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2020-08-01 | Critical cooling rates for amorphous-to-ordered complexion transitions in Cu-rich nanocrystalline alloys | Amorphous complexions in nanocrystalline metals have the potential to improve
mechanical properties and radiation tolerance, as well as resistance to grain
growth. In this study, the stability of amorphous complexions in binary and
ternary Cu-based alloys is investigated by observing the effect of cooling rate
from high temperature on the occurrence of amorphous-to-ordered complexion
transitions. Bulk Cu-Zr and Cu-Zr-Hf alloy samples were annealed to induce
boundary premelting and then quenched through a procedure that induces a
gradient of local cooling rate through the sample height. Amorphous complexion
thickness distributions were found to be invariant to local cooling rate in the
Cu-Zr-Hf alloy, demonstrating enhanced stability of the amorphous complexion
structure compared to the Cu-Zr alloy, which had thinner amorphous complexions
in the regions that were slowly cooled. The experimental results are used to
construct time-temperature-transformation diagrams of the amorphous-to-ordered
complexion transition for both the binary and ternary alloys, enabling a deeper
understanding of the influence of cooling rate and grain boundary chemistry on
complexion transitions. The critical cooling rate necessary to avoid complexion
transitions in the ternary alloy is found to be at least three orders of
magnitude slower than that for the binary alloy. | 2008.00292v2 |
2019-05-30 | Ab initio theory of the spin-dependent conductivity tensor and the spin Hall effect in random alloys | We present an extension of the relativistic electron transport theory for the
standard (charge) conductivity tensor of random alloys within the tight-binding
linear muffin-tin orbital method to the so-called spin-dependent conductivity
tensor, which describes the Kubo linear response of spin currents to external
electric fields. The approach is based on effective charge- and spin-current
operators, that correspond to intersite electron transport and that are
nonrandom, which simplifies the configuration averaging by means of the
coherent potential approximation. Special attention is paid to the Fermi sea
term of the spin-dependent conductivity tensor, which contains a nonzero
incoherent part, in contrast to the standard conductivity tensor. The developed
formalism is applied to the spin Hall effect in binary random nonmagnetic
alloys, both on a model level and for Pt-based alloys with an fcc structure. We
show that the spin Hall conductivity consists of three contributions (one
intrinsic and two extrinsic) which exhibit different concentration dependences
in the dilute limit of an alloy. Results for selected Pt alloys (Pt-Re, Pt-Ta)
lead to the spin Hall angles around 0.2; these sizable values are obtained for
compositions that belong to thermodynamically equilibrium phases. These alloys
can thus be considered as an alternative to other systems for efficient charge
to spin conversion, which are often metastable crystalline or amorphous alloys. | 1905.12855v2 |
2019-08-01 | Valley Phenomena in the Candidate Phase Change Material WSe$_{2(1-x)}$Te$_{2x}$ | Alloyed transition metal dichalcogenides provide an opportunity for coupling
band engineering with valleytronic phenomena in an atomically-thin platform.
However, valley properties in alloys remain largely unexplored. We investigate
the valley degree of freedom in monolayer alloys of the phase change candidate
material WSe$_{2(1-x)}$Te$_{2x}$. Low temperature Raman measurements track the
alloy-induced transition from the semiconducting 1H phase of WSe$_2$ to the
semimetallic 1T$_d$ phase of WTe$_2$. We correlate these observations with
density functional theory calculations and identify new Raman modes from W-Te
vibrations in the 1H alloy phase. Photoluminescence measurements show ultra-low
energy emission features that highlight alloy disorder arising from the large
W-Te bond lengths. Interestingly, valley polarization and coherence in alloys
survive at high Te compositions and are more robust against temperature than in
WSe$_2$. These findings illustrate the persistence of valley properties in
alloys with highly dissimilar parent compounds and suggest band engineering can
be utilized for valleytronic devices. | 1908.00506v2 |
2020-05-30 | Tribocorrosion under galvanic interaction of Ti6Al4V and NiCr implant alloys | Micromovements that occur in the joint between dental prostheses and implants
can lead to wear-induced degradation. This process can be enhanced by corrosion
in the oral environment influenced by the presence of solutions containing
fluoride. Moreover, the eventual galvanic interactions between NiCr and Ti
alloys can accelerate the wear-corrosion process. In this work, the
tribocorrosion process of Ti6Al4V and NiCr alloys used in dental implant
rehabilitations immersed in fluoride solutions at different pH values was
investigated. The galvanic interaction effect between the alloys was also
assessed. Tribocorrosion tests in corrosive media were performed with isolated
Ti6Al4V and NiCr alloys, followed by testing with both alloys in contact. The
media selected were based on fluoride concentrations and pH values that are
possible to be found in oral environments. Analysis of the surfaces after the
tribocorrosion tests was carried out using confocal laser microscopy. The wear
profile and volume losses were determined by confocal measurements. It was
concluded that the galvanic interaction between the alloys increased the
tribocorrosion resistance of Ti6Al4V, compared with that of the isolated
Ti6Al4V alloy. Ti6Al4V coupled with NiCr reduced the electrochemical potential
decay during sliding. The increased resistance was explained by the
electrochemical shift of the Ti6Al4V potential from active dissolution to the
passive domain. | 2006.00270v1 |
2021-06-01 | The effect of local chemical ordering on dislocation activity in multi-principle element alloys: a three-dimensional discrete dislocation dynamics study | The exceptional combination of strength and ductility in multi-component
alloys is often attributed to the interaction of dislocations with the various
solute atoms in the alloy. To study these effects on the mechanical properties
of such alloys there is a need to develop a modeling framework capable of
quantifying the effect of these solutes on the evolution of dislocation
networks. Large scale three-dimensional (3D) Discrete dislocation dynamics
(DDD) simulations can provide access to such studies but to date no relevant
approaches are available that aim for a complete representation of real alloys
with arbitrary chemical compositions. Here, we introduce a formulation of
dislocation interaction with substitutional solute atoms in fcc alloys in 3D
DDD simulations that accounts for solute strengthening induced by atomic misfit
as well as fluctuations in the cross-slip activation energy. Using this model,
we show that local fluctuations in the chemical composition of various
CrFeCoNi-based multi-principal element alloys (MPEA) lead to sluggish
dislocation motion, frequent cross-slip and alignment of dislocations with
solute aggregation features, explaining experimental observations related to
mechanical behavior and dislocation activity. It is also demonstrated, that
this behavior observed for certain MPEAs cannot be reproduced by assuming a
perfect solid solution. The developed method also provides a basis for further
investigations of dislocation plasticity in any real arbitrary fcc alloy with
substitutional solutes. | 2106.00823v2 |
2021-11-11 | Controlling transition metal atomic ordering in two-dimensional Mo$_{1-x}$W$_{x}$S$_{2}$ alloys | The unique optical and electronic properties of two-dimensional transition
metal dichalcogenides (2D TMDs) make them promising materials for applications
in (opto-)electronics, catalysis and more. Specifically, alloys of 2D TMDs have
broad potential applications owing to their composition-controlled properties.
Several important challenges remain regarding controllable and scalable
fabrication of these alloys, such as achieving control over their atomic
ordering (i.e. clustering or random mixing of the transition metal atoms within
the 2D layers). In this work, atomic layer deposition (ALD) is used to
synthesize the TMD alloy Mo$_{1-x}$W$_{x}$S$_{2}$ with excellent composition
control along the complete composition range 0 $\leq$ x $\leq$ 1. Importantly,
this composition control allows us to control the atomic ordering of the alloy
from well-mixed to clustered while keeping the alloy composition fixed, as is
confirmed directly through atomic-resolution HAADF-STEM imaging. The control
over atomic ordering leads to tuning of the bandgap, as is demonstrated using
optical transmission spectroscopy. The relation between this tuning of the
electronic structure and the atomic ordering of the alloy was further confirmed
through ab-initio calculations. Furthermore, as the atomic ordering modulates
from clustered to well-mixed, the typical MoS$_{2}$ and WS$_{2}$ A$_{1g}$
vibrational modes converge. Our results demonstrate that atomic ordering is an
important parameter that can be tuned experimentally to finely tune the
fundamental properties of 2D TMD alloys for specific applications. | 2111.06289v1 |
2022-10-21 | Effective Mg Incorporation in CdMgO Alloy on Quartz Substrate Grown by Plasma-assisted MBE | The development of CdMgO ternary alloy with a single cubic phase is
challenging but meaningful work for technological advancement. In this work, we
have grown a series of Cd1-xMgxO ternary random alloys with various Mg
concentrations (x = 0, 30, 32, 45, and 55%) on quartz substrate by
plasma-assisted molecular beam epitaxy (PA-MBE) technique. The structural
investigations of alloys were performed using the X-ray diffraction (XRD)
technique. The decreases in average crystallite size and lattice parameters
were observed with an increase in Mg content in the alloys. XRD analysis
confirms a single cubic phase is obtained for alloy compositions. The elemental
and morphological studies were carried out using energy dispersive x-ray (EDX)
spectroscopy and atomic force microscope (AFM) technique, respectively. The
optical investigation was carried out using UV-Vis spectroscopy. The optical
bandgaps were estimated using the Tauc relation and it was varied from 2.34 eV
to 3.47 eV by varying the Mg content from zero to 55% in the alloys. The Urbach
energy increases from 112 meV to 350 meV which suggests a more disordered
localized state with an increase in Mg incorporation in the alloys. | 2210.11785v1 |
2023-01-04 | Plasticity in irradiated FeCrAl nanopillars investigated using discrete dislocation dynamics | In this paper, we investigate plasticity in irradiated FeCrAl nanopillars
using discrete dislocation dynamics simulations (DDD), with comparisons to
transmission electron microscopic (TEM) in situ tensile tests of ion and
neutron irradiated commercial FeCrAl alloy C35M. The effects of
irradiation-induced defects, such as a/2 111 and a 100 type loops and
composition fluctuations representative of phase separation in irradiated
FeCrAl alloys, are investigated separately as well as superposed together in
simulations. We explore the effects of defects on the stress-strain behavior,
specifically yield strength and hardening response, of FeCrAl nanopillars. Our
simulations confirm the widely accepted fact that irradiated alloys exhibit a
stress-strain response with higher yield strength and hardening as compared to
homogeneous alloys. However, our DDD calculations reveal an atypical
superposition of the hardening contributions due to composition inhomogeneity
and irradiation loops wherein hardening due composition inhomogeneity
counteracts hardening due to irradiation loops at small scales. As a result, we
observe that the yield strength in irradiated alloys, after taking into
consideration the effects of both composition inhomogeneity and irradiation
loops, is smaller than the yield strength of the alloys with only irradiation
loops and is approximately same for the alloy with composition inhomogeneity
alone. We identify this destructive interference in the superposition in our
parallel TEM in situ tensile tests on unirradiated, ion irradiated, and neutron
irradiated C35M FeCrAl alloy as well. This destructive interference in the
hardening contributions contrasts with the widely utilized dispersed barrier
hardening (DBH) models by the experimental community to model the hardening
contributions due to different irradiation induced defects. | 2301.01812v1 |
2023-05-27 | A novel and sustainable method to develop non-equiatomic CoCrFeNiMox high entropy alloys via spark plasma sintering using commercial commodity powders and evaluation of its mechanical behaviour | A novel approach to developing high entropy alloys (HEAs) using spark plasma
sintering (SPS) was explored in this work where a mix of commercial commodity
powders like Ni625, CoCrF75, and 316L was used instead of pre-alloyed powders
avoiding the expensive pre-alloying steps like mechanical alloying or gas
atomizing. Three non-equiatomic HEAs, based on Co, Cr, Fe, Ni, and Mo were
designed and developed by blending the powders which were sintered via SPS and
resulted in a single FCC phase after homogenization. The HEAs were
microstructurally and mechanically characterized with tensile and hot
compression tests up to a temperature of 750oC showing excellent properties.
