties for solid solution alloys | We present a novel deep learning (DL) approach to produce highly accurate
predictions of macroscopic physical properties of solid solution binary alloys
and magnetic systems. The major idea is to make use of the correlations between
different physical properties in alloy systems to improve the prediction
accuracy of neural network (NN) models. We use multitasking NN models to
simultaneously predict the total energy, charge density and magnetic moment.
These physical properties mutually serve as constraints during the training of
the multitasking NN, resulting in more reliable DL models because multiple
physics properties are correctly learned by a single model. Two binary alloys,
copper-gold (CuAu) and iron-platinum (FePt), were studied. Our results show
that once the multitasking NN's are trained, they can estimate the material
properties for a specific configuration hundreds of times faster than
first-principles density functional theory calculations while retaining
comparable accuracy. We used a simple measure based on the root-mean-squared
errors (RMSE) to quantify the quality of the NN models, and found that the
inclusion of charge density and magnetic moment as physical constraints leads
to more stable models that exhibit improved accuracy and reduced uncertainty
for the energy predictions. | 1912.11152v3 |
2019-12-24 | Influence of thermomechanical loads on the energetics of precipitation in magnesium aluminum alloys | We use first principles calculations to study the influence of
thermomechanical loads on the energetics of precipitation in magnesium-aluminum
alloys. Using Density Functional Theory simulations, we present expressions of
the energy of magnesium-aluminum binary solid solutions as a function of
concentration, strain and temperature. Additionally, from these calculations,
we observe an increase in equilibrium volume (and hence the equilibrium lattice
constants) with the increase in concentration of magnesium. We also observe an
increase in the cohesive energy of solutions with increase in concentration,
and also present their dependence on strain. Calculations also show that the
formation enthalpy of $\beta$ phase solutions to be strongly influenced by
hydrostatic stress, however the formation enthalpy of $\alpha$ phase solutions
remain unaffected by hydrostatic stress. We present an expression of the free
energy of any magnesium aluminum solid solution, that takes into account the
contributions of strain and temperature. We note that these expressions can
serve as input to process models of magnesium-aluminum alloys. We use these
expressions to report the influence of strains and temperature on the
solubility limits and equilibrium chemical potential in Mg-Al alloys. Finally,
we report the influence of thermomechanical loads on the growth of
precipitates, where we observe compressive strains along the $c$ axis promotes
growth, whereas strains along the $a$ and $b$ directions do not influence the
growth of precipitates. | 1912.11434v2 |
2020-02-17 | Ductile and brittle crack-tip response in equimolar refractory high-entropy alloys | Understanding the strengthening and deformation mechanisms in refractory
high-entropy alloys (HEAs), proposed as new high-temperature material, is
required for improving their typically insufficient room-temperature ductility.
Here, density-functional theory simulations and a continuum mechanics analysis
were conducted to systematically investigate the competition between cleavage
decohesion and dislocation emission from a crack tip in the body-centered cubic
refractory HEAs HfNbTiZr, MoNbTaVW, MoNbTaW, MoNbTiV, and NbTiVZr. This
crack-tip competition is evaluated for tensile loading and a totality of 15
crack configurations and slip systems. Our results predict that dislocation
plasticity at the crack tip is generally unfavorable -- although the
competition is close for some crack orientations, suggesting intrinsic
brittleness and low crack-tip fracture toughness in these five HEAs at zero
temperature. Fluctuations in local alloy composition, investigated for
HfNbTiZr, can locally reduce the resistance to dislocation emission for a slip
system relative to the configuration average of that slip system, but do not
change the dominant crack-tip response. In the case of single-crystal MoNbTaW,
where an experimental, room-temperature fracture-toughness value is available
for a crack on a \{100\} plane, theoretical and experimental results agree
favorably. Factors that may limit the agreement are discussed. We survey the
effect of material anisotropy on preferred crack tip orientations, which are
found to be alloy specific. Mixed-mode loadings are found to shift the
competition in favor of cleavage or dislocation nucleation, depending on crack
configuration and amplified by the effect of material anisotropy on crack tip
stresses. | 2002.07013v1 |
2020-06-09 | Orbital-enhanced Warping Effect in P\textsubscript{x},P\textsubscript{y}-derived Rashba Spin Splitting of Monatomic Bismuth Surface Alloy Surface Alloy | Spin-split Rashba bands have been exploited to efficiently control the spin
degree of freedom of moving electrons, which possesses a great potential in
frontier applications of designing spintronic devices and processing spin-based
information. Given that intrinsic breaking of inversion symmetry and sizeable
spin-orbit interaction, two-dimensional (2D) surface alloys formed by heavy
metal elements exhibit a pronounced Rashba-type spin splitting of the surface
states. Here, we have revealed the essential role of atomic orbital symmetry in
the hexagonally warped Rashba spin-split surface state of
$\sqrt{3}\times\sqrt{3} R30^{\circ}$ BiCu$_{2}$ monatomic alloy by scanning
tunneling spectroscopy (STS) and density functional theory (DFT). From
$\mathrm{d}I/\mathrm{d}U$ spectra and calculated band structures, three
hole-like Rashba-split bands hybridized from distinct orbital symmetries have
been identified in the unoccupied energy region. Because of the hexagonally
deformed Fermi surface, quasi-particle interference (QPI) mappings have
resolved scattering channels opened from interband transitions of
\textit{p$_{x},$p$_{y}$}($m_{j}=1/2$) band. In contrast to the
\textit{s,p$_{z}$}-derived band, the hexagonal warping predominately is
accompanied by substantial out-of-plane spin polarization $S_{z}$ up to 24\% in
the dispersion of \textit{p$_{x}$,p$_{y}$}($m_{j}=1/2$) band with an in-plane
orbital symmetry. | 2006.05024v1 |
2020-07-30 | Elastic dipole tensors and relaxation volumes of point defects in concentrated random magnetic Fe-Cr alloys | Point defects in body-centred cubic Fe, Cr and concentrated random magnetic
Fe-Cr are investigated using density functional theory and theory of
elasticity. The volume of a substitutional Cr atom in ferromagnetic bcc Fe is
approximately 18\% larger than the volume of a host Fe atom, whereas the volume
of a substitutional Fe atom in antiferromagnetic bcc Cr is 5\% smaller than the
volume of a host Cr atom. Elastic dipole $\boldsymbol{P}$ and relaxation volume
$\boldsymbol{\Omega}$ tensors of vacancies and self-interstitial atom (SIA)
defects exhibit large fluctuations, with vacancies having negative and SIA
large positive relaxation volumes. Dipole tensors of vacancies are nearly
isotropic across the entire alloy composition range, with diagonal elements
$P_{ii}$ decreasing as a function of Cr content. Fe-Fe and Fe-Cr SIA dumbbells
are more anisotropic than Cr-Cr dumbbells. Fluctuations of elastic dipole
tensors of SIA defects are primarily associated with the variable
crystallographic orientations of the dumbbells. Statistical properties of
tensors $\boldsymbol{P}$ and $\boldsymbol{\Omega}$ are analysed using their
principal invariants, suggesting that point defects differ significantly in
alloys containing below and above 10\% at. Cr. The relaxation volume of a
vacancy depends sensitively on whether it occupies a Fe or a Cr lattice site. A
correlation between elastic relaxation volumes and magnetic moments of defects
found in this study suggests that magnetism is a significant factor influencing
elastic fields of defects in Fe-Cr alloys. | 2007.15424v1 |
2020-09-17 | Hydride growth mechanism in Zircaloy-4: investigation of the partitioning of alloying elements | The long-term safety of water-based nuclear reactors relies in part on the
reliability of zirconium-based nuclear fuel. Yet the progressive ingress of
hydrogen during service makes zirconium alloys subject to delayed hydride
cracking. Here, we use a combination of electron back-scattered diffraction and
atom probe tomography to investigate specific microstructural features from the
as-received sample and in the blocky-alpha microstructure, before and after
electrochemical charging with hydrogen or deuterium followed by a low
temperature heat treatment at 400C for 5 hours followed by furnace cooling at a
rate of 0. 5C per min. Specimens for atom probe were prepared at cryogenic
temperature to avoid the formation of spurious hydrides. We report on the
compositional evolution of grains and grain boundaries over the course of the
sample's thermal history, as well as the ways the growth of the hydrides
modifies locally the composition and the structure of the alloy. We observe a
significant amount of deuterium left in the matrix, even after the slow cooling
and growth of the hydrides. Stacking faults form ahead of the growth front and
Sn segregates at the hydride-matrix interface and on these faults. We propose
that this segregation may facilitate further growth of the hydride. Our
systematic investigation enables us discuss how the solute distribution affects
the evolution of the alloy's properties during its service lifetime. | 2009.08073v1 |
2020-09-27 | Exploring In$_2$(Se$_{1-x}$Te$_x$)$_3$ alloys as photovoltaic materials | In$_2$Se$_3$ in the three-dimensional (3D) hexagonal crystal structure with
space group $P6_1$ ($\gamma$-In$_2$Se$_3$) has a direct band gap of $\sim$1.8
eV and high absorption coefficient, making it a promising semiconductor
material for optoelectronics. Incorporating Te allows for tuning the band gap,
adding flexibility to device design and extending the application range. Here
we report the growth and characterization of $\gamma$-In$_2$Se$_3$ thin films,
and results of hybrid density functional theory calculations to assess the
electronic and optical properties of $\gamma$-In$_2$Se$_3$ and
$\gamma$-In$_2$(Se$_{1-x}$Te$_x$)$_3$ alloys. The calculated band gap of 1.84
eV for $\gamma$-In$_2$Se$_3$ is in good agreement with data from the absorption
spectrum, and the absorption coefficient is found to be as high as that of
direct band gap conventional III-V and II-VI semiconductors. Incorporation of
Te in the form of $\gamma$-In$_2$(Se$_{1-x}$Te$_x$)$_3$ alloys is an effective
way to tune the band gap from 1.84 eV down to 1.23 eV, thus covering the
optimal band gap range for solar cells. We also discuss band gap bowing and
mixing enthalpies, aiming at adding $\gamma$-In$_2$Se$_3$ and
$\gamma$-In$_2$(Se$_{1-x}$Te$_x$)$_3$ alloys to the available toolbox of
materials for solar cells and other optoelectronic devices. | 2009.12885v2 |
2020-10-22 | Computational property predictions of Ta-Nb-Hf-Zr high-entropy alloys | Refractory high entropy alloys (R-HEAs) are having properties and uses as
high strength and high hardness materials for ambient and high temperature,
aerospace and nuclear radiation tolerance applications, orthopedic applications
etc. The mechanical properties like yield strength and ductility of TaNbHfZr
R-HEA depend on the local nanostructure and chemical ordering. In this study we
have computationally obtained various properties of the TaNbHfZr alloy like the
role of configurational entropy in the thermodynamic property, rate of
evolution of nanostructure morphology in thermally annealed systems,
dislocation simulation based quantitative prediction of yield strength, nature
of dislocation movement through short range clustering (SRC) and qualitative
prediction of ductile to brittle transition behavior. The simulation starts
with hybrid Monte Carlo/ Molecular Dynamics (MC/MD) based nanostructure
evolution of an initial random solid solution alloy structure with BCC lattice
structure created with principal axes along [1 1 1], [-1 1 0] and [-1 -1 2]
directions suitable for simulation of 1/2[1 1 1] edge dislocations.
Thermodynamic properties are calculated from the change in enthalpy and the
configurational entropy by next-neighbor bond counting statistics. The MC/MD
evolved structures mimic the annealing treatment at 1800{\deg}C and the output
structures are replicated in periodic directions to make larger 384000 atom
structures used for dislocation simulations. Edge dislocations were utilized to
obtain and explain for the extra strengthening observed because of the
formations of SRCs. Lastly the MC/MD evolved structures containing dislocations
are subjected to a high shear stress beyond CRSS to investigate the stability
of the dislocations and the lattice structures to explain the experimentally
observed transition from ductile to brittle behavior for the TaNbHfZr R-HEA. | 2010.11772v1 |
2020-12-03 | Effect of ageing on the properties of the W-containing IRIS-TiAl alloy | The effects of ageing at 800 $^\circ$C on the properties of the IRIS alloy
(Ti$_{49.9}$Al$_{48}$W$_2$B$_{0.1}$) are studied. The initial microstructure of
this alloy densified by Spark Plasma Sintering (SPS) is mainly composed of
lamellar colonies which are surrounded by $\gamma$ grains. The evolutions of
the alloy strength and creep resistance resulting from this ageing treatment
are measured by the related mechanical tests. The microstructural changes are
investigated by scanning and transmission electron microscopies and by X-ray
diffraction. The main structural evolutions consist in a shrinkage of the
lamellar areas and in a precipitation of $\beta_0$ phase, which is accompanied
by a moderate segregation of tungsten and a decrease of the $\alpha_2$ lamellar
width. However, these evolutions are relatively limited and the microstructural
stability is found to result mainly from the low diffusivity of tungsten.
Conversely, a moderate effect of this ageing treatment on mechanical
properties, at room and high temperatures, is measured. Such experimental
results are interpreted and discussed in terms of the microstructural
evolutions and of the deformation mechanisms which are activated at different
temperatures under various solicitations. | 2012.01760v1 |
2020-12-18 | Aging behavior of Haynes 282 wrought nickel superalloy subjected to a cold pre-deformation by differential speed rolling | The mechanical response of Haynes 282, a wrought gamma prime strengthened
nickel based superalloy, is tailored by using both a cold deformation
processing and a post straining heat treatment including a two step aging. In
this work, the effect of the applied cold pre deformation by conventional equal
speed rolling ESR or differential speed rolling DSR methods on the aging
behavior of Haynes 282 superalloy, is examined for the first time. For this
reason, the solid solution annealed alloy was subjected to 50 pct. nominal cold
thickness reduction by either ESR or DSR methods, followed by a two step heat
treatment at 1010C for 2h and 780C for 8h. The material structural evolution on
each processing step was characterized by using scanning electron microscopy,
electron backscatter diffraction and transmission electron microscopy methods.
Corresponding mechanical properties were examined in static tensile tests
performed on non standard miniaturized specimens. It was found that, the
introduction of a shear strain by rolls speed differentiation in the cold
rolling process results in a more efficient strain hardening of the alloy in
the as deformed state and an intensive grain refinement upon the post
deformation annealing. Consequently, the DSR provides a higher room temperature
mechanical strength obtained in both aging steps, as compared to the non
deformed and conventionally cold rolled counterparts. Furthermore, noticeable
differences in terms of stability of crystallographic orientations for ESRed or
DSRed and heat treated alloy, were noted. | 2012.10114v1 |
2021-01-12 | Reactivity of transition-metal alloys to oxygen and sulphur | Oxidation and tarnishing are the two most common initial steps in the
corrosive process of metals at ambient conditions. These are always initiated
with O and S binding to a metallic surface, so that one can use the binding
energy as a rough proxy for the metal reactivity. With this in mind, we present
a systematic study of the binding energy of O and S across the entire
transition-metals composition space, namely we explore the binding energy of
{\bf 88} single-phase transition metals and of {\bf 646} transition-metal
binary alloys. The analysis is performed by defining a suitable descriptor for
the binding energy. This is here obtained by fitting several schemes, based on
the original Newns-Anderson model, against density-functional-theory data for
the 4$d$ transition metal series. Such descriptor is then applied to a vast
database of electronic structures of transition-metal alloys, for which we are
able to predict the range of binding energies across both the compositional and
the structural space. Finally, we extend our analysis to ternary
transition-metal alloys and identify the most resilient compounds to O and S
binding. | 2101.04767v2 |
2021-05-01 | Density functional simulations of pressurized Mg-Zn and Al-Zn alloys | The Mg-Zn and Al-Zn binary alloys have been investigated theoretically under
static isotropic pressure. The stable phases of these binaries on both
initially hexagonal-close-packed (HCP) and face-centered-cubic (FCC) lattices
have been determined by utilizing an iterative approach that uses a
configurational cluster expansion method, Monte Carlo search algorithm, and
density functional theory (DFT) calculations. Based on 64-atom models, it is
shown that the most stable phases of the Mg-Zn binary alloy under ambient
condition are $\rm MgZn_3$, $\rm Mg_{19}Zn_{45}$, $\rm MgZn$, and $\rm
Mg_{34}Zn_{30}$ for the HCP, and $\rm MgZn_3$ and $\rm MgZn$ for the FCC
lattice, whereas the Al-Zn binary is energetically unfavorable throughout the
entire composition range for both the HCP and FCC lattices under all
conditions. By applying an isotropic pressure in the HCP lattice, $\rm
Mg_{19}Zn_{45}$ turns into an unstable phase at P$\approx$$10$~GPa, a new
stable phase $\rm Mg_{3}Zn$ appears at P$\gtrsim$$20$~GPa, and $\rm
Mg_{34}Zn_{30}$ becomes unstable for P$\gtrsim$$30$~GPa. For FCC lattice, the
$\rm Mg_{3}Zn$ phase weakly touches the convex hull at P$\gtrsim$$20$~GPa while
the other stable phases remain intact up to $\approx$$120$~GPa. Furthermore,
making use of the obtained DFT results, bulk modulus has been computed for
several compositions up to pressure values of the order of $\approx$$120$~GPa.
The findings suggest that one can switch between $\rm Mg$-rich and $\rm
Zn$-rich early-stage clusters simply by applying external pressure. $\rm
Zn$-rich alloys and precipitates are more favorable in terms of stiffness and
stability against external deformation. | 2105.00209v1 |
2021-05-10 | A Novel Smart Memory Alloy Re-centering Damper for Passive Protection of Structures Subjected to Seismic Excitations Using High-Performance NiTiHfPd Material | This research proposes and evaluates a superelastic memory alloy re-centering
damper system for improving the reaction of steel frame buildings that have
been exposed to several levels of seismic threat. The planned superelastic
memory alloy re-centering damper (SMARD) relies on high-performance shape
memory alloy (SMA) bars for its abilities of recentering and augments its
deformation potential with friction springs. To begin, this study investigates
the superelastic reaction of NiTiHfPd SMAs under a variety of conditions and
shows how they can be used in seismic applications. To gather experimental
results, uniaxial experiments on superelastic NiTiHfPd SMAs are performed at
temperatures ranging from -35 to 25 oC and loading frequencies ranging from
0.05 to 1 Hz with four distinct strain amplitudes. We explore the impact of
loading rate and temperature on the superelastic properties of NiTiHfPd SMAs.
The complex answer of 6-floor and 9-floor steel special moment frame buildings
with built SMARDs is then determined using an empirical model. Finally,
nonlinear reaction time background simulations are used to characterize the
actions of managed and unregulated buildings while 44 ground motion data are
used. The results indicate that SMARDs will significantly reduce the dynamic
behavior of steel-frame buildings at various seismic threat levels while
simultaneously improving their post-earthquake functionality. | 2105.04081v1 |
2021-05-10 | The high temperature mechanical properties and the correlated microstructure/texture evolutions of a TWIP high entropy alloy | The present work deals with the microstructure and texture evolutions of a
TWIP high entropy alloy at various temperatures. Toward this end, the tensile
tests were conducted at temperatures ranging from 298 to 873 K under the strain
rate of 0.001s-1. The experimented material exhibited an extraordinary room
temperature work hardening behavior, in respect of both the instantaneous
strain hardening exponent and the work hardening rate values. This resulted in
an acceptable ultimate tensile strength /ductility balance and was justified
considering the high activity of twinning and planar slip. Such work hardening
capacity deteriorated at higher temperatures due to increasing the dislocation
annihilation rate and the stacking fault energy values that suppressed twin
formation and dislocation accumulation. However, the decrease in amplitude and
extent of the work hardening region was insignificant compared with other
single-phase high entropy alloys. This was attributed to the high capacity of
the alloy for dislocation storage at various deformation temperatures. The
serrated flow was observed at 773 and 873 K due to the dynamic strain aging
(DSA) mechanism. Interestingly, in this case, the DSA mechanism has a
beneficial effect on overall ductility, which was attributed to the increase in
the strain hardening exponent result in a delay of necking. Texture examination
also reveals the formation of a double fiber texture due to the simultaneous
contribution of twinning and octahedral slip. | 2105.04597v1 |
2021-05-17 | Thermo-magnetic characterization of phase transitions in a Ni-Mn-In metamagnetic shape memory alloy | The partially overlapped ferroelastic/martensitic and para-ferromagnetic
phase transitions of a Ni$_{50.53}$Mn${33.65}$In$_{15.82}$ metamagnetic shape
memory alloy have been studied from calorimetric, magnetic and acoustic
emission measurement. We have taken advantage of the existence of thermal
hysteresis of the first order ferroelastic/martensitic phase transition
($\sim2.5$K) to discriminate the latent heat contribution $\Delta$Ht = 7.21(15)
kJ/kg and the specific heat contribution $\Delta$Hc = 216(1) J/kg to the total
excess enthalpy of the phase transition. The specific heat was found to follow
a step-like behavior at this phase transition. The intermittent dynamics of the
ferroelastic/martensitic transition has been characterized as a series of
avalanches detected both from acoustic emission and calorimetric measurements.
The energy distribution of these avalanche events was found to follow a power
law with a characteristic energy exponent $\epsilon\sim2$ which is in agreement
with the expected value for a system undergoing a symmetry change from cubic to
monoclinic. Finally, the critical behavior of the para-ferromagnetic austenite
phase transition that takes place at $\sim 311$K has been studied from the
behavior of the specific heat. A critical exponent $\alpha\sim0.09$ has been
obtained, which has been shown to be in agreement with previous values reported
for Ni-Mn-Ga alloys but different from the critical divergence reported for
pure Ni. | 2105.07916v2 |
2021-05-31 | Tunable Two-Dimensional Group-III Metal Alloys | Chemically stable quantum-confined 2D metals are of interest in
next-generation nanoscale quantum devices. Bottom-up design and synthesis of
such metals could enable the creation of materials with tailored, on-demand,
electronic and optical properties for applications that utilize tunable
plasmonic coupling, optical non-linearity, epsilon-near-zero behavior, or
wavelength-specific light trapping. In this work, we demonstrate that the
electronic, superconducting and optical properties of air-stable
two-dimensional metals can be controllably tuned by the formation of alloys.
Environmentally robust large-area two-dimensional InxGa1-x alloys are
synthesized by Confinement Heteroepitaxy (CHet). Near-complete solid solubility
is achieved with no evidence of phase segregation, and the composition is
tunable over the full range of x by changing the relative elemental composition
of the precursor. The optical and electronic properties directly correlate with
alloy composition, wherein the dielectric function, band structure,
superconductivity, and charge transfer from the metal to graphene are all
controlled by the indium/gallium ratio in the 2D metal layer. | 2106.00117v1 |
2021-06-18 | Design Principles for High Temperature Superconductors with Hydrogen-based Alloy Backbone at Moderate Pressure | Hydrogen-based superconductors provide a route to the long-sought goal of
room-temperature superconductivity, but the high pressures required to
metallize these materials limit their immediate application. For example,
carbonaceous sulfur hydride, the first room-temperature superconductor, can
reach a critical temperature (Tc) of 288 K only at the extreme pressure of 267
GPa. The next recognized challenge is the realization of room-temperature
superconductivity at significantly lower pressures. Here, we propose a strategy
for the rational design of high-temperature superconductors at low pressures by
alloying small-radius elements and hydrogen to form ternary hydride
superconductors with alloy backbones. We identify a hitherto unknown
fluorite-type backbone in compositions of the form AXH8, which exhibit high
temperature superconductivity at moderate pressures. The Fm-3m phase of LaBeH8,
with a fluorite-type H-Be alloy backbone, is predicted to be metastable and
superconducting with a Tc ~ 191 K at 50 GPa; a substantially lower pressure
than that required by the geometrically similar clathrate hydride LaH10 (170
GPa). Our approach paves the way for finding high-Tc ternary hydride
superconductors at conditions close to ambient pressures. | 2106.09879v2 |
2021-07-23 | Favorable Interfacial Chemomechanics Enables Stable Cycling of High Li-Content Li-In/Sn Anodes in Sulfide Electrolyte Based Solid-State Batteries | Solid-state batteries (SSBs) can offer a paradigm shift in battery safety and
energy density. Yet, the promise hinges on the ability to integrate
high-performance electrodes with state-of-the-art solid electrolytes. For
example, lithium (Li) metal, the most energy-dense anode candidate, suffers
from severe interfacial chemomechanical issues that lead to cell failure. Li
alloys of In/Sn are attractive alternatives, but their exploration has mostly
been limited to the low capacity(low Li content)and In rich Li$_x$In
(x$\leq$0.5). Here, the fundamental electro-chemo-mechanical behavior of Li-In
and Li-Sn alloys of varied Li stoichiometries is unravelled in sulfide
electrolyte based SSBs. The intermetallic electrodes developed through a
controlled synthesis and fabrication technique display impressive
(electro)chemical stability with Li$_6$PS$_5$Cl as the solid electrolyte and
maintain nearly perfect interfacial contact during the electrochemical Li
insertion/deinsertion under an optimal stack pressure. Their intriguing
variation in the Li migration barrier with composition and its influence on the
observed Li cycling overpotential is revealed through combined computational
and electrochemical studies. Stable interfacial chemomechanics of the alloys
allow long-term dendrite free Li cycling (>1000 h) at relatively high current
densities (1 mA cm$^{-2}$) and capacities (1 mAh cm$^{-2}$), as demonstrated
for Li$_{13}$In$_3$ and Li$_{17}$Sn$_4$, which are more desirable from a
capacity and cost consideration compared to the low Li content analogues. The
presented understanding can guide the development of high-capacity Li-In/Sn
alloy anodes for SSBs. | 2108.00843v1 |
2021-08-08 | Reduced-Order Multiscale Modeling of Plastic Deformations in 3D Alloys with Spatially Varying Porosity by Deflated Clustering Analysis | Aluminum alloys are increasingly utilized as lightweight materials in the
automobile industry due to their superior capability in withstanding high
mechanical loads. A significant challenge impeding the large-scale use of these
alloys in high-performance applications is the presence of
manufacturing-induced, spatially varying porosity defects. In order to
understand the impacts of these defects on the macro-mechanical properties of
cast alloys, multiscale simulations are often required. In this paper, we
introduce a computationally efficient reduced-order multiscale framework to
simulate the behavior of metallic components containing process-induced
porosity under irreversible nonlinear deformations. In our approach, we start
with a data compression scheme that significantly reduces the number of unknown
macroscale and microscale variables by agglomerating close-by finite element
nodes into a limited number of clusters. Then, we use deflation methods to
project these variables into a lower-dimensional space where the material
elastoplastic behaviors are approximated. Finally, we solve for the unknown
variables and map them back to the original, high-dimensional space. We call
our method deflated clustering analysis and by comparing it to direct numerical
simulations we demonstrate that it accurately captures macroscale deformations
and microscopic effective responses. To illustrate the effect of microscale
pores on the macroscopic response of a cast component, we conduct multi-scale
simulations with spatially varying local heterogeneities that are modeled with
a microstructure characterization and reconstruction algorithm. | 2108.03742v4 |
2021-08-30 | Co-deformation Between the Metallic Matrix and Intermetallic Phases in a Creep-Resistant Mg-3.68Al-3.8Ca Alloy | The microstructure of Mg-Al-Ca alloys consists of a hard intra- and
intergranular eutectic Laves phase network embedded in a soft $\alpha$-Mg
matrix. For such heterogeneous microstructures, the mechanical response and
co-deformation of both phases under external load are not yet fully understood.
We therefore used nano- and microindentation in combination with electron
microscopy to study the deformation behaviour of an Mg-3.68Al-3.8Ca alloy.
We found that the hardness of the Mg$_2$Ca phase was significantly larger
than the $\alpha$-Mg phase and stays constant within the measured temperature
range. The strain rate sensitivity of the softer $\alpha$-Mg phase and of the
interfaces increased while activation volume decreased with temperature. The
creep deformation of the Mg$_2$Ca Laves phase was significantly lower than the
$\alpha$-Mg phase at 170 $^{\circ}$C. Moreover, the deformation zone around and
below microindents depends on the matrix orientation and is influenced by the
presence of Laves phases. Most importantly, slip transfer from the $\alpha$-Mg
phase to the (Mg,Al)$_2$Ca Laves phase occurred, carried by the basal planes.
Based on the observed orientation relationship and active slip systems, a slip
transfer mechanism from the soft $\alpha$-Mg phase to the hard Laves phase is
proposed. Further, we present implications for future alloy design strategies. | 2108.13125v1 |
2021-09-05 | Atomistic transport modeling, design principles and empirical rules for Low Noise III-V Digital Alloy Avalanche Photodiodes | A series of III-V ternary and quarternary digital alloy avalanche photodiodes
(APDs) have recently been seen to exhibit very low excess noise. Using band
inversion of an environment-dependent atomistic tight binding description of
short period superlattices, we argue that a combination of increased effective
mass, minigaps and band split-off are primarily responsible for the observed
superior performance. These properties significantly limit the ionization rate
of one carrier type, either holes or electrons, making the avalanche
multiplication process unipolar in nature. The unipolar behavior in turn
reduces the stochasticity of the multiplication gain. The effects of band
folding on carrier transport are studied using the Non-Equilibrium Green's
Function Method that accounts for quantum tunneling, and Boltzmann Transport
Equation model for scattering. It is shown here that carrier transport by
intraband tunneling and optical phonon scattering are reduced in materials with
low excess noise. Based on our calculations, we propose five simple
inequalities that can be used to approximately evaluate the suitability of
digital alloys for designing low noise photodetectors. We evaluate the
performance of multiple digital alloys using these criteria and demonstrate
their validity. | 2109.01995v1 |
2021-09-18 | Efficiency and Forward Voltage of Blue and Green Lateral LEDs with V-shape defects and Random Alloy Fluctuation in Quantum Wells | For nitride-based blue and green light-emitting diodes (LEDs), the forward
voltage $V_\text{for}$ is larger than expected, especially for green LEDs. This
is mainly due to the large barriers to vertical carrier transport caused by the
total polarization discontinuity at multiple quantum well and quantum barrier
interfaces. The natural random alloy fluctuation in QWs has proven to be an
important factor reducing $V_\text{for}$. However, this does not suffice in the
case of green LEDs because of their larger polarization-induced barrier.