The maximum room temperature tensile strength and ductility demonstrated was
712 MPa and 62% respectively, by the alloy Co23.28Cr28.57Fe25.03Ni21.01Mo2.1.
Moreover, the same alloy exhibited a compression strength greater than 640 MPa
with a ductility above 45% at a temperature of 750oC. Also, this study paves
the way for a novel fabrication route that offers more flexibility to develop
new HEAs cost-effectively and efficiently which is crucial for the discovery of
new materials over high-throughput techniques. Using such commodity alloys also
opens the possibility of developing ingot casting from recycled scraps avoiding
the direct use of critical metals. | 2305.17407v1 |
2023-06-14 | Machine learning-based prediction of elastic properties of amorphous metal alloys | The Young's modulus $E$ is the key mechanical property that determines the
resistance of solids to tension/compression. In the present work, the
correlation of the quantity $E$ with such characteristics as the total molar
mass $M$ of alloy components, the number of components $n$ forming an alloy,
the yield stress $\sigma_{y}$ and the glass transition temperature $T_{g}$ has
been studied in detail based on a large set of empirical data for the Young's
modulus of different amorphous metal alloys. It has been established that the
values of the Young's modulus of metal alloys under normal conditions correlate
with such a mechanical characteristic as the yield stress as well as with the
glass transition temperature. As found, the specificity of the ``chemical
formula'' of alloy, which is determined by molar mass $M$ and number of
components $n$, does not affect on elasticity of the material. The machine
learning algorithm identified both the quantities $M$ and $n$ as insignificant
factors in determining $E$. A simple non-linear regression model is obtained
that relates the Young's modulus with $T_{g}$ and $\sigma_{y}$, and this model
correctly reproduces the experimental data for metal alloys of different types.
This obtained regression model generalizes the previously presented empirical
relation $E\simeq49.8\sigma_{y}$ for amorphous metal alloys. | 2306.08387v1 |
2023-09-07 | Enhanced strength-ductility combination by introducing bimodal grains structures in high-density oxide dispersion strengthened FeCrAl alloys fabricated by spark plasma sintering technology | Oxide dispersion strengthened FeCrAl alloys dispersed high-density
nano-oxides in the matrix show outstanding corrosion resistance and mechanical
properties. However, ODS FeCrAl alloys achieve the high strength generally at
the expense of ductility in some way. Here, a method by introducing a bimodal
grain structure was designed to overcome the strength-ductility tradeoff. In
this work, ODS FeCrAl alloys were successfully fabricated through various
mechanical alloying time, combined with spark plasma sintering under the vacuum
of less than 4Pa. Microstructural characterization showed that the average
grains size and nano-oxides size decrease gradually, and the density of
nano-oxides increases, as the milling time increases. Mechanical properties
revealed that both the strength and ductility were significantly synergistic
enhanced with increasing milling time. The bimodal grain distribution
characterized by electron backscatter diffraction (EBSD) (vacuum degree was
less than 5E-5pa) was beneficial for the activation of the back stress
strengthening and the annihilation of these microcracks, thus achieving the
excellent ductility (27.65%). In addition, transmission electron microscope
(TEM) characterization under the vacuum degree of less than 10-6pa illustrated
that ultra-high-density nano-oxides (9.61E22/m3) was crucial for enhancing the
strength of ODS FeCrAl alloys (993MPa). The strengthening mechanism
superposition, based on the model of nano-oxides interrelated with the
dislocation, illustrated an excellent agreement with experimental results from
yield strength strengthening mechanisms. To our best knowledge, H40 (milled for
40h, and sintered at 1100C) alloy presents the outstanding strength with the
exceptional ductility among all studied ODS FeCrAl alloys, which makes it the
promising cladding materials for the accident tolerant fuel cladding. | 2309.03703v1 |
2024-01-23 | Thermal emissivity spectra and structural phase transitions of the eutectic Mg-51%Zn alloy: A candidate for thermal energy storage | The thermal emissivity spectrum in the mid infrared range (3 to 21 $\mu$m) as
well as its dependence on temperature between 225 and 320 $^{\circ}$C has been
obtained for the Mg-51%Zn (weight %) eutectic alloy, a candidate for thermal
storage. The spectral curves show the typical behaviour of metals and alloys,
with emissivity values between 0.05 and 0.2. It was also found that the
emissivity spectrum shows variations in each heating cycle during the first few
cycles. These changes are associated with the presence of metastable phases in
the solid solid phase transition, present in the alloy below the melting point.
The absence of signs of oxidation in air is very favourable for the use of this
alloy in thermal energy storage systems. Moreover, the total normal emissivity
curves obtained from dynamic spectral measurements have allowed analysing the
behaviour phase transition sequence present in this alloy. These experimental
results indicate that accurate emissivity measurements can be sensitive enough
to account for the structural phase transitions in metals and alloys. | 2401.13102v2 |
2024-02-16 | Electronic structure-property relationship in an Al0.5TiZrPdCuNi high-entropy alloy | The valence band (VB) structure of an Al0.5TiZrPdCuNi high-entropy alloy
(HEA) obtained from X-ray photoelectron spectroscopy has been compared to that
recently calculated by Odbadrakh et al, 2019. Both experimental and theoretical
VBs show split-band structures typical of alloys composed from the early (TE)
and late (TL) transition metals. Accordingly, several properties of this alloy
(both in the glassy and crystalline state) associated with the electronic
structure (ES), are compared with those of similar TE-TL alloys. The comparison
shows in addition to the usual dependence on the total TL content strong effect
of alloying with Al on the density of states at the Fermi level, N(EF) and on
the magnetic susceptibility of Al0.5TiZrPdCuNi HEA, which is like that of
conventional glassy alloys, such as Zr-Cu-Al ones. Despite some similarity
between the shapes of theoretical and corresponding experimental VBs there are
significant quantitative differences between them which should be taken into
account in any future studies of ES in HEAs and other compositionally complex
alloys (CCA). | 2402.10490v1 |
2024-03-25 | Facile synthesis of CoSi alloy with rich vacancy for base- and solvent-free aerobic oxidation of aromatic alcohols | Rational design and green synthesis of low-cost and robust catalysts
efficient for the selective oxidation of various alcohols are full of
challenges. Herein, we report a fast and solvent-free arc-melting (AM) method
to controllably synthesize semimetal CoSi alloy (abbreviated as AM-CoSi) that
is efficient for the base- and solvent-free oxidation of six types of aromatic
alcohols. X-ray absorption fine structure (XAFS), electron paramagnetic
resonance (EPR), and aberration corrected high angle annular dark field
scanning transmission electron microscope (AC HAADF-STEM) confirmed the
successful synthesis of AM-CoSi with rich Si vacancy (Siv). The as-prepared
CoSi alloy catalysts exhibit an order of magnitude activity enhancement in the
oxidation of model reactant benzyl alcohol (BAL) to benzyl benzoate (BBE)
compared with its mono counterparts, whereas 70 % yield of BBE which is the
highest yield to date. Experimental results and DFT calculations well verify
that the CoSi alloy structure improves the BAL conversion and Si vacancy mainly
contributes to the generation of BBE. After that, CoSi alloy maintains high
stability and a potential pathway is rationally proposed. Besides, CoSi alloy
also efficiently works for the selective oxidation of various alcohols with
different groups. This work demonstrates for the first time that semimetal CoSi
alloy is robust for the green oxidation of various alcohols and provides a vast
opportunity for reasonable design and application of other semimetal alloy
catalysts. | 2403.16708v1 |
2024-03-28 | AlloyBERT: Alloy Property Prediction with Large Language Models | The pursuit of novel alloys tailored to specific requirements poses
significant challenges for researchers in the field. This underscores the
importance of developing predictive techniques for essential physical
properties of alloys based on their chemical composition and processing
parameters. This study introduces AlloyBERT, a transformer encoder-based model
designed to predict properties such as elastic modulus and yield strength of
alloys using textual inputs. Leveraging the pre-trained RoBERTa encoder model
as its foundation, AlloyBERT employs self-attention mechanisms to establish
meaningful relationships between words, enabling it to interpret human-readable
input and predict target alloy properties. By combining a tokenizer trained on
our textual data and a RoBERTa encoder pre-trained and fine-tuned for this
specific task, we achieved a mean squared error (MSE) of 0.00015 on the Multi
Principal Elemental Alloys (MPEA) data set and 0.00611 on the Refractory Alloy
Yield Strength (RAYS) dataset. This surpasses the performance of shallow
models, which achieved a best-case MSE of 0.00025 and 0.0076 on the MPEA and
RAYS datasets respectively. Our results highlight the potential of language
models in material science and establish a foundational framework for
text-based prediction of alloy properties that does not rely on complex
underlying representations, calculations, or simulations. | 2403.19783v1 |
2011-05-02 | Design scheme of new multifunctional Heusler compounds for spin-transfer torque applications | This paper has been withdrawn. | 1105.0337v2 |
2018-01-10 | Prediction of a magnetic Weyl semimetal without spin-orbit coupling and strong anomalous Hall effect in the Heusler compensated ferrimagnet Ti2MnAl | We predict a magnetic Weyl semimetal in the inverse Heusler Ti2MnAl, a
compensated ferrimagnet with a vanishing net magnetic moment and a Curie
temperature of over 650 K. Despite the vanishing net magnetic moment, we
calculate a large intrinsic anomalous Hall effect (AHE) of about 300 S/cm. It
derives from the Berry curvature distribution of the Weyl points, which are
only 14 meV away from the Fermi level and isolated from trivial bands.
Different from antiferromagnets Mn3X (X= Ge, Sn, Ga, Ir, Rh, and Pt), where the
AHE originates from the non-collinear magnetic structure, the AHE in Ti2MnAl
stems directly from the Weyl points and is topologically protected. The large
anomalous Hall conductivity (AHC) together with a low charge carrier
concentration should give rise to a large anomalous Hall angle. In contrast to
the Co-based ferromagnetic Heusler compounds, the Weyl nodes in Ti2MnAl do not
derive from nodal lines due to the lack of mirror symmetries in the inverse
Heusler structure. Since the magnetic structure breaks spin-rotation symmetry,
the Weyl nodes are stable without SOC. Moreover, because of the large
separation between Weyl points of opposite topological charge, the Fermi arcs
extent up to 75% of the reciprocal lattice vectors in length. This makes
Ti2MnAl an excellent candidate for the comprehensive study of magnetic Weyl
semimetals. It is the first example of a material with Weyl points, large
anomalous Hall effect and angle despite a vanishing net magnetic moment. | 1801.03273v3 |
2022-02-14 | $Ab$ $Initio$ Study of Magnetic Tunnel Junctions Based on Half-Metallic and Spin-Gapless Semiconducting Heusler Compounds: Reconfigurable Diode and Inverse Tunnel-Magnetoresistance Effect | Magnetic tunnel junctions (MTJs) have attracted strong research interest
within the last decades due to their potential use as nonvolatile memory such
as MRAM as well as for magnetic logic applications. Half-metallic magnets
(HMMs) have been suggested as ideal electrode materials for MTJs to achieve an
extremely large tunnel-magnetoresistance (TMR) effect. Despite their high TMR
ratios, MTJs based on HMMs do not exhibit current rectification, i.e., a diode
effect, which was achieved in a magnetic tunnel junction concept based on HMMs
and type-II spin-gapless semiconductors (SGSs). The proposed concept has
recently been experimentally demonstrated using Heusler compounds. In the
present work, we investigate from first-principles MTJs based on type-II SGS
and HMM quaternary Heusler compounds FeVTaAl, FeVTiSi, MnVTiAl, and CoVTiSb.