V-defects have been proposed as another key factor in reducing $V_\text{for}$
to allow laterally injection into multiple quantum wells (MQWs), thus bypassing
the multiple energy barriers incurred by vertical transport. In this paper, to
model carrier transport in the whole LED, we consider both random-alloy and
V-defect effects. A fully two-dimensional drift-diffusion charge-control solver
is used to model both effects. The results indicate that the turn-on voltages
for blue and green LEDs are both affected by random alloy fluctuations and
V-defect density. For green LEDs, $V_\text{for}$ decreases more due to
V-defects, where the smaller polarization barrier at the V-defect sidewall is
the major path for lateral carrier injection. Finally, we discuss how V-defect
density and size affects the results. | 2109.08824v2 |
2021-09-29 | Structure-property relations characterizing the devitrification of Ni-Zr glassy alloy thin films | The investigation of devitrification in thermally annealed nanodimensional
glassy alloy thin films provides a comprehensive understanding of their thermal
stability, which can be used to explore potential applications. The amorphous
to crystalline polymorphous transformation of cosputtered NiZr alloy (Ni78Zr22
at%) films, with a thickness lower than the reported critical limit of
devitrification, was studied through detailed structural characterization and
molecular dynamics (MD) simulations. Devitrification to a nanocrystalline state
(Ni7Zr2 structure) was observed at 800 degC, with an increase in density
(approx 3.6%) much higher than that achieved in bulk alloys. Variation in the
magnetic property of the films and the overall physical structure including
morphology and composition were examined before and after annealing. MD
simulations were employed to effectively elucidate not only the high
densification but also the increased magnetic moment after annealing, which was
correlated with the simulated change in the coordination number around Ni
atoms. The structural relaxation process accompanying devitrification was
described as a disorder-to-order transformation while highlighting the crucial
role played by chemical short range order prevalent in glassy materials. | 2109.14226v1 |
2021-09-30 | On the influence of alloy composition on the additive manufacturability of Ni-based superalloys | The susceptibility of nickel-based superalloys to processing-induced crack
formation during laser powder-bed additive manufacturing is studied. Twelve
different alloys -- some of existing (heritage) type but also other
newly-designed ones -- are considered. A strong inter-dependence of alloy
composition and processability is demonstrated. Stereological procedures are
developed to enable the two dominant defect types found -- solidification
cracks and solid-state ductility dip cracks -- to be distinguished and
quantified. Differential scanning calorimetry, creep stress relaxation tests at
1000$^\circ$C and measurements of tensile ductility at 800$^\circ$C are used to
interpret the effects of alloy composition. A model for solid-state cracking is
proposed, based on an incapacity to relax the thermal stress arising from
constrained differential thermal contraction; its development is supported by
experimental measurements using a constrained bar cooling test. A modified
solidification cracking criterion is proposed based upon solidification range
but including also a contribution from the stress relaxation effect. This work
provides fundamental insights into the role of composition on the additive
manufacturability of these materials. | 2109.15274v1 |
2021-10-24 | Isotropic finite-difference approximations for phase-field simulations of polycrystalline alloy solidification | Phase-field models of microstructural pattern formation during alloy
solidification are commonly solved numerically using the finite-difference
method, which is ideally suited to carry out computationally efficient
simulations on massively parallel computer architectures such as Graphic
Processing Units. However, one known drawback of this method is that the
discretization of differential terms involving spatial derivatives introduces a
spurious lattice anisotropy that can influence the solid-liquid interface
dynamics. We find that this influence is significant for the case of
polycrystalline dendritic solidification, where the crystal axes of different
grains do not generally coincide with the reference axes of the
finite-difference lattice. In particular, we find that with the commonly used
finite-difference implementation of the quantitative phase-field model of
binary alloy solidification, both the operating state of the dendrite tip and
the dendrite growth orientation are strongly affected by the lattice
anisotropy. To circumvent this problem, we use known methods in both real and
Fourier space to derive finite-difference approximations of leading
differential terms in 2D and 3D that are isotropic at order $h^2$ of the
lattice spacing $h$. Importantly, those terms include the divergence of the
anti-trapping current that is found to have a critical influence on pattern
selection. The 2D and 3D discretizations use an approximated form of the
anti-trapping current that facilitates the Fourier-space derivation of the
associated isotropic differential operator at $O(h^2)$, but we also derive a 2D
discretization of the standard form of this current. Finally, we present 2D and
3D phase-field simulations of alloy solidification, showing that the isotropic
finite-difference implementations can dramatically reduce spurious lattice
anisotropy effects. | 2110.12448v1 |
2021-10-25 | Atomistic mechanisms of binary alloy surface segregation from nanoseconds to seconds using accelerated dynamics | Although the equilibrium composition of many alloy surfaces is well
understood, the rate of transient surface segregation during annealing is not
known, despite its crucial effect on alloy corrosion and catalytic reactions
occurring on overlapping timescales. In this work, CuNi bimetallic alloys
representing (100) surface facets are annealed in vacuum using atomistic
simulations to observe the effect of vacancy diffusion on surface separation.
We employ multi-timescale methods to sample the early transient, intermediate,
and equilibrium states of slab surfaces during the separation process,
including standard MD as well as three methods to perform atomistic, long-time
dynamics: parallel trajectory splicing (ParSplice), adaptive kinetic Monte
Carlo (AKMC), and kinetic Monte Carlo (KMC). From nanosecond (ns) to second
timescales, our multiscale computational methodology can observe rare
stochastic events not typically seen with standard MD, closing the gap between
computational and experimental timescales for surface segregation. Rapid
diffusion of a vacancy to the slab is resolved by all four methods in tens of
ns. Stochastic re-entry of vacancies into the subsurface, however, is only seen
on the microsecond timescale in the two KMC methods. Kinetic vacancy trapping
on the surface and its effect on the segregation rate are discussed. The
equilibrium composition profile of CuNi after segregation during annealing is
estimated to occur on a timescale of seconds as determined by KMC, a result
directly comparable to nanoscale experiments. | 2110.13249v1 |
2021-11-10 | In-situ measurements of dendrite tip shape selection in a metallic alloy | The size and shape of the primary dendrite tips determine the principal
length scale of the microstructure evolving during solidification of alloys.
In-situ X-ray measurements of the tip shape in metals have been unsuccessful so
far due to insufficient spatial resolution or high image noise. To overcome
these limitations, high-resolution synchrotron radiography and advanced image
processing techniques are applied to a thin sample of a solidifying Ga-35wt.%In
alloy. Quantitative in-situ measurements are performed of the growth of
dendrite tips during the fast initial transient and the subsequent steady
growth period, with tip velocities ranging over almost two orders of magnitude.
The value of the dendrite tip shape selection parameter is found to be
$\sigma^* = 0.0768$, which suggests an interface energy anisotropy of
$\varepsilon_4 = 0.015$ for the present Ga-In alloy. The non-axisymmetric
dendrite tip shape amplitude coefficient is measured to be $A_4 \approx 0.004$,
which is in excellent agreement with the universal value previously established
for dendrites. | 2111.05658v4 |
2021-11-22 | Nucleation-growth versus spinodal decomposition in Fe-Cr alloys: an experimental verification by atom probe tomography and small angle neutron scattering | Identifying the operative mode of phase separation (spinodal decomposition
(SD) or nucleation-growth (NG)) remains a largely unexplored area of research
in spite of its importance. The present work examines this critically in Fe-Cr
system using atom probe tomography (APT) and small angle neutron scattering
(SANS), and establishes the framework to distinguish the two different modes of
alpha-prime phase separation in thermally aged Fe-35 at.% Cr and Fe-20 at.% Cr
alloys. Independent APT analysis determines the mode of phase separation on the
basis of: (i) presence / absence of periodic chemical fluctuation through
radial distribution function analysis; and (ii) inter-phase interface
characteristics (diffuse / sharp). SANS analysis, in contrast, yields virtually
indistinguishable correlation peaks for both the modes, which necessitates
further investigation of the several different aspects of SANS profiles in the
light of APT results. For the first time, key features of SANS profiles have
been identified that can unambiguously distinguish SD from NG in Fe-Cr system:
(i) nature of temporal evolution of FWHM of the correlation peak; and (ii)
appropriate value of 'gamma' for fitting with the dynamic scaling model
('gamma'= 6 for SD, Fe-35 at.% Cr alloy; 'gamma'= 4 for NG, Fe-20 at.% Cr
alloy). | 2111.11340v1 |
2021-12-22 | Graphene Layer Morphology as an Indicator of the Metals Alloy Formation at the Interface | The intercalation of different species in graphene-metal interfaces is widely
used to stabilise the artificial phases of different materials. However,
formation of the surface alloys upon the guest-metal intercalation is still an
open question, which is very important for the fabrication of graphene-based
interfaces with desired properties. Here, the widely studied interfaces of
graphene with Ru(0001) and Ir(111) were modified using intercalation of a thin
Mn layer and investigated by means of scanning tunnelling microscopy (STM)
accompanied by density functional theory (DFT) calculations, which reproduce
the observed experimental data. It is found that Mn forms a pseudomorphic layer
on Ru(0001) under a strongly buckled graphene layer. In case of Mn
intercalation in graphene/Ir(111), a buried thin layer of MnIr alloy is formed
beneath the first Ir layer under a flat graphene layer. This unexpected
observation is explained on the basis of phase diagram pictures for the Mn-Ru
and Mn-Ir systems as well as via comparison of calculated total energies for
the respective interfaces. Our results shed light on the understanding of
mechanisms of the alloys formation at the graphene-metal interfaces and
demonstrate their potential for the preparation of tailored interfaces for
future graphene-based applications. | 2112.13733v1 |
2022-05-11 | Correlated subgrain and particle analysis of a recovered Al-Mn alloy by directly combining EBSD and backscatter electron imaging | Correlated analysis of (sub)grains and particles in alloys is important to
understand transformation processes and control material properties. A
multimodal data fusion workflow directly combining subgrain data from electron
backscatter diffraction (EBSD) and particle data from backscatter electron
(BSE) images in the scanning electron microscope is presented. The BSE images
provide detection of particles smaller than the applied step size of EBSD down
to 0.03 $\mu$m in diameter. The workflow is demonstrated on a cold-rolled and
recovered Al-Mn alloy, where constituent particles formed during casting and
dispersoids formed during subsequent heating affect recovery and
recrystallization upon annealing. The multimodal dataset enables statistical
analysis including subgrains surrounding constituent particles and dispersoids'
location with respect to subgrain boundaries. Among the subgrains of
recrystallization texture, Cube{001}$\left<100\right>$ subgrains experience an
increased Smith-Zener drag from dispersoids on their boundaries compared to
CubeND{001}$\left<310\right>$ and P{011}$\left<\bar{5}\bar{6}6\right>$
subgrains, with the latter experiencing the lowest drag. Subgrains at
constituent particles are observed to have a growth advantage due to a lower
dislocation density and higher boundary misorientation angle. The dispersoid
size per subgrain boundary length increases as a function of misorientation
angle. The workflow should be applicable to other alloy systems where there is
a need for analysis correlating grains and grain boundaries with secondary
phases smaller than the applied EBSD step size but resolvable by BSE imaging. | 2205.05514v2 |
2022-06-01 | Non-equilibrium magnetic response in concentrated spin-glass AuFe(11%) alloy | We report a detailed study of dc magnetization and ac susceptibility
performed on the zero field cooled (ZFC) and field cooled (FC) state of
polycrystalline AuFe(11%) alloy. The temperature variation of ZFC and FC dc
magnetization at low fields show a distinct peak around Tf = 33 K, which
indicates the cooperative freezing of the finite size spin clusters. A weak
thermomagnetic irreversibility between ZFC and FC magnetization appears at a
temperature Tir, which is slightly below Tf. The ZFC ac susceptibility shows a
sharp cusp at Tf, which shifts towards higher temperatures with an increase in
the frequency of the ac magnetic field. When ac susceptibility is recorded
after cooling the sample from high temperature in the presence of dc bias
magnetic field, the susceptibility cusp gets broadened. In this case, no
perceptible frequency dependency of the Tf has been observed, but a significant
dispersion in the ac susceptibility is present below Tf. This clearly indicates
the non-equilibrium nature of the FC state.In addition, the FC state of
AuFe(11%) alloy exhibits a pronounced memory effect which further underlines
that the FC state is not an equilibrium state. In contrast to the general
perception obtained through the meanfield theories of thermodynamic phase
transition in spin-glass envisaging the FC state to be an equilibrium state,
the present experimental results clearly indicate that the energy landscape of
the FC state of AuFe(11%) alloy is a nontrivial one | 2206.00286v1 |
2022-07-04 | Dielectric function of CuBr$_\mathrm{x}$I$_{1-\mathrm{x}}$ alloy thin films | We study the dielectric function of CuBr$_\mathrm{x}$I$_{1-\mathrm{x}}$ thin
film alloys using spectroscopic ellipsometry in the spectral range between 0.7
eV to 6.4 eV, in combination with first-principles calculations based on
density functional theory. Through the comparison of theory and experiment, we
attribute features in the dielectric function to electronic transitions at
specific k-points in the Brillouin zone. The observed bandgap bowing as a
function of alloy composition is discussed in terms of different physical and
chemical contributions. The band splitting at the top of the valence band due
to spin-orbit coupling is found to decrease with increasing Br-concentration,
from a value of 660 meV for CuI to 150 meV for CuBr. This result can be
understood considering the contribution of copper d-orbitals to the valence
band maximum as a function of the alloy composition. | 2207.01344v2 |
2022-07-11 | Physics-Based Machine-Learning Approach for Modeling the Temperature-Dependent Yield Strengths of Medium- or High-Entropy Alloys | Machine learning is becoming a powerful tool to predict temperature-dependent
yield strengths (YS) of structural materials, particularly for
multi-principal-element systems. However, successful machine-learning
predictions depend on the use of reasonable machine-learning models. Here, we
present a comprehensive and up-to-date overview of a bilinear log model for
predicting temperature-dependent YS of medium-entropy or high-entropy alloys
(MEAs or HEAs). In this model, a break temperature, Tbreak, is introduced,
which can guide the design of MEAs or HEAs with attractive high-temperature
properties. Unlike assuming black-box structures, our model is based on the
underlying physics, incorporated in form of a priori information. A technique
of global optimization is employed to enable the concurrent optimization of
model parameters over low- and high-temperature regimes, showing that the break
temperature is consistent across YS and ultimate strength for a variety of HEA
compositions. A high-level comparison between YS of MEAs/HEAs and those of
nickel-based superalloys reveal superior strength properties of selected
refractory HEAs. For reliable operations, the temperature of a structural
component, such as a turbine blade, made from refractory alloys may need to
stay below Tbreak. Once above Tbreak, phase transformations may start taking
place, and the alloy may begin losing structural integrity. | 2207.05171v1 |
2022-07-28 | Evolution of short-range magnetic correlations in ferromagnetic Ni-V alloys | We experimentally study how the magnetic correlations develop in a binary
alloy close to the ferromagnetic quantum critical point with small-angle
neutron scattering (SANS). Upon alloying the itinerant ferromagnet nickel with
vanadium, the ferromagnetic order is continuously suppressed. The critical
temperature Tc vanishes when vanadium concentrations reach the critical value
of xc=0.116 indicating a quantum critical point separating the ferromagnetic
and paramagnetic phases. Earlier magnetization and $\mu$SR data have indicated
the presence of magnetic inhomogeneities in Ni(1-x)V(x) and, in particular,
recognize the magnetic clusters close to xc, on the paramagnetic and on the
ferromagnetic sides with nontrivial dynamical properties [R. Wang et al., Phys.