Our $ab$ $initio$ quantum transport calculations based on a nonequilibrium
Green's function method have demonstrated that the MTJs under consideration
exhibit current rectification with relatively high on:off ratios. We show that,
in contrast to conventional semiconductor diodes, the rectification bias
voltage window (or breakdown voltage) of the MTJs is limited by the spin gap of
the HMM and SGS Heusler compounds. A unique feature of the present MTJs is that
the diode effect can be configured dynamically, i.e., depending on the relative
orientation of the magnetization of the electrodes, the MTJ allows the
electrical current to pass either in one or the other direction, which leads to
an inverse TMR effect. The combination of nonvolatility, reconfigurable diode
functionality, tunable rectification voltage window, and high Curie temperature
of the electrode materials makes the proposed MTJs very promising for
room-temperature spintronic applications and opens ways to magnetic memory and
logic concepts as well as logic-in-memory computing. | 2202.06752v2 |
2024-02-08 | Inflation and Isotropization in Quintom Cosmology | This paper studies inflation and isotropization in the quintom model in the
Bianchi I, Bianchi III, and Kantowski-Sachs backgrounds. First, we investigate
inherent properties and generalize Heusler's proposition. Then by the use of
the dynamical system approach, we consider the system in multiplicative and
collective modes of potentials. The conclusions of Collins and Hawking and also
Burd and Barrow are discussed. | 2402.05454v1 |
2001-03-02 | Non-linear macroscopic polarization in III-V nitride alloys | We study the dependence of macroscopic polarization on composition and strain
in wurtzite III-V nitride ternary alloys using ab initio density-functional
techniques. The spontaneous polarization is characterized by a large bowing,
strongly dependent on the alloy microscopic structure. The bowing is due to the
different response of the bulk binaries to hydrostatic pressure, and to
internal strain effects (bond alternation). Disorder effects are instead minor.
Deviations from parabolicity (simple bowing) are of order 10 % in the most
extreme case of AlInN alloy, much less at all other compositions. Piezoelectric
polarization is also strongly non-linear. At variance with the spontaneous
component, this behavior is independent of microscopic alloy structure or
disorder effects, and due entirely to the non-linear strain dependence of the
bulk piezoelectric response. It is thus possible to predict the piezoelectric
polarization for any alloy composition using the piezoelectricity of the parent
binaries. | 0103050v1 |
2001-08-27 | Kinetics of formation of twinned structures under L1_0 type orderings in alloys | The earlier-developed master equation approach and kinetic cluster methods
are applied to study kinetics of L1_0 type orderings in alloys, including the
formation of twinned structures characteristic of cubic-tetragonal-type phase
transitions. A microscopical model of interatomic deformational interactions is
suggested which generalizes a similar model of Khachaturyan for dilute alloys
to the physically interesting case of concentrated alloys. The model is used to
simulate A1->L1_0 transformations after a quench of an alloy from the
disordered A1 phase to the single-phase L1_0 state for a number of alloy models
with different chemical interactions, temperatures, concentrations, and
tetragonal distortions. We find a number of peculiar features in both transient
microstructures and transformation kinetics, many of them agreeng well with
experimental data. The simulations also demonstrate a phenomenon of an
interaction-dependent alignment of antiphase boundaries in nearly-equilibrium
twinned bands which seems to be observed in some experiments. | 0108422v1 |
2002-02-21 | Screened Coulomb interactions in metallic alloys: I. Universal screening in the atomic sphere approximation | We have used the locally self-consistent Green's function (LSGF) method in
supercell calculations to establish the distribution of the net charges
assigned to the atomic spheres of the alloy components in metallic alloys with
different compositions and degrees of order. This allows us to determine the
Madelung potential energy of a random alloy in the single-site mean field
approximation which makes the conventional single-site density-functional-
theory coherent potential approximation (SS-DFT-CPA) method practically
identical to the supercell LSGF method with a single-site local interaction
zone that yields an exact solution of the DFT problem. We demonstrate that the
basic mechanism which governs the charge distribution is the screening of the
net charges of the alloy components that makes the direct Coulomb interactions
short-ranged. In the atomic sphere approximation, this screening appears to be
almost independent of the alloy composition, lattice spacing, and crystal
structure. A formalism which allows a consistent treatment of the screened
Coulomb interactions within the single-site mean-filed approximation is
outlined. We also derive the contribution of the screened Coulomb interactions
to the S2 formalism and the generalized perturbation method. | 0202370v1 |
2003-04-29 | EXAFS, XRD and RMC studies of an Amorphous Ga$_{50}$Se$_{50}$ Alloy Produced by Mechanical Alloying | The local atomic order of an amorphous Ga$_{50}$Se$_{50}$ alloy produced by
Mechanical Alloying (MA) was studied by the Extended X-ray Absorption Fine
Structure (EXAFS) and X-ray Diffraction (XRD) techniques and by Reverse Monte
Carlo (RMC) simulations of its total x-ray structure factor. The coordination
numbers and interatomic distances for the first neighbors were determined by
means of EXAFS analysis and RMC simulations. The RMC simulations also furnished
the partial pair distribution functions $G^{\text{RMC}}_{\text{Ga-Ga}}(r)$,
$G^{\text{RMC}}_{\text{Ga-Se}}(r)$ and $G^{\text{RMC}}_{\text{Se-Se}}(r)$. The
results obtained indicated that there are important differences among the local
structure of the amorphous Ga$_{50}$Se$_{50}$ alloy produced by MA and those of
the corresponding crystals, since there are Se-Se pairs in the first
coordination shell of the amorphous alloy that are forbidden in the
Ga$_{50}$Se$_{50}$ crystals. | 0304666v1 |
2004-01-31 | Reverse Monte Carlo Simulations and Raman Scattering of an Amorphous GeSe$_4$ Alloy Produced by Mechanical Alloying | The short and intermediate range order of an amorphous GeSe$_4$ alloy
produced by Mechanical Alloying were studied by Reverse Monte Carlo simulations
of its x-ray total structure factor and Raman scattering. The simulations were
used to compute the $G^{\text{RMC}}_{\text{Ge-Ge}}(r)$,
$G^{\text{RMC}}_{\text{Ge-Se}}(r)$ and $G^{\text{RMC}}_{\text{Se-Se}}(r)$
partial distribution functions and the ${\cal
S}^{\text{RMC}}_{\text{Ge-Ge}}(K)$, ${\cal S}^{\text{RMC}}_{\text{Ge-Se}}(K)$
and ${\cal S}^{\text{RMC}}_{\text{Se-Se}}(K)$ partial structure factors. We
calculated the coordination numbers and interatomic distances for the first and
second neighbors and the bond-angle distribution functions
$\Theta_{ijl}(\cos\theta)$. The data obtained indicate that the structure of
the alloy has important differences when compared to alloys prepared by other
techniques. There are a high number of Se-Se pairs in the first shell, and some
of the tetrahedral units formed seemed to be connected by Se-Se bridges. | 0402015v1 |
2006-04-04 | Comparative study of the Portevin-Le Chatelier effect in interstitial and substitutional alloy | Tensile tests were carried out by deforming polycrystalline samples of an
interstitial alloy, low carbon steel at room temperature in a wide range of
strain rates where the Portevin-Le Chatelier (PLC) effect was observed. The
observed stress time series data were analyzed using the nonlinear dynamical
methods. From the analyses, we could establish the presence of marginal
deterministic chaos in the PLC effect of the low carbon steel. Moreover, we
made a comparative study of the PLC effect of this interstitial alloy with the
substitutional Al-Mg alloy which shows that the dynamics of the PLC effect in
the interstitial alloy is more complex compared to that of the substitutional
alloy. | 0604080v2 |
2006-05-30 | Complex Precipitation Pathways in Multi-Component Alloys | One usual way to strengthen a metal is to add alloying elements and to
control the size and the density of the precipitates obtained. However,
precipitation in multicomponent alloys can take complex pathways depending on
the relative diffusivity of solute atoms and on the relative driving forces
involved. In Al-Zr-Sc alloys, atomic simulations based on first-principle
calculations combined with various complementary experimental approaches
working at different scales reveal a strongly inhomogeneous structure of the
precipitates: owing to the much faster diffusivity of Sc compared with Zr in
the solid solution, and to the absence of Zr and Sc diffusion inside the
precipitates, the precipitate core is mostly Sc-rich, whereas the external
shell is Zr-rich. This explains previous observations of an enhanced nucleation
rate in Al-Zr-Sc alloys compared with binary Al-Sc alloys, along with much
higher resistance to Ostwald ripening, two features of the utmost importance in
the field of light high-strength materials. | 0605738v1 |
2008-11-22 | A simple theory of the Invar effect in iron-nickel alloys | Certain alloys of iron and nickel (so-called 'Invar' alloys) exhibit almost
no thermal expansion over a wide range of temperature. It is clear that this is
the result of an anomalous contraction upon heating which counteracts the
normal thermal expansion arising from the anharmonicity of lattice vibrations.
This anomalous contraction seems to be related to the alloys' magnetic
properties, since the effect vanishes at a temperature close to the Curie
temperature. However, despite many years of intensive research, a widely
accepted microscopic theory of the Invar effect in face-centered-cubic Fe-Ni
alloys is still lacking. Here we present a simple theory of the Invar effect in
these alloys based on Ising magnetism, ab initio total energy calculations, and
the Debye-Gruneisen model. We show that this theory accurately reproduces
several well known properties of these materials, including Guillaume's famous
plot1 of the thermal expansion coefficient as a function of the concentration
of nickel. Within the same framework, we are able to account in a
straightforward way for experimentally observed deviations from Vegard's law.
Our approach supports the idea that the lattice constant is governed by a few
parameters, including the fraction of iron-iron nearest-neighbour pairs. | 0811.3673v1 |
2010-11-08 | Tuning the spin texture in binary and ternary surface alloys on Ag(111) | Recently, a giant spin splitting has been observed in surface alloys on noble
metal (111) surfaces as a result of a strong structural modification at the
surface as well as the large atomic spin-orbit interaction (SOI) of the alloy
atoms. These surface alloys are an ideal playground to manipulate both the size
of the spin splitting as well as the position of the Fermi level as it is
possible to change the atomic SOI as well as the relaxation by varying alloy
atoms and substrates. Using spin- and angle-resolved photoemission spectroscopy
in combination with quantitative low energy electron diffraction we have
studied the mixed binary Bi(x)Sb(1-x)/Ag(111) and the mixed ternary
Bi(x)Pb(y)Sb(1-x-y)/Ag(111) surface alloys where we observed a continuous
evolution of the band structure with x and y. | 1011.1829v1 |
2011-09-14 | The Effect of dopants on magnetic properties of the ordered Fe_{65-x}Al_{35-y}M_{x,y} (M=Ga,B,V; x,y=5,10) alloys | The results of X-ray diffraction, complex in-field (up to 9 T) and
temperature (5-300 K) Moessbauer and magnetometric studies of the ordered
Fe_{65}Al_{35-x}M_x (M=Ga, B; x=0,5,10) and Fe_{65-x}V_xAl_{35} (x=5,10) alloys
are presented. Analysis of the magnetometry studies shows that the systems
Fe_{65}Al_{35} and Fe_{65}Al_{35-x}Ga_x (x=5, 10) are characterized by two
different magnetic states with essentially distinguishing hysteresis loops and
AC susceptibility values. The temperature and external magnetic field values
inducing the transition from one magnetic state to another are higher in the
Ga-doped alloys than in the reference Fe_{65}Al_{35} alloy. The boron addition
transforms the magnetic state of the initial alloy Fe_{65}Al_{35} into a
ferromagnetic one exhibiting high magnetic characteristics. Substitution of V
for Fe in the ternary alloys Fe_{65-x}V_xAl_{35} results in reduction of
magnetic characteristics and collapsing of 57Fe hyperfine magnetic filed. | 1109.3064v1 |
2011-11-21 | Ab initio theory of galvanomagnetic phenomena in ferromagnetic metals and disordered alloys | We present an ab initio theory of transport quantities of metallic
ferromagnets developed in the framework of the fully relativistic tight-binding
linear muffin-tin orbital method. The approach is based on the Kubo-Streda
formula for the conductivity tensor, on the coherent potential approximation
for random alloys, and on the concept of interatomic electron transport. The
developed formalism is applied to pure 3d transition metals (Fe, Co, Ni) and to
random Ni-based ferromagnetic alloys (Ni-Fe, Ni-Co, Ni-Mn). High values of the
anisotropic magnetoresistance (AMR), found for Ni-rich alloys, are explained by
a negligible disorder in the majority spin channel while a change of the sign
of the anomalous Hall effect (AHE) on alloying is interpreted as a band-filling
effect without a direct relation to the high AMR. The influence of disorder on
the AHE in concentrated alloys is investigated as well. | 1111.4793v2 |
2012-04-20 | First-principles based modeling of hydrogen permeation through Pd-Cu alloys | The solubility and diffusivity of hydrogen in disordered Pd1-xCux alloys are
investigated using a combination of first-principles calculations, a
composition-dependent local cluster expansion (CDLCE) technique, and kinetic
Monte Carlo simulations. We demonstrate that a linear CDCLE model can already
accurately describe interstitial H in Pd1-xCux alloys over the entire
composition range (0\leqx\leq1) with accuracy comparable to that of direct
first-principles calculations. Our predicted H solubility and permeability
results are in reasonable agreement with experimental measurements. The
proposed model is quite general and can be employed to rapidly and accurately
screen a large number of alloy compositions for potential membrane
applications. Extension to ternary or higher-order alloy systems should be
straightforward. Our study also highlights the significant effect of local
lattice relaxations on H energetics in size-mismatched disordered alloys, which
has been largely overlooked in the literature. | 1204.4512v1 |
2012-10-03 | Magnetic anisotropy energy of disordered tetragonal Fe-Co systems from ab initio alloy theory | We present results of systematic fully relativistic first-principles
calculations of the uniaxial magnetic anisotropy energy (MAE) of a disordered
and partially ordered tetragonal Fe-Co alloy using the coherent potential
approximation (CPA). This alloy has recently become a promising system for thin
ferromagnetic films with a perpendicular magnetic anisotropy. We find that
existing theoretical approaches to homogeneous random bulk Fe-Co alloys, based
on a simple virtual crystal approximation (VCA), overestimate the maximum MAE
values obtained in the CPA by a factor of four. This pronounced difference is
ascribed to the strong disorder in the minority spin channel of real alloys,
which is neglected in the VCA and which leads to a broadening of the d-like
eigenstates at the Fermi energy and to the reduction of the MAE. The ordered
Fe-Co alloys with a maximum L1_0-like atomic long-range order can exhibit high
values of the MAE, which, however, get dramatically reduced by small
perturbations of the perfect order. | 1210.1028v2 |
2013-01-10 | First-principles calculation of the Gilbert damping parameter via the linear response formalism with application to magnetic transition-metals and alloys | A method for the calculations of the Gilbert damping parameter $\alpha$ is
presented, which based on the linear response formalism, has been implemented
within the fully relativistic Korringa-Kohn-Rostoker band structure method in
combination with the coherent potential approximation alloy theory. To account
for thermal displacements of atoms as a scattering mechanism, an alloy-analogy
model is introduced. This allows the determination of $\alpha$ for various
types of materials, such as elemental magnetic systems and ordered magnetic
compounds at finite temperature, as well as for disordered magnetic alloys at
$T = 0$ K and above. The effects of spin-orbit coupling, chemical and
temperature induced structural disorder are analyzed. Calculations have been
performed for the 3$d$ transition-metals bcc Fe, hcp Co, and fcc Ni, their
binary alloys bcc Fe$_{1-x}$Co$_{x}$, fcc Ni$_{1-x}$Fe$_x$, fcc
Ni$_{1-x}$Co$_x$ and bcc Fe$_{1-x}$V$_{x}$, and for 5d impurities in
transition-metal alloys. All results are in satisfying agreement with
experiment. | 1301.2114v1 |
2013-11-15 | Effects of Irradiation Temperature and Dose Rate on the Mechanical Properties of Self-Ion Implanted Fe and Fe-Cr Alloys | Pure Fe and model Fe-Cr alloys containing 5, 10 and 14%Cr were irradiated
with Fe+ ions at a maximum energy of 2MeV to the same dose of 0.6dpa at
temperatures of 300 C, 400 C and 500 C, and at dose rates corresponding to 6 x
10-4 dpa/s and 3 x 10-5 dpa/s. All materials exhibited an increase in hardness
after irradiation at 300 C. After irradiation at 400 C, hardening was observed
only in Fe-Cr alloys, and not in the pure Fe. After irradiation at 500 C, no
hardening was observed in any of the materials tested. For irradiations at both
300 C and 400 C, greater hardening was found in the Fe-Cr alloys irradiated at
the lower dose rate. Transmission electron microscopy and atom probe tomography
of Fe 5%Cr identified larger dislocation loop densities and sizes in the alloy
irradiated with the high dose rate and Cr precipitation in the alloy irradiated
with the low dose rate. | 1311.3786v1 |
2014-05-09 | Thermodynamics, kinetics and fragility of bulk metallic glass forming liquids | This review deals with the kinetic and thermodynamic fragility of bulk
metallic glass forming liquids. The experimental methods to determine the
kinetic fragility, relaxation behavior and thermodynamic functions of
undercooled metallic liquids are introduced. Existing data are assessed and
discussed using the Vogel-Fulcher-Tammann equation and in the frameworks of the
Adam-Gibbs as well as the Cohen-Turnbull free volume approach. In contrast to
pure metals and most non glass forming alloys, bulk glass formers are
moderately strong liquids. In general the fragility parameter $D^{*} $
increases with the complexity of the alloy with differences between the alloy
families, e.g. noble-metal based alloys being more fragile than Zr-based
alloys. At least some bulk metallic glass forming liquids, such as Vitreloy 1,
undergo transitions from a fragile state at high temperatures to a strong state
at low temperatures with indications that in Zr-based alloys this behavior is a
common phenomenon. | 1405.2251v1 |
2014-11-01 | Ferromagnetic interactions and martensitic transformation in Fe doped Ni-Mn-In shape memory alloys | The structure, magnetic and martensitic properties of Fe doped Ni-Mn-In
magnetic shape memory alloys have been studied by differential scanning
calorimetry, magnetization, resistivity, X-ray diffraction (XRD) and EXAFS.
While Ni$_{2}$MnIn$_{1-x}$Fe$_{x}$ ($0 \le x \le 0.6$) alloys are ferromagnetic
and non martensitic, the martensitic transformation temperature in
Ni$_{2}$Mn$_{1.5} $In$_{1-y}$Fe$_{y}$ and Ni$_{2}$Mn$ _{1.6}
$In$_{1-y}$Fe$_{y}$ increases for lower Fe concentrations ($y \le 0.05$) before
decreasing sharply for higher Fe concentrations. XRD analysis reveals presence
of cubic and tetragonal structural phases in Ni$_{2}$MnIn$_{1-x}$Fe$_{x}$ at
room temperature with tetragonal phase content increasing with Fe doping. Even
though the local structure around Mn and Ni in these Fe doped alloys is similar
to martensitic Mn rich Ni-Mn-In alloys, presence of ferromagnetic interactions
and structural disorder induced by Fe affect Mn-Ni-Mn antiferromagnetic
interactions resulting in suppression of martensitic transformation in these Fe
doped alloys. | 1411.0058v1 |
2015-05-04 | Analyzing Alloy Formulas using an SMT Solver: A Case Study | This paper describes how Yices, a modern SAT Modulo theories solver, can be
used to analyze the address-book problem expressed in Alloy, a first-order
relational logic with transitive closure. Current analysis of Alloy models - as
performed by the Alloy Analyzer - is based on SAT solving and thus, is done
only with respect to finitized types. Our analysis generalizes this approach by
taking advantage of the background theories available in Yices, and avoiding
type finitization when possible. Consequently, it is potentially capable of
proving that an assertion is a tautology - a capability completely missing from
the Alloy Analyzer. This paper also reports on our experimental results that
compare the performance of our analysis to that of the Alloy Analyzer for
various versions of the address book problem. | 1505.00672v1 |
2015-06-12 | Ab initio study of structural, electronic, and thermal properties of Ir$_{1-x}$Rh$_{x}$ alloys | The structural, electronic, mechanical and thermal properties of
Ir$_{1-x}$Rh$_{x}$ alloys were studied systematically using ab initio density
functional theory at different concentrations (x = 0.00, 0.25, 0.50, 0.75,
1.00). A Special Quasirandom Structure method was used to make alloys having
FCC structure with four atoms per unit cell. The ground state properties such
as lattice constant and bulk modulus were calculated to find the equilibrium
atomic position for stable alloys. The calculated ground state properties are
in good agreement with the experimental and previously presented other
theoretical data. The electronic band structure and density of states were
calculated to study the electronic properties for these alloys at different
concentrations. The electronic properties substantiate the metallic behavior of
alloys. The first principle density functional perturbation theory as
implemented in quasiharmonic approximation was used for the calculation of
thermal properties. We have calculated the thermal properties such as Debye
temperatures, vibration energy, entropy, constant-volume specific heat and
internal energy. The ab initio linear-response method was used to calculate
phonon densities of states. | 1506.03966v1 |
2015-07-23 | Deformation behavior of Mg-8.5wt.%Al alloy under reverse loading investigated by in-situ neutron diffraction and elastic viscoplastic self-consistent modeling | The cyclic deformation behavior of extruded Mg-8.5wt.%Al alloy with a
conventional extrusion texture and a modified texture is systematically
investigated by in-situ neutron diffraction and elastic viscoplastic
self-consistent (EVPSC) modeling incorporating a twinning/de-twinning (TDT)
scheme. The role of twinning and de-twinning on the deformation behavior of