Rev. Lett. 118, 267202 (2017)]. We present the results of SANS study with full
polarization analysis of polycrystalline Ni(1-x)V(x) samples with x=0.10 and
x=0.11 with low critical temperatures Tc below 50 K. For both Ni-V samples
close to xc we find isotropic magnetic short-range correlations in the
nanometer-scale persisting at low temperatures. They are suppressed gradually
in higher magnetic fields. In addition, signatures of long-range ordered
magnetic domains are present below Tc. The fraction of these magnetic clusters
embedded in the ferromagnetic ordered phase grows towards xc and agrees well
with the cluster fraction estimate from the magnetization and $\mu$SR data. Our
SANS studies provide new insights into the nature of the inhomogeneities in a
ferromagnetic alloy close to a quantum critical point. | 2207.14196v2 |
2022-08-11 | Quantitative assessment of the microstructural factors controlling the fatigue crack initiation mechanisms in AZ31 Mg alloy | The deformation and fatigue crack nucleation mechanisms were studied by means
of slip trace analysis and secondary electron microscopy in a textured AZ31B-O
Mg alloy subjected to fully-reversed cyclic deformation at two different cyclic
strain semi-amplitudes. Samples were deformed in two orientations leading to
symmetric and non-symmetric cyclic stress-strain curves due to the activation
of different deformation mechanisms. They were ascertained in longitudinal
sections of the specimens, which included a large number of grains (from 1500
to 4500 for each specimen), to obtain statistically significant results. If the
dominant deformation mechanisms were basal slip and tensile
twinning/detwinning, the most damaging fatigue cracks were nucleated along
twins in large grains, together with cracks parallel to basal slip bands
associated with the localization of deformation in clusters of small grains
suitably oriented for basal slip. If the main deformation mechanisms were
tensile twinning/detwinning and pyramidal slip, the longest fatigue cracks were
nucleated along pyramidal slip bands in large grains. Grain boundary cracks
around small grains were found in all cases, but they were not critical from
the viewpoint of fatigue failure. This information is relevant to assess the
effect of the microstructural features on the fatigue life of Mg alloys and as
input to simulate the fatigue behavior of Mg alloys using fatigue indicator
parameters. | 2208.05861v1 |
2022-09-13 | Strain and composition dependencies of the near bandgap optical transitions in monoclinic (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys with coherent biaxial in-plane strain on (010) Ga$_2$O$_3$ | The bowing of the energy of the three lowest band-to-band transitions in
$\beta$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$ alloys was resolved using a combined
density functional theory (DFT) and generalized spectroscopic ellipsometry
(GSE) approach. The DFT calculations of the electronic band structure of both,
$\beta$-Ga$_2$O$_3$ and $\theta$-Al$_2$O$_3$, allow extracting of the linear
portion of the energy shift in the alloys, and provide a method for quantifying
the role of coherent strain present in the
$\beta$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$ thin films on (010) $\beta$-Ga$_2$O$_3$
substrates. The energies of band-to-band transitions were obtained using the
spectroscopic ellipsometry eigenpolarization model approach [A. Mock et al.,
Phys. Rev. B 95, 165202 (2017)]. After subtracting the effects of strain which
also induces additional bowing and after subtraction of the linear portion of
the energy shift due to alloying, the bowing parameters associated with the
three lowest band-to-band transitions in monoclinic
$\beta$-(Al$_{x}$Ga$_{1-x}$)$_2$O$_3$ are found. | 2209.06025v1 |
2022-11-15 | Enhanced piezoelectric response of AlN via alloying of transitional metals, and influence of type and distribution of transition metals | Aluminum nitride (AlN) is an important piezoelectric material for a wide
range of applications, many efforts are devoted to improving its piezoelectric
response by alloying with transition metals (TMs). In this paper, the influence
of the type and distribution of TM on the piezoelectric response is discussed
for the first time. TM0.0625Al0.9375N with twenty-eight different TMs are
investigated, and most show higher values of piezoelectric strain modulus d33
than that of AlN. This is because the TM introduces weaker TM-N bonds and
locates closer to the centre of three neighbouring N atoms. The location of TM
is determined to be significantly correlated with its group number. Alloys of
TMxAl1-xN (TM=Sc, Cr, Sr, Mo, Ru and Rh) with varying x are further studied. On
basis of the cost of the TMs and piezoelectric performances, the alloy with Mo
is more effective in enhancing d33. A high d33 of 12.3 times that of pure AlN
is realized in a metastable configuration of Mo0.167Al0.833N. The distribution
of Mo plays a key role in the piezoelectric performance. A higher d33 is more
likely to appear in MoxAl1-xN with more Al sublayers containing Mo atoms and
with fewer dimers of Mo atoms along the z-axis. | 2211.07870v1 |
2022-12-16 | Compositional phase stability in medium-entropy and high-entropy Cantor-Wu alloys from an ab initio all-electron, Landau-type theory and atomistic modelling | We describe implementation and analysis of a first-principles theory, derived
in an earlier work, for the leading terms in an expansion of a Gibbs free
energy of a multi-component alloy in terms of order parameters that
characterize potential, compositional phases. The theory includes effects of
rearranging charge and other electronics from changing atomic occupancies on
lattice sites. As well as the rigorous description of atomic short-range order
in the homogeneously disordered phase, pairwise interaction parameters suited
for atomistic modelling in a multicomponent setting can be calculated. From our
study of an indicative series of the Cantor-Wu alloys, NiCo, NiCoCr, NiCoFeCr,
and NiCoFeMnCr, we find that the interactions are not approximated well either
as pseudobinary or restricted to nearest neighbour range. Our computed
order-disorder transition temperatures are low, consistent with experimental
observations, and the nature of the ordering is dominated by correlations
between Ni, Co, and Cr, while Fe and Mn interact weakly. Further atomistic
modelling suggests that there is no true single-phase low-temperature ground
state for these multicomponent systems. Instead the single-phase solid solution
is kept stable to low temperatures by the large configurational entropy and the
Fe, Mn dilution effects. The computationally cost-effectiveness of our method
makes it a good candidate for further exploration of the space of
multicomponent alloys. | 2212.08468v1 |
2022-12-21 | Effect of Ti addition on the structural, thermodynamic, and elastic properties of Ti$_{x}$(HfNbTaZr)$_{(1-x)/4}$ alloys | The structure and thermodynamic properties of Ti$_x$(HfNbTaZr)$_{(1-x)/4}$
from Refractory High Entropy multicomponent Alloys to pure titanium are
investigated employing comprehensive MCSQS realizations of the disordered
atomic structure and DFT calculations. We showed that to model the random
structure in a limited supercell, it is necessary to probe a large space of
random configurations with respect to the nearest neighbor's shells. Mimicking
the randomness with the many-body terms does not lead to significant
improvements in the mixing energy, but modeling the random structure with the
few nearest neighbor pairs leads to improvements in the mixing energy.
Furthermore, we demonstrated the existence of weak to medium SRO for the two
equimolar compositions. Chemical ordering is investigated by associating a
large number of MCSQS realizations to DFT energy calculations, and SRO results
are rationalized in terms of the crystallographic structure of the pairs of
elements and binary phase diagrams. When Ti is added to
Ti$_x$(HfNbTaZr)$_{(1-x)/4}$ alloys, the mixing energy remains slightly
positive for all $x$. For $x$ > 0.4, a phase transition in favor of an hcp
structure is observed in agreement with the predictions of the Bo-Md diagram.
At $x$ = 0.5, a dual phase is predicted. Ti content in this class of alloys
could be a practical way to select phase structure and tailor the elastic
properties to specific applications. | 2212.10977v3 |
2023-01-06 | APC Nb$_3$Sn superconductors based on internal oxidation of Nb-Ta-Hf alloys | In the last few years, a new type of Nb$_3$Sn superconducting composite,
containing a high density of artificial pinning centers (APC) generated via an
internal oxidation approach, has demonstrated a significantly superior
performance relative to present, state-of-the-art commercial Nb$_3$Sn
conductors. This was achieved via the internal oxidation of Nb-4at.%Ta-1at.%Zr
alloy. On the other hand, our recent studies have shown that internal oxidation
of Nb-Ta-Hf alloys can also lead to dramatic improvements in Nb$_3$Sn
performance. In this work we follow up this latter approach, fabricating a
61-stack APC wire based on the internal oxidation of Nb-4at.%Ta-1at.%Hf alloy,
and compare its critical current density (Jc) and irreversibility field (Birr)
with APC wires made using Nb-4at.%Ta-1at.%Zr. A second goal of this work was to
improve the filamentary design of APC wires in order to improve their wire
quality and electromagnetic stability. Our new modifications have led to
significantly improved RRR and stability in the conductors, while still keeping
non-Cu Jc at or above the FCC Jc specification. Further improvement via
optimization of the wire recipe and design is ongoing. Finally, additional work
needed to make APC conductors ready for applications in magnets is discussed. | 2301.02571v2 |
2023-01-10 | Geometric Study on Canonical Nonlinearity for FCC-based Binary Alloys | For classical discrete systems under constant composition (typically reffered
to as substitutional alloys), canonical average phi typically provides a
complicated nonlinear map from a set of potential energy surface to that of
macroscropic structure in thermodynamic equilibrium, the so-called canonical
nonlinearity: CN. Although our recent study reveals that the CN can be
reasonablly addressed for individual microscopic configuration by two different
ways of special vector field on configuration space, anharmonicity in the
structural degree of freedoms (ASDF), and Kullback-Leibler (KL) divergence DKL,
that is the conceptual extention of ASDF to statistical manifold to include
further non-local information about CN, their direct correlation on real
lattices, is still totally unclear. We here tuckle this problem for fcc-based
equiatomic binary alloys that have been most studied in the CN-based context.
We confirm that while one of the contribution to CN of DdG for each
configuration, due to difference in CDOS from Gaussian, exhibits significant
positive correlation with ASDF, another contribution of Dns due to
non-separability in structural degee of freedoms (SDFs) exhibit no effective
correlation with ASDF, which can be naturally accepted since the former
contribution depends on ASDF itself, while the latter is independent. We find
that average of Dns over all configurations for sets of SDFs can be
well-characterized by information about asymmetric Hausdorff distance between
configurational polyhedra (CP) for practical and ideally separable system, and
CP hypervolumes. This fact certainly indicates that non-local information about
CN has profound connection to the geometric configuration for ground-state
structures of alloys on configuration space. | 2301.03741v2 |
2023-01-12 | Effect of hydrostatic pressure and alloying on thermoelectric properties of van der Waals solid KMgSb: An \textit{ab-initio} study | Through a combined first-principles and Boltzmann transport theory, we
systematically investigate the thermal and electrical transport properties of
the unexplored ternary quasi two-dimensional KMgSb system of KMgX (X = P, As,
Sb, and Bi) family. Herein, the transport properties of KMgSb under the
application of hydrostatic pressure and alloy engineering are reported. At a
carrier concentration of $\sim8\times10^{19}~\mathrm{cm^{-3}}$, the figure of
merit zT ($\sim0.75$) for both the $n$-type and $p$-type of KMgSb closely
matched, making it an attractive option for engineering both legs of a
thermoelectric device using the same material. This is particularly desirable
for high-performance thermoelectric applications. Furthermore, the zT value
increases as pressure decreases, further enhancing its potential for use in
thermoelectric devices. In the case of substitutional doping (replacing 50 \%
Sb by Bi atom), we observed $\sim49~\%$ (in-plane) increase in the peak
thermoelectric figure of merit (zT). The maximum zT value obtained after alloy
engineering is $\sim1.45$ at 900~K temperature. Hydrostatic pressure is
observed to be a great tool to tune the lattice thermal conductivity
($\kappa_L$). We observed that the negative pressure-like effects could be
achieved by chemically doping bigger-size atoms, especially when $\kappa_L$ is
a property under investigation. Through our computational investigation, we
explain that hydrostatic pressure and alloy engineering may improve
thermoelectric performance dramatically. | 2301.04969v2 |
2023-03-18 | Rate dependence of damage formation in metallic-intermetallic Mg-Al-Ca composites | We study a cast Mg-4.65Al-2.82Ca alloy with a microstructure containing
$\alpha$-Mg matrix reinforced with a C36 Laves phase skeleton. Such ternary
alloys are targeted for elevated temperature applications in automotive engines
since they possess excellent creep properties. However, in application, the
alloy may be subjected to a wide range of strain rates and in material
development, accelerated testing is often of essence. It is therefore crucial
to understand the effect of such rate variations. Here, we focus on their
impact on damage formation. Due to the locally highly variable skeleton forming
the reinforcement in this alloy, we employ an analysis based on high resolution
panoramic imaging by scanning electron microscopy coupled with automated damage
analysis by deep learning-based object detection and classification
convolutional neural network algorithm (YOLOV5). We find that with decreasing
strain rate the dominant damage mechanism for a given strain level changes: at
a strain rate of $5\cdot10^{-4}/s$ the evolution of microcracks in the C36
Laves phase governs damage formation. However , when the strain rate is
decreased to $5\cdot10^{-6}/s$, interface decohesion at the $\alpha$-Mg/Laves
phase interfaces becomes equally important. We also observe a change in crack
orientation indicating an increasing influence of plastic co-deformation of the
{\alpha}-Mg matrix and Laves phase. We attribute this transition in leading
damage mechanism to thermally activated processes at the interface. | 2303.10477v1 |
2023-03-25 | Prediction of novel final phases in aged uranium-niobium alloys | Ordered intermetallics are long believed to be the final products of the
aging of U-Nb solid solutions at low temperatures, a crucial property for the
practical applications of this alloy in engineering and industry. However, such
conjectured ordered compounds have not been experimentally or theoretically
established. Herein, numerical evidence for ordered intermetallic U-Nb
compounds is presented using thorough first-principles structure predictions up
to 500 GPa. Two stable U2Nb compounds and one metastable U2Nb and one
metastable U3Nb were discovered. A unique hybridized transition driven by
pressure was observed in U2Nb, which is a superposition of one first-order
transition and another second-order transition, leading to striking features
near the transition pressure of 21.6 GPa. The decomposition limit of these
compounds at high temperature was also investigated. The strong stability of
U2Nb in the region of low pressure and high temperature was revealed. This
discovery of ordered U2Nb and its strong stability over a wide pressure range
completely changed the phase diagram of U-Nb alloys and shed new light on the
dynamic response and aging mechanism of U-Nb alloys. | 2303.14442v1 |
2023-03-27 | Microstructural engineering by heat treatments of multi-principal element alloys via spinodal mediated phase transformation pathways | Nanoscale multi-phase microstructures observed in multi-principal element
alloys (MPEAs) such as $\rm AlMo_{0.5}NbTa_{0.5}TiZr$, $\rm
Al_{0.5}NbTa_{0.8}Ti_{1.5}V_{0.2}Zr$, $\rm TiZrNbTa$, $\rm AlCoCrFeNi$ and $\rm
Fe_{15}Co_{15}Ni_{20}Mn_{20}Cu_{30}$ that exhibit promising mechanical or
functional properties may have evolved through spinodal-mediated phase
transformation pathways (PTPs). The microstructures in such MPEA systems could
be further engineered for targeted applications by appropriately designing the
alloy composition and heat-treatment schedule. In this study, we investigate
systematically how different heat treatment schedules such as single-step
isothermal aging, two-step isothermal aging and continuous cooling alter the
interplay among the various factors associated with alloy composition, such as
volume fraction of individual phases, lattice misfit and modulus mismatch
between the co-existing phases. We have determined the degree to which these
factors influence significantly the spinodal-mediated PTPs and the
corresponding microstructures by use of high-throughput phase-field
simulations. In particular, we demonstrate that the microstructural topology
(i.e., which phase forms the continuous matrix and which phase forms discrete
precipitates) in the same MPEA having an asymmetric miscibility gap could be
inverted simply by a continuous cooling heat treatment. Further, we reveal a
rich variety of novel hierarchical microstructures that could be designed using
two-step isothermal aging heat treatments in MPEA systems with symmetric or
asymmetric miscibility gaps. These simulation results may shed light on novel
microstructure design and engineering for the above-mentioned MPEA systems. | 2303.15370v2 |
2023-05-09 | Interaction of $\langle a \rangle$ prismatic screw dislocations with the $α-β$ interface side face in $α-β$ Ti alloys | Slip transmission across $\alpha-\beta$ interfaces is of great significance
in understanding the strength of $\alpha-\beta$ Ti alloys for aerospace
applications. Molecular statics (MS) and molecular dynamics (MD) simulations
were conducted to investigate the mechanisms of slip transmission of $\langle a
\rangle$ prismatic screw dislocations across the $\alpha-\beta$ interface side
face. In these simulations, the $\alpha$ phase consisted of pure HCP Ti whereas
the $\beta$ phase was modeled as Ti$_{60}$Nb$_{40}$ BCC random alloy using a
Ti-Nb interatomic potential. Firstly, the misfit dislocations structure on the
$\alpha-\beta$ interface side face has been characterized from MS simulations.