Mg-8.5wt.% Al alloy is investigated in terms of the macroscopic stress-strain
response, the evolution of the activities of various deformation mechanisms,
the texture evolution, the evolution of the internal elastic strains, and the
evolution of the diffraction peak intensities. The alloy with the conventional
extrusion texture undergoes twinning during initial compression and de-twinning
during reverse tension. The same alloy does not favor twinning during initial
tension, but rather during reverse compression. The alloy with a modified
texture undergoes twinning during initial tension followed by detwinning during
reverse compression. The results provide insights into the effect of initial
texture, loading path, slip, twinning, de-twinning on the cyclic behavior of
magnesium. | 1507.06384v1 |
2016-01-11 | Water-Induced Bimetallic Alloy Surface Segregation: A First Principle Study | Bimetallic alloys have drawn extensive attentions in materials science due to
their widespread applications in electronics, engineering and catalysis. A very
fundamental question of alloy is its surface segregation phenomenon. Many
recent observations have shown that reactive gases or supports may have strong
effects on alloy segregation. However, segregation in water, the most common
solvent and environment, has not received enough attention. In this paper we
give the quantitative descriptions on the surface segregation energies of 23
transition-metal impurities in Cu hosts under the conditions of water
adsorption by performing density functional theory (DFT) calculations. The
general trends in the changes of segregation energies caused by water
adsorption are established. Our results show water adsorption could induce
strong surface segregation tendencies for early and middle transition metals in
Cu alloys. This finding not only prompts us to re-examine the potential effects
of water on bimetallic alloy surfaces, but would be also very helpful as a
guide for the further theoretical and experimental studies in this field. | 1601.02346v1 |
2016-01-27 | On the origin of bulk glass forming ability in Cu-Hf, Zr alloys | Understanding the formation of bulk metallic glasses (BMG) in metallic
systems and finding a reliable criterion for selection of BMG compositions are
among the most important issues in condensed matter physics and material
science. Using the results of magnetic susceptibility measurements performed on
both amorphous and crystallized Cu-Hf alloys (30-70 at% Cu) we find a
correlation between the difference in magnetic susceptibilities of
corresponding glassy and crystalline alloys and the variation in the glass
forming ability (GFA) in these alloys. Since the same correlation can be
inferred from data for the properties associated with the electronic structure
of Cu-Zr alloys, it seems quite general and may apply to other glassy alloys
based on early and late transition metals. This correlation is plausible from
the free energy considerations and provides a simple way to select the
compositions with high GFA. | 1601.07397v2 |
2016-10-12 | Effect of Electron Count and Chemical Complexity in the Ta-Nb-Hf-Zr-Ti High-Entropy Alloy Superconductor | High-entropy alloys are made from random mixtures of principal elements on
simple lattices, stabilized by a high mixing entropy. The recently discovered
BCC Ta-Nb-Hf-Zr-Ti high entropy alloy superconductor appears to display
properties of both simple crystalline intermetallics and amorphous materials,
e.g. it has a well defined superconducting transition along with an exceptional
robustness against disorder. Here we show that the valence-electron count
dependence of the superconducting transition temperature in the high entropy
alloy falls between those of analogous simple solid solutions and amorphous
materials, and test the effect of alloy complexity on the superconductivity. We
propose high-entropy alloys as excellent intermediate systems for studying
superconductivity as it evolves between crystalline and amorphous materials. | 1610.03746v1 |
2016-11-03 | High pressure synthesis of a hexagonal close-packed phase of the high-entropy alloy CrMnFeCoNi | High-entropy alloys, near-equiatomic solid solutions of five or more
elements, represent a new strategy for the design of materials with properties
superior to those of conventional alloys. However, their phase space remains
constrained, with transition metal high-entropy alloys exhibiting only face- or
body-centered cubic structures. Here, we report the high-pressure synthesis of
a hexagonal close-packed phase of the prototypical high-entropy alloy
CrMnFeCoNi. This martensitic transformation begins at 14 GPa and is attributed
to suppression of the local magnetic moments, destabilizing the initial fcc
structure. Similar to fcc-to-hcp transformations in Al and the noble gases, the
transformation is sluggish, occurring over a range of >40 GPa. However, the
behaviour of CrMnFeCoNi is unique in that the hcp phase is retained following
decompression to ambient pressure, yielding metastable fcc-hcp mixtures. This
demonstrates a means of tuning the structures and properties of high-entropy
alloys in a manner not achievable by conventional processing techniques. | 1611.00876v2 |
2017-01-09 | Magnetic properties of ultra-thin 3d transition-metal binary alloys I: spin and orbital moments, anisotropy, and confirmation of Slater-Pauling behavior | The structure and static magnetic properties - saturation magnetization,
perpendicular anisotropy, spectroscopic g-factor, and orbital magnetization -
of thin-film 3d transition metal alloys are determined over the full range of
alloy compositions via X-ray diffraction, magnetometry, and ferromagnetic
resonance measurements. We determine the interfacial perpendicular magnetic
anisotropy by use of samples sets with varying thickness for specific alloy
concentrations. The results agree with prior published data and theoretical
predictions. They provide a comprehensive compilation of the magnetic
properties of thin-film Ni-Co, Ni-Fe and Co-Fe alloys that goes well beyond the
often-cited Slater-Pauling dependence of magnetic moment on alloy
concentration. | 1701.02177v1 |
2017-07-29 | Experimental study of extrinsic spin Hall effect in CuPt alloy | We have experimentally studied the effects on the spin Hall angle due to
systematic addition of Pt into the light metal Cu. We perform spin torque
ferromagnetic resonance measurements on Py/CuPt bilayer and find that as the Pt
concentration increases, the spin Hall angle of CuPt alloy increases. Moreover,
only 28% Pt in CuPt alloy can give rise to a spin Hall angle close to that of
Pt. We further extract the spin Hall resistivity of CuPt alloy for different Pt
concentrations and find that the contribution of skew scattering is larger for
lower Pt concentrations, while the side-jump contribution is larger for higher
Pt concentrations. From technological perspective, since the CuPt alloy can
sustain high processing temperatures and Cu is the most common metallization
element in the Si platform, it would be easier to integrate the CuPt alloy
based spintronic devices into existing Si fabrication technology. | 1707.09525v1 |
2018-03-27 | Evidence of multiband superconductivity in the $β$-phase Mo$_{1-x}$Re$_x$ alloys | We present a detailed study of the superconducting properties in the
beta-phase Mo$_{1-x}$Re$_x$ (x = 0.25 and 0.4) solid solution alloys pursued
through magnetization and heat capacity measurements. The temperature
dependence of the upper critical field H$_{C2}$(T) in these binary alloys shows
a deviation from the prediction of the Werthamer-Helfand-Hohenberg (WHH)
theory. The temperature dependence of superfluid density estimated from the
variation of lower critical field H$_{C1}$ with temperature, cannot be
explained within the framework of a single superconducting energy gap. The heat
capacity also shows an anomalous feature in its temperature dependence. All
these results can be reasonably explained by considering the existence of two
superconducting energy gaps in these Mo$_{1-x}$Re$_x$ alloys. Initial results
of electronic structure calculations and resonant photoelectron spectroscopy
measurements support this possibility and suggest that the Re-5d like states at
the Fermi level may not intermix with the Mo-5p and 5s like states in the
beta-phase Mo$_{1-x}$Re$_x$ alloys and contribute quite distinctly to the
superconductivity of these alloys. | 1803.10315v1 |
2018-11-03 | Modern Data Analytics Approach to Predict Creep of High-Temperature Alloys | A breakthrough in alloy design often requires comprehensive understanding in
complex multi-component/multi-phase systems to generate novel material
hypotheses. We introduce a modern data analytics workflow that leverages
high-quality experimental data augmented with advanced features obtained from
high-fidelity models. Herein, we use an example of a consistently-measured
creep dataset of developmental high-temperature alloy combined with scientific
alloy features populated from a high-throughput computational thermodynamic
approach. Extensive correlation analyses provide ranking insights for most
impactful alloy features for creep resistance, evaluated from a large set of
candidate features suggested by domain experts. We also show that we can
accurately train machine learning models by integrating high-ranking features
obtained from correlation analyses. The demonstrated approach can be extended
beyond incorporating thermodynamic features, with input from domain experts
used to compile lists of features from other alloy physics, such as diffusion
kinetics and microstructure evolution. | 1811.01239v1 |
2020-01-19 | Linking electronic structure calculations to generalized stacking fault energies in multicomponent alloys | The generalized stacking fault energy is a key ingredient to mesoscale models
of dislocations. Here we develop an approach to quantify the dependence of
generalized stacking fault energies on the degree of chemical disorder in
multicomponent alloys. We introduce the notion of a "configurationally-resolved
planar fault" (CRPF) energy and extend the cluster expansion method from alloy
theory to express the CRPF as a function of chemical occupation variables of
sites surrounding the fault. We apply the approach to explore the composition
and temperature dependence of the unstable stacking fault energy (USF) in
binary Mo-Nb alloys. First-principles calculations are used to parameterize a
formation energy and CRPF cluster expansion. Monte Carlo simulations show that
the distribution of USF energies is significantly affected by chemical
composition and temperature. The formalism can be applied to any multicomponent
alloy and will enable the development of rigorous models for deformation
mechanisms in high-entropy alloys. | 2001.06912v1 |
2019-04-23 | Large-area synthesis of continuous two-dimensional MoTexSe2-x alloy films by chemical vapor deposition | Great achievements have been made in alloying of two-dimensional (2D)
semiconducting transition metal dichalcogenides (TMDs), which can allow tunable
band gaps for practical applications in optoelectronic devices. However,
telluride-based TMDs alloys were less studied due to the difficulties of sample
synthesis. Here, in this work we report the large-area synthesis of 2D
MoTexSe2-x alloy films with controllable Te composition by a modified alkali
metal halides assisted chemical vapor deposition method. The as-prepared films
have millimeter-scale transverse size. Raman spectra experiments combining
calculated Raman spectra and vibrational images obtained by density functional
theory (DFT) confirmed the 2H-phase of the MoTexSe2-x alloys. The A1g mode of
MoSe2 shows a significant downshift accompanied by asymmetric broadening to
lower wavenumber with increasing value of x, while E12g mode seems unchanged,
which were well explained by a phonon confinement model. Our work provides a
simple method to synthesize large-scale 2H phase Te-based 2D TMDs alloys for
their further applications. | 1904.10218v1 |
2019-10-28 | The spin Hall effect of Bi-Sb alloys driven by thermally excited Dirac-like electrons | We have studied the charge to spin conversion in Bi$_{1-x}$Sb$_x$/CoFeB
heterostructures. The spin Hall conductivity (SHC) of the sputter deposited
heterostructures exhibits a high plateau at Bi-rich compositions, corresponding
to the topological insulator phase, followed by a decrease of SHC for Sb-richer
alloys, in agreement with the calculated intrinsic spin Hall effect of
Bi$_{1-x}$Sb$_x$ alloy. The SHC increases with increasing thickness of the
Bi$_{1-x}$Sb$_x$ alloy before it saturates, indicating that it is the bulk of
the alloy that predominantly contributes to the generation of spin current; the
topological surface states, if present in the films, play little role.
Surprisingly, the SHC is found to increase with increasing temperature,
following the trend of carrier density. These results suggest that the large
SHC at room temperature, with a spin Hall efficiency exceeding 1 and an
extremely large spin current mobility, is due to increased number of
Dirac-like, thermally-excited electrons in the $L$ valley of the narrow gap
Bi$_{1-x}$Sb$_x$ alloy. | 1910.12433v1 |
2019-10-29 | Improvement on corrosion resistance and biocompability of ZK60 magnesium alloy by carboxyl ion implantation | Magnesium alloys have been considered to be potential biocompatible metallic
materials. Further improvement on the anti-corrosion is expected to make this
type of materials more suitable for biomedical applications in the fields of
orthopedics, cardiovascular surgery and others. In this paper, we introduce a
method of carboxyl ion (COOH+) implantation to reduce the degradation of ZK60
Mg alloy and improve its functionality in physiological environment. X-ray
photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) experiments
show the formation of a smooth layer containing carbaxylic group, carbonate,
metal oxides and hydroxides on the ion implanted alloy surface. Corrosion
experiments and in vitro cytotoxicity tests demonstrate that the ion
implantation treatment can both reduce the corrosion rate and improve the
biocompatibility of the alloy. The promising results indicate that organic
functional group ion implantation may be a practical method of improving the
biological and corrosion properties of magnesium alloys. | 1910.13219v1 |
2019-06-23 | Effect of elastic anisotropy on phase separation in ternary alloys: A phase-field study | The precipitate shape, size and distribution are crucial factors which
determine the properties of several technologically important alloys. Elastic
interactions between the inclusions modify their morphology and align them
along elastically favourable crystallographic directions. Among the several
factors contributing to the elastic interaction energy between precipitating
phases, anisotropy in elastic moduli is decisive in the emergence of modulated
structures during phase separation in elastically coherent alloy systems. We
employ a phase-field model incorporating elastic interaction energy between the
misfitting phases to study microstructural evolution in ternary three-phase
alloy systems when the elastic moduli are anisotropic. The spatiotemporal
evolution of the composition field variables is governed by solving a set of
coupled Cahn-Hilliard equations numerically using a semi-implicit Fourier
spectral technique. We methodically vary the misfit strains, alloy chemistry
and elastic anisotropy to investigate their influence on domain morphology
during phase separation. The coherency strains between the phases and alloy
composition alter the coherent phase equilibria and decomposition pathways. The
degree of anisotropy in elastic moduli modifies the elastic interaction energy
between the precipitates depending on the sign and magnitude of relative
misfits, and thus determines the shape and alignment of the inclusions in the
microstructure. | 1906.09637v1 |
2020-05-20 | Can experiment determine the stacking fault energy of metastable alloys? | Stacking fault energy (SFE) plays an important role in deformation mechanisms
and mechanical properties of face-centered cubic (fcc) metals and alloys. In
metastable fcc alloys, the SFEs determined from density functional theory (DFT)
calculations and experimental methods often have opposite signs. Here, we show
that the negative SFE by DFT reflects the thermodynamic instability of the fcc
phase relative to the hexagonal close-packed one; while the experimentally
determined SFEs are restricted to be positive by the models behind the indirect
measurements. We argue that the common models underlying the experimental
measurements of SFE fail in metastable alloys. In various concentrated solid
solutions, we demonstrate that the SFEs obtained by DFT calculations correlate
well with the primary deformation mechanisms observed experimentally, showing a
better resolution than the experimentally measured SFEs. Furthermore, we
believe that the negative SFE is important for understanding the abnormal
behaviors of partial dislocations in metastable alloys under deformation. The
present work advances the fundamental understanding of SFE and its relation to
plastic deformations, and sheds light on future alloy design by physical
metallurgy. | 2005.09983v1 |
2020-08-26 | Edge Dislocations Can Control Yield Strength in Refractory Body-Centered-Cubic High Entropy Alloys | Energy efficiency is motivating the search for new high-temperature metals.
Some new body-centered-cubic random multicomponent "high entropy alloys (HEAs)"
based on refractory elements (Cr-Mo-Nb-Ta-V-W-Hf-Ti-Zr) possess exceptional
strengths at high temperatures but the physical origins of this outstanding
behavior are not known. Here we show, using integrated neutron-diffraction
(ND), high-resolution transmission electron microscopy (HRTEM), and theory,
that the high strength and strength retention of a NbTaVTi alloy and a new
high-strength/low-density CrMoNbV alloy are attributable to edge dislocations.
This is surprising because plastic-flow in BCC elemental metals and dilute
alloys is universally accepted to be controlled by screw dislocations. We use
the insight and theory to perform a computationally-guided search over $10^7$
BCC HEAs and identify over $10^6$ possible ultra-strong high-temperature alloy
compositions for future exploration. | 2008.11671v1 |
2018-04-25 | A multidisciplinary approach to study precipitation kinetics and hardening in an Al-4Cu (wt. %) alloy | A multidisciplinary approach is presented to analyse the precipitation
process in a model Al-Cu alloy. Although this topic has been extensively
studied in the past, most of the investigations are focussed either on
transmission electron microscopy or on thermal analysis of the processes. The
information obtained from these techniques cannot, however, provide a coherent
picture of all the complex transformations that take place during decomposition
of supersaturated solid solution. Thermal analysis, high resolution
dilatometry, (high resolution) transmission electron microscopy and density
functional calculations are combined to study precipitation kinetics,
interfacial energies, and the effect of second phase precipitates on the
mechanical strength of the alloy. Data on both the coherent and semi-coherent
orientations of the {\theta}"/Al interface are reported for the first time. The
combination of the different characterization and modelling techniques provides
a detailed picture of the precipitation phenomena that take place during aging
and of the different contributions to the strength of the alloy. This strategy
can be used to analyse and design more complex alloys. | 1804.09634v1 |
2019-03-03 | Near-Unity Spin Hall Ratio in Ni$_x$Cu$_{1-x}$ Alloys | We report a large spin Hall effect in the 3$d$ transition metal alloy
Ni$_x$Cu$_{1-x}$ for $x\in\left\{ 0.3,0.75\right\} $, detected via the
ferromagnetic resonance of a Permalloy (Py = Ni$_{80}$Fe$_{20}$) film deposited
in a bilayer with the alloy. A thickness series at $x$ = 0.6, for which the
alloy is paramagnetic at room temperature, allows us to determine the spin Hall
ratio $\theta_{\rm{SH}}\approx1$, spin diffusion length $\lambda_{\rm{s}}$,
spin mixing conductance $G_{\uparrow\downarrow}$, and damping
$\alpha_{\rm{SML}}$ due to spin memory loss . We compare our results with
similar experiments on Py/Pt bilayers measured using the same method. Ab initio
band structure calculations with disorder and spin-orbit coupling suggest an
intrinsic spin Hall effect in Ni$_x$Cu$_{1-x}$ alloys, although the experiments
here cannot distinguish between extrinsic and intrinsic mechanisms. | 1903.00910v2 |
2019-03-19 | Anomalous Ground State in Fe$_{1-x}$Ni$_{x}$ Invar alloys | This paper reports high resolution X-ray photoelectron spectroscopy (XPS)
studies on Fe$_{1-x}$Ni$_x$ (x=0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.9) alloys down
to 10 K temperature. Core levels and Auger transitions of the alloys except the
invar alloy (x=0.4) exhibit no observable temperature induced changes. The
invar alloy exhibits changes in the core levels below 20 K temperature that
strongly depend on the core level. Such core level dependent changes with
temperature were attributed to the precipitation of spin glass like phase below
20 K only in the invar alloy. Ni L$_3$M$_{45}$M$_{45}$ Auger transition also
supported such precipitation below 20 K. | 1903.08221v3 |
2019-08-10 | Multielemental single-atom-thick A layers in nanolaminated V2(Sn, A)C (A=Fe, Co, Ni, Mn) for tailoring magnetic properties | Tailoring of individual single-atom-thick layers in nanolaminated materials
offers atomic-level control over material properties. Nonetheless, multielement
alloying in individual atomic layers in nanolaminates is largely unexplored.
Here, we report a series of inherently nanolaminated V2(A'xSn1-x)C (A'=Fe, Co,
Ni and Mn, and combinations thereof, with x=1/3) synthesized by an alloy-guided
reaction. The simultaneous occupancy of the four magnetic elements and Sn, the
individual single-atom-thick A layers in the compound constitute
high-entropy-alloy analogues, two-dimensional in the sense that the alloying
exclusively occurs in the A layers. V2(A'xSn1-x)C exhibit distinct
ferromagnetic behavior that can be compositionally tailored from the
multielement A-layer alloying. This two-dimensional alloying provides a
structural-design route with expanded chemical space for discovering materials
and exploit properties. | 1908.03709v1 |
2019-12-10 | Grain refinement and enhancement of critical current density in the V_0.60Ti_0.40 alloy superconductors with Gd addition | The V-Ti alloys are promising materials as alternate to the commercial
Nb-based superconductors for high current-high magnetic field applications.
However, the critical current density (J_c) of these alloys are somewhat low
due to their low grain-boundary density. We show here that grain refinement of
the V-Ti alloys and enhancement of the J_c can be achieved by the addition of
Gd into the system, which precipitates as clusters along the grain boundaries.
Both the J_c and the pinning force density (F_P) increase with the increasing
Gd content up to 1 at. % Gd, where they are more than 20 times higher than
those of the parent V_0.60Ti_0.40 alloy. Introduction of Gd into the system
also leads to ferromagnetic (FM) correlations, and the alloys containing more
than 0.5 at. % Gd exhibit spontaneous magnetization. In spite of the FM
correlations, the superconducting transition temperature increases slightly
with Gd-addition. | 1912.04507v2 |
2020-03-20 | Experimental and theoretical study of tracer diffusion in a series of (CoCrFeMn)$_{100-x}$Ni$_x$ alloys | Tracer diffusion of all constituting elements is studied at various
temperatures in a series of (CoCrFeMn)$_{100-x}$Ni$_x$ alloys with compositions
ranging from pure Ni to the equiatomic CoCrFeMnNi high-entropy alloy. At a
given homologous temperature, the measured tracer diffusion coefficients change
non-monotonically along the transition from pure Ni to the concentrated alloys
and finally to the equiatomic CoCrFeMnNi alloy. This is explained by atomistic
Monte-Carlo simulations based on a modified embedded-atom potentials, which
reveal that local heterogeneities of the atomic configurations around a vacancy
cause correlation effects and induce significant deviations from predictions of
the random alloy model. | 2003.09474v1 |
2020-06-13 | Structural properties of Fe-Ni/Cu/Fe-Ni trilayers on Si (100) | We investigate the structural properties of
Fe$_{1-x}$Ni$_x$/Cu/Fe$_{1-x}$Ni$_x$ ( $x=0.5$, non Invar and $x=0.36$, Invar)
trilayers deposited on Si~(100)~at room temperature using dc magnetron
sputtering technique in ultra high vacuum conditions taking high purity Fe, Ni
and Cu metals with Cu layer thickness 4, 6 and 8 nm for each alloy composition.