This predicted dislocation structure is in good agreement with experimental
observations in Ti-alloys. Secondly, MD simulations of the interaction of
$[a_1]$, $[a_2]$, and $[a_3]$ prismatic screw dislocations with the interface
side face in an $\alpha-\beta$ bi-crystal were performed at different
temperatures. A distinct barrier of the interface side face to different types
of dislocation transmission was found. The origin of this anisotropy in the
slip transmission is due to the relative misalignment of the slip systems
between the $\alpha$ and the $\beta$ phases from the Burgers orientation
relationship. Finally, the mechanisms of slip transmission of each dislocation
type were analyzed in a detailed atomistic description and compared to
experimental observations. | 2305.05737v2 |
2023-05-10 | Local Alloy Order in a Ge1-xSnx/Ge Epitaxial Layer | The local ordering of atoms in alloys directly has a strong impact on their
electronic and optical properties. This is particularly relevant in nonrandom
alloys, especially if they are deposited using far from the equilibrium
processes, as is the case of epitaxial Ge1-xSnx layers. In this work, we
investigate the arrangement of Ge and Sn atoms in optoelectronic grade Ge1-xSnx
epitaxial layers featuring a Sn content in the 5-14% range by using
polarization-dependent Raman spectroscopy and density-functional-theory
calculations. The thorough analysis of the polarization-dependent spectra in
parallel and perpendicular configuration allowed us to properly tag all the
observed vibrational modes, and to shed light on that associated to
disorder-assisted Raman transitions. Indeed, with the help of large-scale
atomistic simulations, we were able to highlight how the presence of Sn atoms,
that modify the local environments of Ge atoms, gives rise to two spectral
features at different Raman shifts, corresponding to distortions of the atomic
bonds. This analysis provides a valuable framework for advancing the
understanding of the vibrational properties in Ge1-xSnx alloys, particularly
with regard to the impact of local ordering of the different atomic species. | 2305.06005v2 |
2023-05-25 | Bandgap manipulation of hBN by alloying with aluminum: absorption properties of hexagonal BAlN | The versatile range of applications for two-dimensional (2D) materials has
encouraged scientists to further engineer the properties of these materials.
This is often accomplished by stacking layered materials into more complex van
der Waals heterostructures. A much less popular but technologically promising
approach is the alloying of 2D materials with different element compositions.
In this work, we demonstrate a first step in manipulating the hBN bandgap in
terms of its width and indirect/direct character of the optical transitions. We
present a set of aluminum alloyed hexagonal boron nitride (hBAlN) samples that
were grown by metal organic vapor phase epitaxy (MOVPE) on 2-inch sapphire
substrates with different aluminum concentration. Importantly, the obtained
samples revealed a sp$^2$-bonded crystal structure. Optical absorption
experiments disclosed two strong peaks in the excitonic spectral range with
absorption coefficient $\alpha \sim 10^6$ cm$^{-1}$. Their energies correspond
very well with the energies of indirect and direct bandgap transitions in hBN.
However, they are slightly redshifted. This observation is in agreement with
predictions that alloying with Al leads to a decrease of the bandgap energy.
The observation of two absorption peaks can be explained in terms of mixing
electronic states in the K and M conduction band valleys, which leads to a
significant enhancement of the absorption coefficient for indirect transitions. | 2305.15810v1 |
2023-07-08 | Towards reducing tension-compression yield and cyclic asymmetry in pure magnesium and magnesium-aluminum alloy with cerium addition | In this study, we report the effect of cerium (Ce) addition on the
tension-compression yield and cyclic asymmetry in commercially pure magnesium
(Cp-Mg) and Mg-Al alloy at room temperature (RT). The investigated materials
Cp-Mg, Mg-0.5Ce, and Mg-3Al-0.5Ce were extruded at 400{\deg}C, followed by
annealing at the same temperature for one hour. Incorporating 0.5wt.% Ce in
pure Mg results in the weakening of its basal texture, uniform distribution of
Mg12Ce precipitates, and grain size refinement. Consequently, the tensile yield
strength and ductility of pure Mg increased, and tension-compression yield
asymmetry was eliminated. However, the presence of 3wt.% Al in Mg suppresses
the beneficial effects of Ce addition. The formation of complex precipitates,
such as Mg-Al-Ce and Al11Ce3, limits the weakening of the basal texture,
reduction in grain size, improvement in ductility, and elimination of
tension-compression yield asymmetry observed in Mg-0.5Ce. Nevertheless, Al
contributes to the solid solution strengthening in Mg and possibly lowers the
critical stress required for twinning in Mg, resulting in the highest tensile
strength of Mg-3Al-0.5Ce. Finally, the addition of 0.5wt.% Ce enhances the
cyclic strength, stabilizes cyclic stress response, reduces inelastic strain,
and minimizes cyclic asymmetry in both pure Mg and Mg-Al alloy while
maintaining a comparable fatigue life. Overall, Ce addition positively impacts
the microstructure and mechanical behavior of pure Mg and its investigated
alloy. The reasons for these improvements are discussed in detail. | 2307.04051v3 |
2023-07-15 | Exploring the Impact of Configurational Entropy on the Design and Development of CoNi-Based Superalloys for Sustainable Applications | A comprehensive literature review on recently rediscovered Co- and/or
CoNi-based superalloys, strengthened by the {\gamma}' phase, revealed a
relationship between the configurational entropy of the system and the
{\gamma}' solvus temperature. This study was conducted on a high Cr CoNi-based
superalloy system with high configurational entropy to test our hypothesis
based on the sustainable metallurgy framework. Thermodynamic calculations were
performed to design the chemical compositions, followed by vacuum casting and
heat treatments to produce the desired alloys. The microstructures were
characterized using a scanning electron microscope, electron backscattered
diffraction, transmission electron microscope, and differential thermal
analysis. Microhardness and nanoindentation tests were employed to measure the
mechanical properties. The results showed that both the configurational entropy
and the type of alloying elements determine the final high-temperature
performance of the alloys. We found that to enhance the higher {\gamma}' solvus
temperature, the configurational entropy should be increased by adding
{\gamma}' stabilizing elements. The microstructural and mechanical
characteristics of the designed alloys before and after heat treatments are
discussed in detail. The outcome of this study is beneficial for developing
cobalt-based high-entropy superalloys with appropriate processing windows and
freezing ranges for advanced sustainable manufacturing purposes, such as using
powder bed fusion technologies. | 2307.07731v1 |
2023-08-10 | Exploring Solute Behavior and Texture Selection in Magnesium Alloys at the Atomistic Level | This study advances our understanding of how chemical binding and solute
distribution impact grain boundary segregation behavior and subsequent
annealing texture modification in lean Mg-X-Zn alloys (X = RE or Ca). Notably,
differences in Ca and Gd solute behavior at grain boundaries were revealed,
where Ca exhibited stronger binding to vacancy sites than Gd, resulting in
elevated Ca segregation and an RD-TD-type texture. The introduction of Zn
showed significant synergistic effects on solute clustering, with Gd-Zn pairs
forming more favorably than Ca-Zn pairs, leading to a strong synergy between Zn
and Gd. This promoted their co-segregation and high concentration at the grain
boundary, generating a unique TD-spread texture. In contrast, weaker binding in
Ca-Zn pairs did not affect Ca segregation but influenced Zn segregation, which
underscores the importance of solute binding behavior in alloy design concepts.
Additionally, the combined atomicscale experiments and ab initio predictions
provide strong evidence that selective texture development in Mg alloys is tied
to heterogeneous solute-boundary interactions, where the sensitivity of the
binding energy to volumetric strain affects solute segregation at grain
boundaries, resulting in varying grain boundary mobilities and specific texture
component growth. It also emphasizes that solute behavior in clustering and
segregation is influenced not only by atomic size but also by chemical binding
strength with vacancies or co-added Zn. | 2308.05811v2 |
2023-08-17 | On the role of selective nucleation and growth to recrystallization texture development in a Mg-Gd-Zn alloy | One of the main material properties altered by rare earth additions in
magnesium alloys is texture, which can be specifically adjusted to enhance
ductility and formability. The current study aims at illuminating the texture
selection process in a Mg-0.073at%Gd-0.165at%Zn alloy by investigating
recrystallization nucleation and early nucleus growth during static
recrystallization. An as-cast sample of the investigated alloy was deformed in
uniaxial compression at 200{\deg}C till 40% strain and was then cut into two
halves for subsequent microstructure characterization via ex-situ and quasi
in-situ EBSD investigations. In order to gain insights into the evolution of
texture during recrystallization, the contributions from dynamic and static
recrystallization were initially separated and the origin of the non-basal
orientation of recrystallization nuclei was traced back to several potential
nucleation sites within the deformed matrix. Considering the significant role
of double-twin band recrystallization in determining the recrystallization
texture, this type of recrystallization nucleation was further investigated via
quasi-in-situ EBSD on a deformed sample, annealed at 400{\deg} for different
annealing times. With progressive annealing a noticeable trend was observed, in
which the basal nuclei gradually diminished and eventually vanished from the
annealed microstructure. In contrast, the off-basal nuclei exhibited continuous
growth, ultimately becoming the dominant contributors to the recrystallization
texture. The study therefore emphasizes the importance of particular nucleation
sites that generate favorably oriented off-basal nuclei, which over the course
of recrystallization outcompete the neighboring basal-oriented nuclei in terms
of growth, and thereby dominate the recrystallization texture. | 2308.08916v1 |
2023-08-28 | Defects and Oxygen Impurities in Ferroelectric Wurtzite Al$_{1-x}$Sc$_x$N Alloys | III-nitrides and related alloys are widely used for optoelectronics and as
acoustic resonators. Ferroelectric wurtzite nitrides are of particular interest
because of their potential for direct integration with Si and wide bandgap
semiconductors, and unique polarization switching characteristics; such
interest has taken off since the first report of ferroelectric
Al$_{1-x}$Sc$_x$N alloys. However, the coercive fields needed to switch
polarization are on the order of MV/cm, which is 1-2 orders of magnitude larger
than oxide perovskite ferroelectrics. Atomic-scale point defects are known to
impact the dielectric properties, including breakdown fields and leakage
currents, as well as ferroelectric switching. However, very little is known
about the native defects and impurities in Al$_{1-x}$Sc$_x$N, and their effect
on the dielectric properties. In this study, we use first-principles
calculations to determine the formation energetics of native defects and
unintentional oxygen incorporation in Al$_{1-x}$Sc$_x$N. We find that nitrogen
vacancies are the dominant native defects, and that they introduce multiple
mid-gap states that can lead to premature dielectric breakdown in
ferroelectrics and carrier recombination in optoelectronics. Growth under
N-rich conditions will reduce the concentration of these deep defects. We also
investigate unintentional oxygen incorporation on the nitrogen site and find
that the substitutional defect is present in high concentrations, which can
contribute to increased temperature-activated leakage currents. Our findings
provide fundamental understanding of the defect physics in Al$_{1-x}$Sc$_x$N
alloys, which is critical for future deployment of ferroelectric devices. | 2308.14310v1 |
2023-08-30 | Strengthening from dislocation restructuring and local climb at platelet linear complexions in Al-Cu alloys | Stress-driven segregation at dislocations can lead to structural transitions
between different linear complexion states. In this work, we examine how
platelet array linear complexions influence dislocation motion and quantify the
associated strengthening effect in Al-Cu alloys using atomistic simulations.
The presence of platelet complexions leads to faceting of the dislocations,
with nanoscale segments climbing upwards along the platelet growth direction,
resulting in a complex non-planar configuration that restricts subsequent
dislocation motion. Upon deformation, the leading partial dislocation must
climb down from the platelet complexions first, followed by a similar sequence
at the trailing partial dislocation, in order to overcome the precipitates and
commence plastic slip. The dislocation depinning mechanism of linear
complexions is strikingly different from traditional precipitation-strengthened
alloys, where dislocations overcome obstacles by either shearing through or
looping around obstacles. The critical shear stress required to unpin
dislocations from platelet complexions is found to be inversely proportional to
precipitate spacing, which includes not just the open space (as observed in
Orowan bowing) but also the region along the platelet particle where climb
occurs. Thus, platelet linear complexions provide a new way to modify
dislocation structure directly and improve the mechanical properties of metal
alloys. | 2308.16117v2 |
2023-09-23 | Enhanced radiation damage tolerance of amorphous interphase and grain boundary complexions in Cu-Ta | Amorphous interfacial complexions are particularly resistant to radiation
damage and have been primarily studied in alloys with good glass-forming
ability, yet recent reports suggest that these features can form even in
immiscible alloys such as Cu-Ta under irradiation. In this study, the
mechanisms of damage production and annihilation due to primary knock-on atom
collisions are investigated for amorphous interphase and grain boundaries in a
Cu-Ta alloy using atomistic simulations. Amorphous complexions, in particular
amorphous interphase complexions that separate Cu and Ta grains, result in less
residual defect damage than their ordered counterparts. Stemming from the
nanophase chemical separation in this alloy, the amorphous complexions exhibit
a highly heterogeneous distribution of atomic excess volume, as compared to a
good glass former like Cu-Zr. Complexion thickness, a tunable structural
descriptor, plays a vital role in damage resistance. Thicker interfacial films
are more damage-tolerant because they alter the defect production rate due to
differences in intrinsic displacement threshold energies during the collision
cascade. Overall, the findings of this work highlight the importance of
interfacial engineering in enhancing the properties of materials operating in
radiation-prone environments and the promise of amorphous complexions as
particularly radiation damage-tolerant microstructural features. | 2309.13474v2 |
2023-10-16 | Variations of Interatomic Force Constants in the Topological Phonon Phase Transition of AlGaN | The topological effects of phonons have been extensively studied in various
materials, particularly in the wide-bandgap semiconductor GaN, which has the
potential to improve heat dissipation in power electronics due to its
intrinsic, topologically-protected, non-dissipative phonon surface states.
Nevertheless, the phase transition of the Weyl phonons in nitrides and their
composite alloys has yet to be elucidated. To unveil the microscale origin,
topological phonon properties in AlGaN alloys are investigated using the
virtual crystal approximation (VCA) and special quasi-random structure (SQS)
approaches in this work. It is found that phase transitions in Weyl phonons are
evidently present in AlGaN alloys and nitride single crystals. Under strain
states, both GaN and AlN show a more prominent phase transition of Weyl phonons
when subjected to biaxial compressive and uniaxial tensile strains. And it has
been observed that the zz components in the self-term and the transverse 1NN
force constants (FCs) are the most influential during the phase transition. The
nonlinear Weyl phonon transition in AlGaN alloys, as modeled by the VCA, is
reflected in the normalized self-term and first-nearest-neighbor (1NN) FCs,
which vary in a nonlinear fashion with an increasing magnitude. This nonlinear
phenomenon is also confirmed in the SQS modeling, where the unfolded phonon
dispersions are consistent with those in the VCA modeling. With increased
branches, hundreds of Weyl phonons are present accompanied by significant
disorders in normalized FCs, which mainly occur for N atoms in self-terms and
for all components in normalized 1NN FCs. | 2310.09996v1 |
2023-10-19 | Automated Repair of Declarative Software Specifications in the Era of Large Language Models | The growing adoption of declarative software specification languages, coupled
with their inherent difficulty in debugging, has underscored the need for
effective and automated repair techniques applicable to such languages.
Researchers have recently explored various methods to automatically repair
declarative software specifications, such as template-based repair,
feedback-driven iterative repair, and bounded exhaustive approaches. The latest
developments in large language models provide new opportunities for the
automatic repair of declarative specifications. In this study, we assess the
effectiveness of utilizing OpenAI's ChatGPT to repair software specifications
written in the Alloy declarative language. Unlike imperative languages,
specifications in Alloy are not executed but rather translated into logical
formulas and evaluated using backend constraint solvers to identify
specification instances and counterexamples to assertions. Our evaluation
focuses on ChatGPT's ability to improve the correctness and completeness of
Alloy declarative specifications through automatic repairs. We analyze the
results produced by ChatGPT and compare them with those of leading automatic
Alloy repair methods. Our study revealed that while ChatGPT falls short in
comparison to existing techniques, it was able to successfully repair bugs that
no other technique could address. Our analysis also identified errors in
ChatGPT's generated repairs, including improper operator usage, type errors,
higher-order logic misuse, and relational arity mismatches. Additionally, we
observed instances of hallucinations in ChatGPT-generated repairs and
inconsistency in its results. Our study provides valuable insights for software
practitioners, researchers, and tool builders considering ChatGPT for
declarative specification repairs. | 2310.12425v2 |
2023-11-30 | Criteria to observe single-shot all-optical switching in Gd-based ferrimagnetic alloys | Single-shot all-optical helicity-independent switching (AO-HIS) induced by a
femto-second laser pulse has been mainly reported in Gadolinium based rare
earth-transition metal (RE-TM) alloys such as GdFeCo or GdCo, but the mechanism
leading to magnetization switching is a hotly debated topic. Here, we elaborate
on a large number of GdyRE1-x-yCox (RE = Dy, Tb, Ho) alloys to tune various
magnetic parameters in order to define what the criteria are for observing
AO-HIS in such systems. The state diagrams show that two laser fluences
thresholds must be considered:the fluence which induces the single laser pulse
switching (FSwitch) and the fluence at which the material breaks into a
multi-domain state (FMulti). Those two fluences are shown to behave very
differently as a function of the material properties and the laser pulse
duration. Taking into account the parameters defining the conditions for which
multi-domain states are created and considering only the angular momentum
transfer from the Gd sublattice to the rest of the system explains in large our
experimental results. The importance of the compensation in the ferrimagnetic
alloys is also discussed. We believe the defined criteria will be an important
tool for designing new ultra-fast spintronic devices based on all optical
switching. | 2311.18359v1 |
2024-01-01 | Tuning Thermal Conductivity of Hybrid Perovskites through Halide Alloying | Tuning the thermal transport properties of hybrid halide perovskites is
critical for their applications in optoelectronics, thermoelectrics, and
photovoltaics. Here, we demonstrate an effective strategy to modulate the
thermal transport property of hybrid perovskites by halide alloying. A highly
tunable thermal conductivity of mixed-halide hybrid perovskites is achieved due
to halide-alloying and structural distortion. Our experimental measurements
show that the room temperature thermal conductivity of MAPb(BrxI1-x)3 (x = 0-1)
can be largely modulated from 0.27 W/mK (x = 0.5) to 0.47 W/mK (x = 1).
Molecular dynamics simulations further demonstrate that the thermal
conductivity reduction of hybrid halide perovskites results from the
suppression of the mean free paths of the low-frequency acoustic and optical
phonons. It is found that halide alloying and the induced structural distortion
can largely increase the scatterings of optical and acoustic phonons,
respectively. The confined diffusion of MA+ cations in the octahedra cage is
found to act as an additional thermal transport channel in hybrid perovskites
and can contribute around 10-20% of the total thermal conductivity. Our
findings provide a strategy for tailoring the thermal transport in hybrid
halide perovskites which may largely benefit their related applications. | 2401.00647v1 |
2024-02-02 | Predictive Models based on Deep Learning Algorithms for Tensile Deformation of AlCoCuCrFeNi High-entropy alloy | High-entropy alloys (HEAs) stand out between multi-component alloys due to
their attractive microstructures and mechanical properties. In this
investigation, molecular dynamics (MD) simulation and machine learning were
used to ascertain the deformation mechanism of AlCoCuCrFeNi HEAs under the
influence of temperature, strain rate, and grain sizes. First, the MD
simulation shows that the yield stress decreases significantly as the strain
and temperature increase. In other cases, changes in strain rate and grain size
have less effect on mechanical properties than changes in strain and
temperature. The alloys exhibited superplastic behavior under all test
conditions. The deformity mechanism discloses that strain and temperature are
the main sources of beginning strain, and the shear bands move along the
uniaxial tensile axis inside the workpiece. Furthermore, the fast phase shift
of inclusion under mild strain indicates the relative instability of the
inclusion phase of HCP. Ultimately, the dislocation evolution mechanism shows
that the dislocations are transported to free surfaces under increased strain
when they nucleate around the grain boundary. Surprisingly, the ML prediction
results also confirm the same characteristics as those confirmed from the MD
simulation. Hence, the combination of MD and ML reinforces the confidence in
the findings of mechanical characteristics of HEA. Consequently, this
combination fills the gaps between MD and ML, which can significantly save time
human power and cost to conduct real experiments for testing HEA deformation in
practice. | 2402.01578v1 |
2024-02-03 | Lattice-dynamics and in-plane antiferromagnetism in MnxZn1_xPS3 across the entire composition range | Alloyed MnxZn1_xPS3 samples have been grown covering the whole compositional
range and studied by means of Raman spectroscopy at temperatures covering from
4K up to 850K. Our results, supported by SQUID magnetic measurements, allowed,
from one hand, to complete the magnetic phase diagram of MnxZn1_xPS3 and
establish x>0.3 as the composition at which the alloy retains
antiferromagnetism and, from the other hand, to identify the Raman signatures
indicative of a magnetic transition. The origin of these Raman signatures is
discussed in terms of spin-phonon coupling resulting in the appearance of low-
and high-frequency zone-folded phonon modes. For the alloy, an assignment of
the 1st and 2nd order modes is provided with the aid of first-principle
lattice-dynamical calculations. The compositional dependence of all phonon
modes is described and the presence of zone-folded modes is shown to take place
for both, the alloy and MnPS3. Finally, a comparison of the Raman spectra of
ZnPS3 to other compounds of the transition-metal phosphorous trisulfide family
allowed shows that low-frequency phonon peaks exhibit an abnormally large
broadening. This is consistent with previous claims on the occurrence of a
second-order Jahn-Teller effect that takes place for ZnPS3 and Zn-rich
MnxZn1_xPS3. | 2402.02102v1 |
2024-02-04 | Origin of the low precipitation hardening in magnesium alloys | In this work electron backscattered diffraction (EBSD)-assisted slip trace
analysis and transmission electron microscopy have been utilized to investigate
the interaction of basal dislocations with precipitates in the Mg alloys
Mg-1%wt.Mn-0.7%wt.Nd (MN11) and Mg-9%wt.Al-1%wt.Zn (AZ91), with the ultimate
aim of determining the origin of their poor precipitation hardening.
Precipitates in these alloys have a plate-shaped morphology, with plates being,
respectively, perpendicular (MgxNdy) and parallel (Mg17Al12) to the basal plane
of the magnesium matrix. Mechanical tests were carried out in solid solution
and peak-aged samples, in tension and compression, both at RT and at moderate
temperature (250C). EBSD-assisted slip trace analysis revealed a clear
dominance of basal slip under a wide range of testing conditions in the
peak-aged MN11 and AZ91 alloys. At room temperature, the origin of the low
precipitation hardening observed lies at the easiness with which precipitates
are sheared by basal dislocations, due to the lattice matching at the interface
with the Mg matrix. At high temperature, dislocation-precipitate interactions
are highly dependent on the deformation mode. In tension, enhanced basal slip
localization gives rise to high stress concentrations at the intersection
between coarse slip traces and particle interfaces, leading to precipitate
fracture; in compression, a more homogenous distribution of basal slip leads to
the dominance of particle shearing. Our study demonstrates experimentally that
basal dislocations are able to shear, and even fracture, the MgxNdy and
Mg17Al12 plates when, for appropriate testing conditions, the local stress due
to dislocation accumulation at particle interfaces exceeds the precipitate
strength. | 2402.06652v1 |
2024-02-13 | Chemical tuning of photo- and persistent luminescence of Cr3+-activated beta-Ga2O3 by alloying with Al2O3 and In2O3 | An effect of alloying of the monoclinic beta- Ga2O3 with Al2O3 and In2O3 on
the photoluminescent, thermoluminescent and persistent luminescent properties
of Cr3+ ions has been comprehensively investigated. For this purpose, various
series of Cr3+ and Ca2+ co-doped microcrystalline phosphors were synthesized by
the solution combustion method, including pseudobinary compounds like
(Ga-Al)2O3 with up to 20% Al and (Ga-In)2O3 with up to 50% In as well as
pseudoternary compounds (Ga Al In)2O3 with balanced proportion of Al, Ga and
In. The phase composition and crystal structure of the obtained materials were
examined by X-ray powder diffraction technique. Detailed luminescence studies
were conducted for the (Ga-Al)2O3 and (Ga-In)2O3 compounds which exhibited a
single-phase monoclinic structure. Low-temperature and time-resolved
photoluminescence investigations of the Cr-doped pseudobinary compounds
unveiled several types of Cr3+ centres, attributed to the Al-, Ga- and
In-centred octahedra in the studied alloys. The obtained results underscore the
benefit of bandgap engineering through alteration in the host lattice chemical
composition for efficient tuning of the thermoluminescent and persistent
luminescent properties of the near-infrared-emitting beta Ga2O3:Cr based
phosphors. Furthermore, it was demonstrated that modification of the chemical
composition of the host lattice also adjusts the thermometric performance of
the studied phosphors. Indeed, the specific sensitivity of the beta- Ga2O3:Cr3+
decay time luminescence thermometer showed nearly twofold enhancement when the
host lattice was alloyed with 30% of In2O3. | 2402.08542v2 |
2024-02-14 | Towards a dynamically reconfigurable pixelated reflective display: Focused ion beam for phase-change metapixel structures | The switching and optical properties of phase-change thin films are actively
investigated for future smart optical devices. The possibility of having more
than one stable state, the large optical contrast between phases, and the fast
and reversible switching are some attractive properties driving the research
interest. Optical devices based on phase change alloys are considered the
frontier contenders for tunable photonics. The combination of vivid structural
color formation, with partial amorphization/crystallization of phase change
alloys, and the associated optical tunability could be integrated into an
energy-efficient reflective display device with high pixel density. This work
demonstrates a contrast formation due to relative height differences from
isolated pixelated structures. A reflective heterostructure device consisting
of a low-loss Sb2Se3 alloy on a gold substrate was produced. With a focused ion
beam, a pixelated metasurface structure was produced. Moreover, the ability to
create local height differences using an ion beam was employed to create a
structural color combination mimicking traditional LED like RGB pixels. We
believe our approach in creating metapixels on phase change thin film surfaces
could open up research interest in phase change alloys and moving away from
semi/static plasmonic systems into truly dynamic display devices. | 2402.08891v1 |
2024-02-19 | Data-driven study of composition-dependent phase compatibility in NiTi shape memory alloys | The martensitic transformation in NiTi-based Shape Memory Alloys (SMAs)
provides a basis for shape memory effect and superelasticity, thereby enabling
applications requiring solid-state actuation and large recoverable shape
changes upon mechanical load cycling. In order to tailor the transformation to
a particular application, the compositional dependence of properties in
NiTi-based SMAs, such as martensitic transformation temperatures and
hysteresis, has been exploited. However, the compositional design space is
large and complex, and experimental studies are expensive. In this work, we
develop an interpretable piecewise linear regression model that predicts the
$\lambda_2$ parameter, a measure of compatibility between austenite and
martensite phases, and an (indirect) factor that is well-correlated with
martensitic transformation hysteresis, based on the chemical features derived
from the alloy composition. The model is capable of predicting, for the first
time, the type of martensitic transformation for a given alloy chemistry. The
proposed model is validated by experimental data from the literature as well as
in-house measurements. The results show that the model can effectively
distinguish between $B19$ and $B19^{\prime}$ regions for any given composition
in NiTi-based SMAs and accurately estimate the $\lambda_2$ parameter. Our
analysis also reveals that the weighted average of the quotient of the first
ionization energy and the Voronoi coordination number is a key compositional
characteristic that correlates with the $\lambda_2$ parameter and thermodynamic
responses, including the transformation hysteresis, martensite start
temperature, and critical temperature. The work herein demonstrates the
potential of data-driven methodologies for understanding and designing
NiTi-based SMAs with desired transformation characteristics. | 2402.12520v1 |
2024-03-06 | Collision Cascade-Driven Evolution of Vacancy Defects in Ni-Based Concentrated Solid-Solution Alloys | Concentrated solid--solution alloys (CSAs) in single--phase form have
recently garnered considerable attention owing to their potential for
exceptional irradiation resistance. This computational study delves into the
intricate interplay of alloying elements on the generation, recombination, and
evolution of irradiation-induced defects. Molecular dynamics simulations were
conducted for collision cascades at room temperature, spanning a range of
primary knock-on atom energies from 1 to 10 keV. The investigation encompasses
a series of model crystals, progressing from pure Ni to binary CSAs such as
NiFe$_{20}$, NiFe, NiCr$_{20}$, and culminating in the more intricate
NiFeCr$_{20}$ CSA. We observe that materials rich in chromium actively
facilitate dislocation emissions and induce the nucleation of stacking fault
tetrahedra in the proximity of nanovoids, owing to Shockley partial
interactions. This result is validated by molecular static simulations, which
calculate the surface, vacancy, and defect formation energies. Among various
shapes considered, the spherical void proves to be the most stable, followed by
the truncated octahedron and octahedron shapes. On the other hand, the
tetrahedron cubic shape is identified as the most unstable, and stacking fault
tetrahedra exhibit the highest formation energy. Notably, among the materials
studied, NiCr$_{20}$ and NiFeCr$_{20}$ CSAs stood out as the sole alloys
capable of manifesting this mechanism, mainly observed at high impact energies. | 2403.03922v1 |
2024-03-14 | A general-purpose neural network potential for Ti-Al-Nb alloys towards large-scale molecular dynamics with ab initio accuracy | High Nb-containing TiAl alloys exhibit exceptional high-temperature strength
and room-temperature ductility, making them widely used in hot-section
components of automotive and aerospace engines. However, the lack of accurate
interatomic interaction potentials for large-scale modeling severely hampers a
comprehensive understanding of the failure mechanism of Ti-Al-Nb alloys and the
development of strategies to enhance the mechanical properties. Here, we
develop a general-purpose machine-learned potential (MLP) for the Ti-Al-Nb
ternary system by combining the neural evolution potentials framework with an
active learning scheme. The developed MLP, trained on extensive
first-principles datasets, demonstrates remarkable accuracy in predicting
various lattice and defect properties, as well as high-temperature
characteristics such as thermal expansion and melting point for TiAl systems.