The structure of the alloy films of the trilayers was investigated using x-ray
diffraction and the thickness \& roughness of the layers were obtained by x-ray
reflectivity measurement. Both the as prepared and annealed trilayers exhibit
layered structure. The as deposited Fe-Ni alloy in non Invar trilayer exhibits
only fcc structure whereas in Invar alloy it exhibits a mixed fcc and bcc
phases. Interestingly after annealing at 425$^0$C in ultra high vacuum, the
Invar alloy completely transformed to fcc structure for all Cu thicknesses. In
both Invar and non Invar trilayers, the Bragg reflections corresponding to
Fe-Ni alloy layers become sharp after annealing. The induced structural
transformation in Invar trilayer is explained using enhanced diffusion of Fe
and Ni atoms at high temperatures. | 2006.07662v1 |
2020-07-02 | Free-standing 2D metals from binary metal alloys | Recent experiment demonstrated the formation of free-standing Au monolayers
by exposing Au-Ag alloy to electron beam irradiation. Inspired by this
discovery, we used semi-empirical effective medium theory simulations to
investigate monolayer formation in 30 different binary metal alloys composed of
late d-series metals Ni, Cu, Pd, Ag, Pt, and Au. In qualitative agreement with
the experiment, we find that the beam energy required to dealloy Ag atoms from
Au-Ag alloy is smaller than the energy required to break the dealloyed Au
monolayer. Our simulations suggest that similar method could also be used to
form Au monolayers from Au-Cu alloy and Pt monolayers from Pt-Cu, Pt-Ni, and
Pt-Pd alloys. | 2007.01291v1 |
2020-07-22 | The origin of jerky dislocation motion in high-entropy alloys | Dislocations in single-phase concentrated random alloys, including high-
entropy alloys (HEAs), repeatedly encounter pinning during glide, resulting in
jerky dislocation motion. While solute-dislocation interaction is well
understood in conventional alloys, the origin of individual pinning points in
concentrated random alloys is a matter of debate. In this work, we investigate
the origin of dislocation pinning in the CoCrFeMnNi HEA. In- situ transmission
electron microscopy studies reveal wavy dislocation lines and a jagged glide
motion under external loading, even though no segregation or clustering is
found around Shockley partial dislocations. Atomistic simulations reproduce the
jerky dislocation motion and link the repeated pinning to local fluctuations in
the Peierls friction. We demonstrate that the density of high local Peierls
friction is proportional to the critical stress required for dislocation glide
and the dislocation mobility. | 2007.11489v3 |
2020-07-22 | An atomistic description of alloys and core shells nanoparticles | Using the extended discrete interaction model we investigate the tuneabilty
of surface plasmon resonance in alloys and core-shell nanoparticles made from
silver and gold. We show that the surface plasmon resonance of these alloys and
core-shell particles to a large extent follow Vegard's law irrespective of the
geometry of the nanoparticle. We show the evolution of the polarizability with
size and demonstrate the highly non-linear behaviour of the polarizability with
the ratio of the constituents and geometry in alloys and core-shell
nanoparticles, with the exception for nanorod alloys. A thorough statistical
investigation reveals that there is only a small dependence of the surface
plasmon resonance on atomic arrangement and exact distribution in a
nanoparticle and that the standard deviation decrease rapidly with the size of
the nanoparticles. The physical reasoning for the random distribution algorithm
for alloys in discrete interaction models is explained in details and verified
by the statistical analysis. | 2007.11688v1 |
2020-09-15 | Alloy Engineering of Polar (Si,Ge)2N2O System for Controllable Second Harmonic Performance | Although silicon oxynitrides are important semiconductors for many practical
applications, their potential second-order nonlinear optical (NLO)
applications, regardless of balanced or controllable performance, have never
been systemically explored. Using the first-principles calculations, in this
article, we discover that the sinoite (i.e., typical silicon oxynitride Si2N2O)
can simultaneously exhibit wide optical bandgap, strong second-harmonic
generation (SHG) effect, and large birefringence, which are further confirmed
by our preliminary experimental data. Importantly, we propose that alloying
engineering can be further applied to control the balanced NLO properties in
the Si2N2O system. Combining first-principles calculations and cluster
expansion theory, we demonstrate that alloying Ge into Si2N2O can easily form
low formation energy Si2(1-x)Ge2xN2O alloys, which can in turn achieve
controllable phase-matching harmonic output with high SHG efficiency at
different energy ranges. Therefore, alloy engineering could provide a unique
approach to effectively control the balanced NLO performance of
Si2(1-x)Ge2xN2O, making this polar alloy system holding potential applications
in tunable laser frequency conversion and controllable all-optical devices. | 2009.06932v2 |
2020-10-29 | Plasma frequency in doped highly mismatched alloys | Highly mismatched alloys (HMAs) have band structures strongly modified due to
the introduction of the alloying element. We consider HMAs where the isolated
state of the alloying element is near the host conduction band, which causes
the conduction band to split into two bands. We determine the bulk plasma
frequency when the lower-energy band is partially occupied, as by doping, using
a semi-analytical method based on a disorder-averaged Green's function. We
include the nontrivial effects of interband transitions to the higher-energy
band, which limit the plasma frequency to be less than an effective band gap.
We show that the distribution of states in the split bands causes plasmons in
HMAs to behave differently than plasmons in standard metals and semiconductors.
The effective mass of the lower split band $m^*$ changes with alloy fraction,
and we find that the plasmon frequency with small carrier concentration $n$
scales with $\sqrt{n}/m^*$ rather than the $\sqrt{n/m^*}$ that is expected in
standard materials. We suggest experiments to observe these phenomena.
Considering the typical range of material parameters in this group of alloys
and taking a realistic example, we suggest that HMAs can serve as highly
tunable low-frequency plasmonic materials. | 2010.15953v1 |
2020-12-01 | The Benefits of Trace Cu in Wrought Al-Mg Alloys | The softening and strengthening contributions in pre-deformed and aged
Al-Mg-Cu alloys containing 3wt.%Mg and 0.5wt.%Cu are evaluated by a combination
of microscopy, mechanical testing and modelling. A refined phenomenological
model for the work hardening response, accounting for the separate effects of
recovery and precipitation, is shown to be suitable for an unambiguous
determination of the precipitation hardening contribution in these alloys.
Significantly, it is found that the mechanical response of these alloys is not
strongly impacted by Cu content (in the low Cu content regime), pre-deformation
level or aging temperature meaning that the alloys are robust with respect to
variations in composition. This is interesting from the perspective of alloy
design concepts based on `recycling friendly' compositions in applications that
include paint-baking. | 2012.00277v1 |
2021-01-14 | Neural-networks model for force prediction in multi-principal-element alloys | Atomistic simulations can provide useful insights into the physical
properties of multi-principal-element alloys. However, classical potentials
mostly fail to capture key quantum (electronic-structure) effects. We present a
deep 3D convolutional neural network (3D CNN) based framework combined with a
voxelization technique to design interatomic potentials for chemically complex
alloys. We highlight the performance of the 3D CNN model and its efficacy in
computing potentials using the medium-entropy alloy TaNbMo. In order to provide
insights into the effect of voxel resolution, we implemented two approaches
based on the inner and outer bounding boxes. An efficient 3D CNN model, which
is as accurate as the density-functional theory (DFT) approach, for calculating
potentials will provide a promising schema for accurate atomistic simulations
of structure and dynamics of general multi-principle element alloys. | 2101.05867v2 |
2021-01-29 | Surface lattice Green's functions for high-entropy alloys | We study the surface elastic response of pure Ni, the random alloy FeNiCr and
an average FeNiCr alloy in terms of the surface lattice Green's function. We
propose a scheme for computing per-site Green's function and study their
per-site variations. The average FeNiCr alloy accurately reproduces the mean
Green's function of the full random alloy. Variation around this mean is
largest near the edge of the surface Brillouin-zone and decays as $q^{-2}$ with
wavevector $q$ towards the $\Gamma$-point. We also present expressions for the
continuum surface Green's function of anisotropic solids of finite and infinite
thickness and show that the atomistic Green's function approaches continuum
near the $\Gamma$-point. Our results are a first step towards efficient contact
calculations and Peierls-Nabarro type models for dislocations in high-entropy
alloys. | 2101.12519v1 |
2021-02-26 | Data Analytics Approach to Predict High-Temperature Cyclic Oxidation Kinetics of NiCr-based Alloys | Although of practical importance, there is no established modeling framework
to accurately predict high-temperature cyclic oxidation kinetics of
multi-component alloys due to the inherent complexity. We present a data
analytics approach to predict the oxidation rate constant of NiCr-based alloys
as a function of composition and temperature with a highly consistent and
well-curated experimental dataset. Two characteristic oxidation models, i.e., a
simple parabolic law and a statistical cyclic-oxidation model, have been chosen
to numerically represent the high-temperature oxidation kinetics of commercial
and model NiCr-based alloys. We have successfully trained machine learning (ML)
models using highly ranked key input features identified by correlation
analysis to accurately predict experimental parabolic rate constants (kp). This
study demonstrates the potential of ML approaches to predict oxidation kinetics
of alloys over a wide composition and temperature ranges. This approach can
also serve as a basis for introducing more physically meaningful ML input
features to predict the comprehensive cyclic oxidation behavior of
multi-component high-temperature alloys with proper constraints based on the
known underlying mechanisms. | 2102.13261v1 |
2021-04-12 | CALPHAD-informed phase-field modeling of grain boundary microchemistry and precipitation in Al-Zn-Mg-Cu alloys | The grain boundary (GB) microchemistry and precipitation behaviour in
high-strength Al-Zn-Mg-Cu alloys has an important influence on their mechanical
and electrochemical properties. Simulation of the GB segregation,
precipitation, and solute distribution in these alloys requires an accurate
description of the thermodynamics and kinetics of this multi-component system.
CALPHAD databases have been successfully developed for equilibrium
thermodynamic calculations in complex multi-component systems, and in recent
years have been combined with diffusion simulations. In this work, we have
directly incorporated a CALPHAD database into a phase-field framework, to
simulate, with high fidelity, the complex kinetics of the non-equilibrium GB
microstructures that develop in these important commercial alloys during heat
treatment. In particular, the influence of GB solute segregation, GB diffusion,
precipitate number density, and far-field matrix composition, on the growth of
a population of GB precipitates, was systematically investigated in a model
Al-Zn-Mg-Cu alloy of near AA7050 composition. The simulation results were
compared with scanning transmission electron microscopy and atom probe
tomography characterisation of alloys of the similar composition, with good
agreement. | 2104.05791v1 |
2021-04-15 | Optimization of High Entropy Alloy Catalyst for Ammonia Decomposition and Ammonia Synthesis | The successful synthesis of high entropy alloy (HEA) nanoparticles, a
long-sought goal in materials science, opens a new frontier in materials
science with applications across catalysis, electronics, structural alloys, and
energetic materials. Recently, a Co25Mo45Fe10Ni10Cu10 HEA made of
earth-abundant elements was shown to have a high catalytic activity for ammonia
decomposition, which rivals that of state-of-the-art, but prohibitively
expensive, ruthenium catalyst. Using a computational approach based on
first-principles calculations in conjunction with data analytics and machine
learning, we build a model to rapidly compute the adsorption energy of H, N,
and NHx (x=1,3) species on CoMoFeNiCu alloy surfaces with varied alloy
compositions and atomic arrangement. We show that the 25/45 Co/Mo ratio
identified experimentally as the most active composition for ammonia
decomposition increases the likelihood that the surface adsorbs nitrogen
equivalently to that of ruthenium while at the same time interacting moderately
strongly with intermediates. Our study underscores the importance of
computational modeling and machine learning to identify and optimize HEA alloys
across their near-infinite materials design space. | 2104.07827v2 |
2021-05-06 | Low temperature annealing method for fabricating alloy nanostructures and metasurfaces: Unlocking a novel degree of freedom | The material and exact shape of a nanostructure determine its optical
response, which is especially strong for plasmonic metals. Unfortunately, only
a very few plasmonic metals are available, which limits the spectral range
where these strong optical effects can be utilized. Alloying different
plasmonic metals can overcome this limitation, at the expense of using a high
temperature alloying process, which adversely destroys the nanostructure shape.