Notably, this potential can effectively describe the key effect of Nb doping on
stacking fault energies and formation energies. Of practical importance is that
our MLP enables large-scale molecular dynamics simulations involving tens of
millions of atoms with ab initio accuracy, achieving an outstanding balance
between computational speed and accuracy. These results pave the way for
studying micro-mechanical behaviors in TiAl lamellar structures and developing
high-performance TiAl alloys towards applications at elevated temperatures. | 2403.09529v1 |
2024-03-22 | Single-layer of Bi$_{1-x}$Sb$_x$ grown on Ag(111) | In this work, we report the growth of a single mixed Bi$_{1-x}$Sb$_x$ layer,
with diverse stoichiometries, on a Ag(111) substrate. The atomic geometry has
been thoroughly investigated by low energy electron diffraction, scanning
tunneling microscopy, and X-ray photoelectron spectroscopy experiments, as well
as calculations based on density functional theory (DFT). We first determined
that both pure systems (Bi/Ag(111) and Sb/Ag(111)) show similar behaviors: they
form surface alloys with ($\sqrt{3}\times\sqrt{3}$)R30$^\circ$ periodicity for
coverages lower than 1/3 ML, and undergo a dealloying transition for higher
coverages up to 2/3 ML. We then established a simple preparation procedure to
obtain a mixed Bi-Sb overlayer on Ag(111): it is essential to start with a
surface completely covered by either of the two pure surface alloys and then
deposit the other element on it. The energetics derived from DFT calculations
provide insight into the systems preference towards the formation of this
phase, and also predict a pathway to the formation of Bi-rich non-alloyed
phases. The obtained mixed Bi-Sb phase has a lateral atomic arrangement very
similar to the one in the non-alloyed phase observed for Sb on Ag(111), with Sb
and Bi atoms distributed disorderly, and presents a significant vertical
corrugation, promising considerable Rashba effects. | 2403.15242v1 |
2024-04-09 | A Machine Learning Framework for the Prediction of Grain Boundary Segregation in Chemically Complex Environments | The discovery of complex concentrated alloys has unveiled materials with
diverse atomic environments, prompting the exploration of solute segregation
beyond dilute alloys. Data-driven methods offer promising for modeling
segregation in such chemically complex environments, and are employed in this
study to understand segregation behavior of a refractory complex concentrated
alloy, NbMoTaW. A flexible methodology is developed that uses composable
computational modules, with different arrangements of these modules employed to
obtain site availabilities at absolute zero and the corresponding density of
states beyond the dilute limit, resulting in an extremely large dataset
containing 10 million data points. The artificial neural network developed here
can rely solely on descriptions of local atomic environments to predict
behavior at the dilute limit with very small errors, while the addition of
negative segregation instance classification allows any solute concentration
from zero up to the equiatomic concentration for ternary or quaternary alloys
to be modeled at room temperature. The machine learning model thus achieves a
significant speed advantage over traditional atomistic simulations, being four
orders of magnitude faster, while only experiencing a minimal reduction in
accuracy. This efficiency presents a powerful tool for rapid microstructural
and interfacial design in unseen domains. Scientifically, our approach reveals
a transition in the segregation behavior of Mo from unfavorable in simple
systems to favorable in complex environments. Additionally, increasing solute
concentration was observed to cause anti-segregation sites to begin to fill,
challenging conventional understanding and highlighting the complexity of
segregation dynamics in chemically complex environments. | 2404.06499v1 |
2024-04-10 | The effects of V doping on the intrinsic properties of SmFe10Co2 alloys: a theoretical investigation | The present study focuses on the intrinsic properties of the SmFe10Co2-xVx (x
= 0-2) alloys, which includes the SmFe10Co2 alloy, one of the most promising
permanent magnets with the ThMn12 type of structure due to its large saturation
magnetization (1.78 T), high Curie temperature (Tc = 859 K), and anisotropy
field (12 T) experimentally obtained. Unfortunately, its low coercivity (<0.4
T) hinders its use in permanent magnet applications. The effect of V-doping on
magnetization, magnetocrystalline anisotropy energy, and Curie temperature is
investigated by electronic band structure calculations. The spin-polarized
fully relativistic Korringa-Kohn-Rostoker (SPR-KKR) band structure method,
which employs the coherent potential approximation (CPA) to deal with
substitutional disorder, has been used. The Hubbard-U correction to local spin
density approximation (LSDA +U) was used to account for the large correlation
effects due to the 4f electronic states of Sm. The computed magnetic moments
and magnetocrystalline anisotropy energies were compared with existing
experimental data to validate the theoretical approach's reliability. The
exchange-coupling parameters from the Heisenberg model were used for obtaining
the mean-field estimated Curie temperature. The magnetic anisotropy energy was
separated into contributions from transition metals and Sm, and its
relationships with the local environment, interatomic distances, and valence
electron delocalization were analyzed. The suitability of the hypothetical
SmFe10CoV alloy for permanent magnet manufacture was assessed using the
calculated anisotropy field, magnetic hardness, and intrinsic magnetic
properties. | 2404.06897v1 |
2024-04-15 | Deep image learning of quantitative structure-property relationships of cooper alloys via feature augmentation on Geodesic curve in shape space | Understanding how the structure of materials affects their properties is a
cornerstone of materials science and engineering. However, traditional methods
have struggled to accurately describe the quantitative structure-property
relationships for complex structures. In our study, we bridge this gap by
leveraging machine learning to analyze images of materials' microstructures,
thus offering a novel way to understand and predict the properties of materials
based on their microstructures. We introduce a method known as FAGC (Feature
Augmentation on Geodesic Curves), specifically demonstrated for Cu-Cr-Zr
alloys. This approach utilizes machine learning to examine the shapes within
images of the alloys' microstructures and predict their mechanical and
electronic properties. This generative FAGC approach can effectively expand the
relatively small training datasets due to the limited availability of materials
images labeled with quantitative properties. The process begins with extracting
features from the images using neural networks. These features are then mapped
onto the Pre-shape space to construct the Geodesic curves. Along these curves,
new features are generated, effectively increasing the dataset. Moreover, we
design a pseudo-labeling mechanism for these newly generated features to
further enhance the training dataset. Our FAGC method has shown remarkable
results, significantly improving the accuracy of predicting the electronic
conductivity and hardness of Cu-Cr-Zr alloys, with R-squared values of 0.978
and 0.998, respectively. These outcomes underscore the potential of FAGC to
address the challenge of limited image data in materials science, providing a
powerful tool for establishing detailed and quantitative relationships between
complex microstructures and material properties. | 2404.09515v1 |
2015-03-01 | Anomalous Hall effect and current spin polarization in Co$_2$FeX (X = Al, Ga, In, Si, Ge, and Sn) Heusler compounds: A systematic {\it ab initio} study | In this paper, we perform a systematic {\it ab initio} study of two principal
spin-related phenomena, namely, anomalous Hall effect and current spin
polarization, in Co$_2$Fe-based Heusler compounds Co$_2$FeX (X = Al, Ga, In,
Si, Ge, Sn) within the generalized gradient approximation (GGA). The accurate
full-potential linearized augmented plane-wave method is used. We find that the
spin-polarization of the longitudinal current ($P^L$) in Co$_2$FeX (X = Al, Ga,
In, Al$_{0.5}$Si$_{0.5}$ and Sn) is $\sim$100 \% even though that of the
electronic states at the Fermi level ($P^D$) is not. Further, the other
compounds also have a high current spin polarization with $P^L > 85$ \%. This
indicates that all the Co$_2$FeX compounds considered are promising for
spin-transport devices. Interestingly, $P^D$ is negative in Co$_2$FeX (X = Si,
Ge and Sn), differing in sign from the $P^L$ as well as that from the transport
experiments. Secondly, the calculated anomalous Hall conductivities (AHCs) are
moderate, being within 200 S/cm, and agree well with the available experiments
on highly L2$_1$ ordered Co$_2$FeSi specimen although they differ significantly
from the reported experiments on other compounds where the B2 antisite
disorders were present. Surprisingly, the AHC in Co$_2$FeSi decreases and then
changes sign when Si is replaced by Ge and finally by Sn. Third, the calculated
total magnetic moments agree well with the corresponding experimental ones in
all the studied compounds except Co$_2$FeSi where a difference of 0.3
$\mu_B$/f.u. exists. We also perform the GGA plus on-site Coulomb interaction
$U$ calculations in the GGA+$U$ scheme. We find that including the $U$ affects
the calculated total magnetic moment, spin polarization and AHC significantly,
and in most cases, results in a disagreement with the available experimental
results. | 1503.00204v1 |
2019-04-08 | Two functionals approach in DFT for the prediction of thermoelectric properties of Fe$_{2}$ScX (X = P, As, Sb) full Heusler compounds | In the quest of new thermoelectric (TE) materials with high power factors,
full-Heusler compounds having flat band are found to be promising candidates.
In this direction, Fe$_{2}$ScX (X=P,As,Sb) compounds are investigated using mBJ
for the band gap and SCAN to describe the electronic bands and phonon
properties for TE applications. The band gaps obtained from mBJ are 0.81 eV,
0.69 eV and 0.60 eV for Fe$_{2}$ScX compounds, respectively. The phonon
dispersion, phonon density of states (DOS) and partial DOS are calculated. The
phonon contributions to specific heat are obtained as a function of temperature
under harmonic approximation. The electronic band structutre calculated from
mBJ and SCAN functionals are qualitatively compared. The TE parameters are
calculated for both hole and electron dopings under semiclassical theory. We
use simple, but reasonable method to estimate phonon relaxation time
($\tau_{ph}$). Using the specific heat, estimated $\tau_{ph}$ and slopes (phase
velocity) of acoustic branches in the linear region, lattice thermal
conductivity ($\kappa_{ph}$) at 300 K is calculated for three compounds. The
obtained values of $\kappa_{ph}$ with constant $\tau_{ph}$ are 18.2, 13.6 and
10.3 $Wm^{-1}K^{-1}$, respectively. Finally, the temperature dependent figure
of merit $ZT$ values are calculated for optimal carrier concentrations in the
doping range considered, to evaluate the materials for TE application. The $ZT$
values for n-type Fe$_{2}$ScX, in 900-1200 K, are 0.34-0.43, 0.40-0.48 and
0.45-0.52, respectively. While, the p-type Fe$_{2}$ScX have $ZT$ of 0.25-0.34,
0.20-0.28 and 0.18-0.26, respectively in the same temperature range. The $ZT$
values suggest that, Fe$_{2}$ScX compounds can be promising materials in high
temperature power generation application on successful synthesis and further
$\kappa_{ph}$ reduction by methods like nanostructuring. | 1904.04322v1 |
2019-10-11 | Band structure tuning of Heusler compounds revisited: Spin- and momentum-resolved electronic structure analysis of compounds with different band filling | Spin-filtered time-of-flight photoelectron momentum microscopy reveals a
systematic variation of the band structure within a series of highly
spin-polarized ferromagnetic Heusler compounds with increasing number of
valence electrons (Co2MnGa, Co2MnSi and Co2Fe0.4Mn0.6Si). The positions of the
Fermi energy for minority and majority electrons deviate strongly from a simple
band-filling model. Photoexcitation at h$\nu$=6.05 eV (4th harmonic of a
Ti:sapphire laser) gives access to the spin-polarization texture P(EB,kx,ky) of
the bulk bands in a (kx,ky)-range with diameter 1.4{\AA}$^{-1}$ and energies
from the Fermi energy EF to a binding energy of EB=2 eV. The minority bands of
Co2MnGa cross the Fermi level, inhibiting half-metallicity; the crossing points
allow a precise adjustment of experimental and theoretical majority and
minority bands, requiring shifts in opposite directions. The top of the
minority band lies only 0.15 eV above EF, i.e. Co2MnGa is much closer to being
half-metallic than predicted by calculations. For half-metallic Co2MnSi and
Co2Fe0.4Mn0.6Si clear minority band gaps are visible, the topmost occupied
minority bands lie 0.5 and 0.35 eV below EF, in reasonable agreement with
theory; the exchange splitting is significantly smaller than in theory. The
comparison of all three compounds uncovers the surprising fact that with
increasing number of valence electrons the frontier majority bands (close to
EF) exhibit an increasing deficiency in filling, in comparison with the
prediction of a DFT calculation. The same trend is visible in comparison with a
DMFT calculation. For s-polarized excitation both half-metallic compounds
exhibit nearly complete positive spin polarization close to EF, consistent with
previous work in literature. | 1910.05205v1 |
2020-08-26 | First-principles electronic structure, phonon properties, lattice thermal conductivity and prediction of figure of merit of FeVSb half-Heusler | In this work, we have studied the electronic structure of a promising
thermoelectric half-Heusler FeVSb using FP-LAPW method and SCAN meta-GGA
including spin-orbit coupling. Using the obtained electronic structure and
transport calculations we try to address the experimental Seebeck coefficient
$S$ of FeVSb samples. The good agreement between the experimental and
calculated $S$ suggests the band gap could be $\sim$0.7 eV. This is supported
by the obtained mBJ band gap of $\sim$0.7 eV. Further, we study and report the
phonon dispersion, density of states and thermodynamic properties. The effect
of long range Coulomb interactions on phonon frequencies are also included by
non-analytical term correction. Under quasi-harmonic approximation, the thermal
expansion behaviour upto 1200 K is calculated. Using the first-principles
anharmonic phonon calculations, the lattice thermal conductivity $\kappa_{ph}$
of FeVSb is obtained under single-mode relaxation time approximation
considering the phonon-phonon interaction. At 300 K, the calculated
$\kappa_{ph}$ is $\sim$18.6 W$m^{-1}K^{-1}$ which is higher compared to
experimental value. But, above 500 K the calculated $\kappa_{ph}$ is in good
agreement with experiment. A prediction of figure of merit $ZT$ and efficiency
for p-type and n-type FeVSb is made by finding out optimal carrier
concentration. At 1200 K, a maximum $ZT$ of $\sim$0.66 and $\sim$0.44 is
expected for p-type and n-type FeVSb, respectively. For p-type and n-type
materials, maximum efficiency of $\sim$12.2 \% and $\sim$6.0 \% are estimated
for hot and cold temperature of 1200 K and 300 K, respectively. A possibility
of achieving n-type and p-type FeVSb by various elemental doping/vacancy is
also discussed. Our study is expected to help in further exploring the
thermoelectric material FeVSb. | 2008.11693v1 |
2020-12-23 | Identifying the fingerprints of topological states by tuning magnetoresistance in a semimetal: the case of topological half-Heusler Pt1-xAuxLuSb | Topological materials often exhibit remarkably linear, non-saturating
magnetoresistance (LMR), which is both of scientific and technological
importance. However, the role of topologically non-trivial states in the
emergence of such a behaviour has eluded clear demonstration in experiments.
Here, by reducing the coupling between the topological surface states (TSS) and
the bulk carriers we controllably tune the LMR behavior in Pt1-xAuxLuSb into
distinct plateaus in Hall resistance, which we show arise from a quantum Hall
phase. This allowed us to reveal how smearing of the Landau levels, which
otherwise give rise to a quantum Hall phase, results in an LMR behavior due to
strong interaction between the TSS with a positive g-factor and the bulk
carriers. We establish that controlling the coupling strength between the
surface and the bulk carriers in topological materials can bring about dramatic
changes in their magnetotransport behavior. In addition, our work outlines a
strategy to reveal macroscopic physical observables of TSS in compounds with a
semi-metallic bulk band structure, as is the case in multi-functional Heusler
compounds, thereby opening up opportunities for their utilization in hybrid
quantum structures. | 2012.12633v5 |
2021-01-23 | Studying the lifetime of charge and heat carriers due to intrinsic scattering mechanisms in FeVSb half-Heusler thermoelectric | This work, presents a study of lifetime of carriers due to intrinsic
scattering mechanisms $viz.$ electron-electron (EEI), electron-phonon (EPI) and
phonon-phonon interactions (PPI) in a promising half-Heusler thermoelectric
FeVSb. Using the full-$GW$ method, the effect of EEI and temperature on the
valence and conduction band extrema and band gap are studied. The lifetime of
carriers with temperature are estimated at these band extrema. At 300 K,
estimated value of lifetime at VBM (CBM) is $\sim$1.91 x10$^{-14}s$ ($\sim$2.05
x10$^{-14}s$). The estimated ground state band gap considering EEI is $\sim$378
meV. Next, the effect of EPI on the lifetime of electrons and phonons with
temperature are discussed. The comparison of two electron lifetimes suggests
that EEI should be considered in transport calculations along with EPI. The
average acoustic, optical and overall phonon lifetimes due to EPI are studied
with temperature. Further, the effect of PPI is studied by computing average
phonon lifetime for acoustic and optical phonon branches. The lifetime of the
acoustic phonons are higher compared to optical phonons which indicates
acoustic phonons contribute more to lattice thermal conductivity
($\kappa_{ph}$). The comparison of phonon lifetime due to EPI and PPI suggests
that, above 500 K EPI is the dominant phonon scattering mechanism and cannot be
ignored in $\kappa_{ph}$ calculations. Lastly, a prediction of the power factor
and figure of merit of n-type and p-type FeVSb is made by considering the
temperature dependent carrier lifetime for the electronic transport terms. This
study shows the importance of considering EEI in electronic transport
calculations and EPI in phonon transport calculations. Our study is expected to
provide results to further explore the thermoelectric transport in this
material. | 2101.09555v1 |
2022-06-22 | Non-trivial topological phases in transition metal rich half-Heusler Oxides | Topological Insulators with gapless surface states and insulating bulk in
non-centrosymmetric cubic systems have been extensively explored following the
discovery of two-dimensional quantum spin hall effect in zincblende HgTe. In
such systems the negative band inversion strength E$_{BIS}$ ($=$ E$_{\Gamma_6}
-$ E$_{\Gamma_8} <$ 0) governs the robustness of the non-trivial topological
states at ambient conditions. Hence, realizing large negative values of
E$_{BIS}$ has been a guiding motivation of several investigations reported in
literature. Here, we present a material design approach which can be employed
to realize large negative values of E$_{BIS}$ in cubic materials such as
half-Heusler (HH) oxides with 18 valence electron configurations. We explore 27
HH oxides of the form ABO (A = Li, K, Rb; B = Cu, Ag, Au) in $\alpha$-,
$\beta$-, and $\gamma$-phase (by placing transition metal atom at different
Wyckoff positions) for their non-trivial topological phase. Off these three
phases, we found that, the $\alpha$-phase of nine HH oxides (wherein the
transition metal atoms occupy 4a Wyckoff positions in the crystal structure) is
the most promising with non-trivial topological phase which is governed by the
mass-darwin relativistic effects enhancing E$_{BIS}$. Whereas the other phases
were found to be either trivial semiconductors or semimetals or metals and most
of them being dynamically unstable. We focus on RbAuO in $\alpha$-phase with
E$_{BIS}$ of $-$ 1.29 eV and the effect of strain fields on the topological
surface states of this compound. We conclude that the $\alpha$-phase of HH
oxide presented here can be synthesized experimentally for diverse room
temperature applications in spintronics and nanoelectronics. | 2206.10976v2 |
1995-08-17 | Weighted two particle Green's functions in the CPA | We extend the two particle theory of disordered systems within the coherent
potential approximation CPA to obtain weighted contributions to averaged two
particle resolvents which arise from separate alloy components. Starting from
first principles in a model of diagonal disorder and the single site
approximation for a binary substitutional alloy $A_cB_{1-c}$ we extend the
approach of a fundamental paper by Velick{\'y} to evaluate various weighted
forms of a general class of two particle Green's functions. Applications in a
wide range of linear response theory are discussed in detail as well as the
behavior of the weighted functions in a strong disorder limit. To exemplify our
analytic calculations the optical absorption in a disordered model- alloy is
studied numerically. | 9508068v1 |
1996-04-24 | Monte Carlo study of the growth of $L1_2$ ordered domains in fcc $A_3B$ binary alloys | A Monte Carlo study of the late time growth of $L1_2$ ordered domains on a
fcc $A_3B$ binary alloy is presented. The energy of the alloy has been modeled
by a nearest neighbor interaction Ising hamiltonian. The system exhibits a
fourfold degenerated ground-state and two kinds of interfaces separating
ordered domains: flat and curved antiphase boundaries. Two different dynamics
are used in the simulations: the standard atom-atom exchange mechanism and the
more realistic vacancy-atom exchange mechanism. The results obtained by both
methods are compared. In particular we study the time evolution of the excess
energy, the structure factor and the mean distance between walls. In the case
of atom-atom exchange mechanism anisotropic growth has been found: two
characteristic lengths are needed in order to describe the evolution.
Contrarily, with the vacancy-atom exchange mechanism scaling with a single
length holds. Results are contrasted with existing experiments in $Cu_3Au$ and
theories for anisotropic growth. | 9604150v1 |
1996-09-19 | Disorder-driven non-Fermi liquid behavior in Kondo alloys | We demonstrate that a model of disordered Anderson lattices can account for
many non-Fermi liquid features observed in a number of Kondo alloys. Due to the
exponential nature of the Kondo temperature scale, even moderate disorder leads
to a rather broad distribution of Kondo temperatures, inducing strong effective
disorder seen by the conduction electrons. Spins with very small Kondo
temperatures remain unquenched and dominate the low temperature properties. The
model predicts logarithmic divergences in thermodynamic quantities at low
temperatures. We also find a linear temperature dependence of the resistivity,
a feature that remained a stumbling block in previous theoretical attempts. We
argue that for realistic amounts of disorder, such marginal Fermi liquid
behavior is a very robust feature of disordered Kondo alloys. | 9609193v1 |
1996-09-20 | Electronic structure of the valence band of the II--VI wide band gap binary/ternary alloy interfaces | We present an electronic structure calculation of the valence band for some
II--VI binary/ternary alloy interfaces. We use the empirical tight-binding
method and the surface Green's function matching method. For the ternary alloys
we use our previously set Hamiltonians they describe well the band gap change
with composition obtained experimentally. At the interface domain, we find
three non-dispersive and two interface states besides the known bulk bands. The
non-dispersive states are reminiscent of the ones already obtained
experimentally as well as theoretically, in (001)-oriented surfaces. We make
use of the available theoretical calculations for the (001)-oriented surfaces
of the binary compounds and for the binary/binary interfaces to compare our new
results with. | 9609196v1 |
1996-11-28 | Thermal and Mechanical Properties of Pt-Rh Alloys | We utilize the many-body potentials developed by Sutton and Chen(1990) within
the context of the tight-binding approach to study the bulk properties of
metals and metal alloys in molecular dynamics (MD) simulations. In the
simulations of Pt-Rh alloys we used the MD algorithms based on an extended
Hamiltonian formalism from the works of Andersen(1980), Parrinello and
Rahman(1980), Nose(1984), Hoover(1985) and Cagin(1988).The simulator program
that we use generates information about various physical properties during the
run time, along with critical trajectory and stepwise information which need to
be analysed post production. The thermodynamical and mechanical properties are
calculated using the statistical fluctuation expressions over the MD. | 9611241v1 |
1998-08-05 | Theoretical search for Chevrel phase based thermoelectric materials | We investigate the thermoelectric properties of some semiconducting Chevrel
phases. Band structure calculations are used to compute thermopowers and to
estimate of the effects of alloying and disorder on carrier mobility. Alloying
on the Mo site with transition metals like Re, Ru or Tc to reach a
semiconducting composition causes large changes in the electronic structure at
the Fermi level. Such alloys are expected to have low carrier mobilities.
Filling with transition metals was also found to be incompatible with high
thermoelectric performance based on the calculated electronic structures.
Filling with Zn, Cu, and especially with Li was found to be favorable. The
calculated electronic structures of these filled Chevrel phases are consistent
with low scattering of carriers by defects associated with the filling. We
expect good mobility and high thermopower in materials with the composition
close to (Li,Cu)$_4$Mo$_6$Se$_8$, particularly when Li-rich, and recommend this
system for experimental investigation. | 9808056v1 |
1999-03-07 | Correlation between Spin Polarization and Magnetic Moment in Ferromagnetic Alloys | The correlation between the magnetic moment in ferromagnetic alloys and the
tunneling spin polarization in ferromagnet-insulator-superconductor tunneling
experiments has been a mystery. The measured spin polarization for Fe, Co, Ni,
and various Ni alloys is positive and roughly proportional to their magnetic
moments, which can not be explained by considering the net density of states.
Using a tight-binding coherent potential approximation (CPA) model, we show
that while the polarization of the net density of states is not correlated with
the magnetic moment, the polarization of the density of states of {\it s}
electrons is correlated with the magnetic moment in the same manner as observed
by the tunneling experiments.
We also discuss the spin polarization measurements by Andreev reflection
experiments, some of which obtained different results from the tunneling
experiments and our calculations. | 9903118v2 |
1999-10-26 | Depletion of density of states near Fermi energy induced by disorder and electron correlation in alloys | We have performed high resolution photoemission study of substitutionally
disordered alloys Cu-Pt, Cu-Pd, Cu-Ni, and Pd-Pt. The ratios between alloy
spectra and pure metal spectra are found to have dips at the Fermi level when
the residual resistivity is high and when rather strong repulsive
electron-electron interaction is expected. This is in accordance with Altshuler
and Aronov's model which predicts depletion of density of states at the Fermi
level when both disorder and electron correlation are present. | 9910411v1 |
2000-08-02 | Superconducing Alloys with Weak and Strong Scattering: Anderson's Theorem and a Superconductor-Insulator Transition | We have studied the effects of strong impurity scattering on disordered
superconductors beyond the low impurity concentration limit. By applying the
full CPA to a superconductiong A-B binary alloy, we calculated the fluctuations
of the local order parameters $\Delta_{A}, \Delta_{B}$ and charge densities,
$n_{A}, n_{B}$ for weak and strong on site disorder. We find that for narrow
band alloy s-wav e superconductors the conditions for Anderson's theorem are
satisfied in general only for the case of particle-hole symmetry. In this case
it is satisfied regardless whether we are in the weak or strong scattering
regimes. Interestingly, we find that strong scattering leads to band splitting
and in this regime for any band filling we have a critical concentration where
a superconductor-insulator quantum phase transition occurs at T=0. | 0008031v1 |
2000-08-04 | Local structure study of In_xGa_(1-x)As semiconductor alloys using High Energy Synchrotron X-ray Diffraction | Nearest and higher neighbor distances as well as bond length distributions
(static and thermal) of the In_xGa_(1-x)As (0<x<1) semiconductor alloys have
been obtained from high real-space resolution atomic pair distribution
functions (PDFs). Using this structural information, we modeled the local
atomic displacements in In_xGa_(1-x)As alloys. From a supercell model based on
the Kirkwood potential, we obtained 3-D As and (In,Ga) ensemble averaged
probability distributions. This clearly shows that As atom displacements are
highly directional and can be represented as a combination of <100> and <111>
displacements. Examination of the Kirkwood model indicates that the standard
deviation (sigma) of the static disorder on the (In,Ga) sublattice is around
60% of the value on the As sublattice and the (In,Ga) atomic displacements are
much more isotropic than those on the As sublattice. The single crystal diffuse
scattering calculated from the Kirkwood model shows that atomic displacements
are most strongly correlated along <110> directions. | 0008079v1 |
2000-08-30 | Ferromagnetism in the Periodic Anderson Model - a Modified Alloy Analogy | We introduce a new aproximation scheme for the periodic Anderson model (PAM).
The modified alloy approximation represents an optimum alloy approximation for
the strong coupling limit, which can be solved within the CPA-formalism.
Zero-temperature and finite-temperature phase diagrams are presented for the
PAM in the intermediate-valence regime. The diversity of magnetic properties
accessible by variation of the system parameters can be studied by means of
quasiparticle densities of states: The conduction band couples either ferro- or
antiferromagneticaly to the f-levels. A finite hybridization is a necessary
precondition for ferromagnetism. However, too strong hybridization generally
suppresses ferromagnetism, but can for certain system parameters also lead to a
semi-metallic state with unusual magnetic properties. By comparing with the
spectral density approximation, the influence of quasiparticle damping can be
examined. | 0008441v1 |
2000-11-03 | Charge transfer electrostatic model of compositional order in perovskite alloys | We introduce an electrostatic model including charge transfer, which is shown
to account for the observed B-site ordering in Pb-based perovskite alloys. The
model allows charge transfer between A-sites and is a generalization of
Bellaiche and Vanderbilt's purely electrostatic model. The large covalency of
Pb^{2+} compared to Ba^{2+} is modeled by an environment dependent effective
A-site charge. Monte Carlo simulations of this model successfully reproduce the
long range compositional order of both Pb-based and Ba-based complex
A(BB^{'}B^{''})O_3 perovskite alloys. The models are also extended to study
systems with A-site and B-site doping, such as
(Na_{1/2}La_{1/2})(Mg_{1/3}Nb_{2/3})O_3,
(Ba_{1-x}La_{x})(Mg_{(1+x)/3}Nb_{(2-x)/3})O_3 and
(Pb_{1-x}La_{x})(Mg_{(1+x)/3}Ta_{(2-x)/3})O_3. General trends are reproduced by
purely electrostatic interactions, and charge transfer effects indicate that
local structural relaxations can tip the balance between different B-site
orderings in Pb based materials. | 0011063v1 |
2000-12-13 | Strain Relaxation Mechanisms and Local Structural Changes in Si_{1-x}$Ge_{x} Alloys | In this work, we address issues pertinent to the understanding of the
structural and electronic properties of Si_{1-x} Ge_{x}alloys, namely, (i) how
does the lattice constant mismatch between bulk Si and bulk Ge manifests itself
in the alloy system? and (ii) what are the relevant strain release mechanisms?
To provide answers to these questions, we have carried out an in-depth study of
the changes in the local geometric and electronic structures arising from the
strain relaxation in Si_{1-x} Ge_{x} alloys using an ab initio molecular
dynamics scheme. The optimized lattice constant, while exhibiting a general
trend of linear dependence on the composition (Vegard's law), shows a negative
deviation from Vegard's law in the vicinity of x=0.5. We delineate the
mechanisms responsible for each one of the above features. We show that the
radial-strain relaxation through bond stretching is responsible for the overall
trend of linear dependence of the lattice constant on the composition. On the
other hand, the negative deviation from Vegard's law is shown to arise from the
angular-strain relaxation. | 0012220v1 |
2001-08-09 | Elastic and thermodynamic properties of the shape-memory alloy AuZn | The current work reports on the elastic shear moduli, internal friction, and
the specific heat of the B2 cubic ordered alloy AuZn as a function of
temperature. Measurements were made on single-crystal and polycrystalline
samples using Resonant Ultrasound Spectroscopy (RUS), semi-adiabatic
calorimetry and stress-strain measurements. Our results confirm that this alloy
exhibits the shape-memory effect and a phase transition at 64.75 K that appears
to be continuous (second-order) from the specific heat data. It is argued that
the combination of equiatomic composition and a low transformation temperature
constrain the chemical potential and its derivatives to exhibit behavior that
lies at the borderline between that of a first-order (discontinuous) and a
continuous phase transition. The acoustic dissipation does not peak at the
transtion temperature as expected, but shows a maximum well into the
low-temperature phase. The Debye temeprature value of 219 K, obtained from the
low-temperature specific heat data is in favorable agreement with that
determined from the acoustic data (207 K) above the transition. | 0108168v1 |
2001-08-27 | Master equation approach to configurational kinetics of non-equilibrium alloys and its application to studies of L1_0 type orderings | We review a series of works where the fundamental master equation is used to
develop a microscopical description of evolution of non-equilibrium atomic
distributions in alloys. We describe exact equations for temporal evolution of
local concentrations and their correlators as well as approximate methods to
treat these equations, such as the kinetic mean-field and the kinetic cluster
methods. We also describe an application of these methods to studies of
kinetics of L1_0 type orderings in FCC alloys which reveal a number of peculiar
microstructural effects, many of them agreeing well with experimental
observations. | 0108441v1 |
2001-10-27 | Dynamics of a single electron in the disordered Holstein model | We study, at zero temperature, the dynamics of a single electron in a
Holstein model augmented by site-diagonal, binary-alloy type disorder. The
average over the phonon vacuum and the alloy configurations is performed within
a generalized dynamical coherent potential approximation. We present numerical
results for a Bethe lattice with infinite coordination number. In particular,
we investigate, in the intermediate electron-phonon coupling regime, the
spectral and diffusion properties in the vicinity of the high-energy edge of
the lowest polaronic subband. To characterize the diffusion properties, we
define a spectrally resolved delocalization time, which is, for a given energy,
the characteristic time scale on which the electron leaves a given site. We
find the delocalization times substantially enhanced for states with a large
phonon content, i.e., in the absence (presence) of alloy-type disorder at the
high-energy edge(s) of the polaronic subband (mini-subbands). According to
their delocalization times, we discriminate between ``fast''
quasi-particle-like and ``sluggish'' defect-like polaron states and
qualitatively address the issue of trapping of an electronic carrier. | 0110577v1 |
2001-11-01 | Metal-insulator transition in amorphous alloys | We focus on the central problem of discriminating between metallic and
insulating behaviour in amorphous alloys formed between a semiconductor and a
metal. For this, the logarithmic temperature derivative of the conductivity, w
= d ln sigma / d ln T, has proved over recent years to be very helpful in
determining the critical value x_c of the metal content x for the
metal-insulator transition (MIT). We show that, for various amorphous alloys,
recent experimental results on w(T,x) are qualitatively inconsistent with the
usual assumptions of continuity of the MIT at T = 0 and of sigma(T,x_c) being
proportional to a power of T. These results suggest that w(T,x_c) tends to 0 as
T -> 0, in which case the MIT should be discontinuous at T = 0 (but only
there), in agreement with Mott's hypothesis of a finite minimum metallic
conductivity. | 0111017v1 |
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