Here, we develop a low temperature alloying process at only 300{\deg}C and
fabricate Au-Ag nanostructures with a broad diversity of shapes, aspect ratios
and stoichiometries. EDX and XPS analyses confirm the homogeneous alloying
through the entire sample. Varying the alloy stoichiometry tunes the optical
response of the nanostructure and controls spectral features such as Fano
resonances. Binary metasurfaces that combine nanostructures with different
stoichiometries are fabricated using multiple-step electron beam lithography,
and their optical function as hologram or Fresnel zone plate is demonstrated at
the visible wavelength of 532 nm. This low temperature annealing technique
provides a versatile and cost-effective way of fabricating complex Au-Ag
nanostructures with arbitrary stoichiometry. | 2105.02461v1 |
2021-05-11 | Exploring the Correlation between Solvent Diffusion and Creep Resistance of Mg-Ga HCP Alloys from High Throughput Liquid-Solid Diffusion Couple | The liquid-solid diffusion couple technique, supported by phenomenological
analysis and nano-indentation tests, is proposed on account of the relatively
low melting points of Mg to explore the diffusion mobility and creep
deformation. The potential of this strategy is demonstrated in Mg-Ga hcp alloys
where Ga solute (i.e. impurity) and Mg solvent diffusions in hcp Mg-Ga alloys
were both unveiled. It was followed by mapping the compressive creep behavior
via nanoindentation along the composition arrays within the same Mg-Ga couple
sample. The compressive creep resistance of Mg-Ga hcp alloys increased with the
Ga content, and this enhancement was similar to the one found in Mg-Zn alloys
and superior to the one reported in Mg-Al alloys though Al is a slower impurity
diffuser in hcp-Mg than Zn and Ga. Thereby, the solvent diffusion and its
variation with the composition, rather than the solute diffusion, was suggested
to govern the creep properties at high temperatures and low stresses. | 2105.05096v1 |
2021-05-31 | Aluminium Alloy Design and Discovery using Machine Learning | The traditional design and development of metallic alloys has taken a
hill-climbing approach to date, with incremental advances. Throughout the last
century, aluminium (Al) alloy design has been essentially empirical and
iterative, based on lessons learned from in service use and human experience.
Incremental alloy development is costly, slow, and doesn't fully harness the
data that exists in the field of Al-alloy metallurgy. In the present work, an
attempt has been made to utilise a data science approach to develop a machine
learning (ML) model for Al-alloy design. An objective-optimisation process has
also been developed, to exploit the ML model, for user experience and practical
application. A successful model was developed and presented herein, along with
the open-access software. | 2105.14806v2 |
2021-06-23 | First-principles study of the robust superconducting state of NbTi alloys under ultrahigh pressures | A recent experiment reported that robust superconductivity appears in NbTi
alloys under ultrahigh pressures with an almost constant superconducting $T_c$
of ~19 K from 120 to 261.7 GPa [J. Guo et al., Adv. Mater. 31, 1807240 (2019)],
which is very rare among the known superconductors. We investigate the origin
of this novel superconducting behavior in NbTi alloys based on density
functional theory and density functional perturbation theory calculations. Our
results indicate that the pressure tends to transform NbTi alloys from a random
phase to a uniformly ordered crystal phase, and the exotic robust
superconductivity of NbTi alloys can still be understood in the framework of
BCS theory. The Nb element in NbTi alloys plays a dominant role in the
superconductivity at low pressure, while the NbTi crystal with an alternative
and uniform Nb and Ti atomic arrangement may be responsible for the stable
superconductivity under high pressures. The robust superconducting transition
temperature of NbTi under ultrahigh pressure can be explained by a synergistic
effect of the enhanced phonon frequency, the modestly reduced total
electron-phonon coupling, and the pressure-dependent screened Coulomb
repulsion. | 2106.12371v1 |
2021-11-05 | Towards Stacking Fault Energy Engineering in FCC High Entropy Alloys | Stacking Fault Energy (SFE) is an intrinsic alloy property that governs much
of the plastic deformation mechanisms observed in fcc alloys. While SFE has
been recognized for many years as a key intrinsic mechanical property, its
inference via experimental observations or prediction using, for example,
computationally intensive first-principles methods is challenging. This
difficulty precludes the explicit use of SFE as an alloy design parameter. In
this work, we combine DFT calculations (with necessary configurational
averaging), machine-learning (ML) and physics-based models to predict the SFE
in the fcc CoCrFeMnNiV-Al high-entropy alloy space. The best-performing ML
model is capable of accurately predicting the SFE of arbitrary compositions
within this 7-element system. This efficient model along with a recently
developed model to estimate intrinsic strength of fcc HEAs is used to explore
the strength-SFE Pareto front, predicting new-candidate alloys with
particularly interesting mechanical behavior. | 2111.03591v1 |
2022-01-20 | Coexistence of two types of short-range order in SiGeSn medium-entropy alloys | Short-range chemical order (SRO) has been recently demonstrated to play a
decisive role in modulating a wide range of physical properties in
medium-entropy alloy (MEA) and high-entropy alloy (HEA). The enormous
configurational space of these alloys implies multiple forms of SRO are likely
to develop concurrently but such structural diversity has not been reported.
Here we show, through extensive {\em ab initio}-based sampling study, that
SiGeSn medium-entropy alloys spontaneously develop two distinct forms of SRO.
Remarkably, the two types of SROs, which carry different energies, distinct
degrees of local ordering, and dissimilar electronic structures, are found to
co-exist in a wide range of compositions of SiGeSn alloys. The co-existence of
two SROs is rationalized through their virtual degeneracy of thermodynamic
stability, due to the subtle balance in the change of enthalpy and
configurational entropy upon the transformation between the two SROs. Such
co-existence of SROs thus suggests an inherent structural heterogeneity, a
diffuse electronic structure, and a new route for band engineering in SiGeSn
MEA. More generally, our finding indicates the possible ubiquity of the
co-existence of multiple forms of SRO in a broad range of MEAs and HEAs, which
has profound implications on their diverse physical properties. | 2201.08256v1 |
2022-08-24 | Low-temperature ordering in a substitutional alloy with injecting nonequilibrium vacancies: The FePt case | Achieving the compositionally ordered state in a substitutional alloy of two
or more species can often be even critical for improving its functional
properties. To produce a highly ordered alloy, a longtime high-temperature (up
to T=1000 K) treatment of the alloy is typically necessary because of
insufficient vacancy concentration (c_v) and their mobility. However, such
processing affects the morphology and complicates the technology of functional
alloys. We show theoretically that the ordering in the practically important
FePt system (Fe_xPt_1-x with x being close to 0.5) is already achievable at
T=450 K for reasonable times t<10^3 s due to frozen nonequilibrium vacancies.
Our simulation is based on the Dienes equation for relaxation of the long-range
order parameter (S), with taking additionally into account that the ordering
kinetics in the alloy is mediated by vacancies. Importantly, the results of
such simulation are in good agreement with previous experimental data on the
ordering kinetics. We also find that nanosecond laser pulses can be employed to
achieve a sufficient level of c_v=10^-5 for effective low-temperature ordering. | 2208.11720v3 |
2022-10-07 | Radiation-resistant aluminium alloy for space missions in the extreme environment of the solar system | Future human-based exploration of our solar system requires the invention of
materials that can resist harsh environments. Age-hardenable aluminium alloys
would be attractive candidates for structural components in long-distance
spacecrafts, but their radiation resistance to solar energetic particles is
insufficient. Common hardening phases dissolve and displacement damage occurs
in the alloy matrix, which strongly degrades properties. Here we present an
alloy where hardening is achieved by T-phase, featuring a giant unit cell and
highly-negative enthalpy of formation. The phase shows record radiation
survivability and can stabilize an ultrafine-grained structure upon temperature
and radiation in the alloy, therby successfully preventing displacement damage
to occur. Such concept can be considered ideal for the next-generation space
materials and the design of radiation resistant alloy. | 2210.03397v3 |
2022-12-31 | Investigating representation schemes for surrogate modeling of High Entropy Alloys | The design of new High Entropy Alloys that can achieve exceptional mechanical
properties is presently of great interest to the materials science community.
However, due to the difficulty of designing these alloys using traditional
methods, machine learning has recently emerged as an essential tool.
Particularly, the screening of candidate alloy compositions using surrogate
models has become a mainstay of materials design in recent years. Many of these
models use the atomic fractions of the alloying elements as inputs. However,
there are many possible representation schemes for encoding alloy compositions,
including both unstructured and structured variants. As the input features play
a critical role in determining surrogate model performance, we have
systematically compared these representation schemes on the basis of their
performance in single-task deep learning models and in transfer learning
scenarios. The results from these tests indicate that compared to the
unstructured and randomly ordered schemes, chemically meaningful arrangements
of elements within spatial representation schemes generally lead to better
models. However, we also observed that tree-based models using only the atomic
fractions as input were able to outperform these models in transfer learning. | 2301.00179v1 |
2023-03-08 | Atomic Representations of Local and Global Chemistry in Complex Alloys | The exceptional properties observed in complex concentrated alloys (CCAs)
arise from the interplay between crystalline order and chemical disorder at the
atomic scale, complicating a unique determination of properties. In contrast to
conventional alloys, CCA properties emerge as distributions due to varying
local chemical environments and the specific scale of measurement. Currently
there are few ways to quantitatively define, track, and compare local alloy
compositions (versus a global label, i.e. equiatomic) contained in a CCA.
Molecular dynamics is used here to build descriptive metrics that connect a
global alloy composition to the diverse local alloy compositions that define
it. A machine-learned interatomic potential for MoNbTaTi is developed and we
use these metrics to investigate how property distributions change with
excursions in global-local composition space. Short-range order is examined
through the lens of local chemistry for the equiatomic composition,
demonstrating stark changes in vacancy formation energy with local chemistry
evolution. | 2303.04311v3 |
2023-05-20 | A Computational Approach for Mapping Electrochemical Activity of Multi-Principal Element Alloys | Multi principal element alloys (MPEAs) comprise a unique class of metal
alloys. MPEAs have been demonstrated to possess several exceptional properties,
including, as most relevant to the present study, a high corrosion resistance.
In the context of MPEA design, the vast number of potential alloying elements
and the staggering number of elemental combinations favours a computational
alloy design approach. In order to computationally assess the prospective
corrosion performance of MPEA, an approach was developed in this study. A
density functional theory (DFT) based Monte Carlo method was used for the
development of MPEA structure, with the AlCrTiV alloy used as a model.
High-throughput DFT calculations were performed to create training datasets for
surface activity towards different adsorbate species: O2-, Cl- and H+. Machine
learning (ML) with combined representation was then utilised to predict the
adsorption and vacancy energies as descriptors for surface activity. The
capability of the combined computational methods of MC, DFT and ML, as a
virtual electrochemical performance simulator for MPEAs was established and may
be useful in exploring other MPEAs. | 2305.12059v1 |
2023-06-29 | Structure-Dynamics Relationship in Al-Mg-Si Liquid Alloys | Enhancing properties and performances of aluminium alloys by a control of
their solidification is pivotal in automotive and aerospace industries. The
fundamental role of the structure-diffusion relationship is investigated for
Al-Mg-Si liquid alloys taken as a prototype of Al-6xxx. For this purpose, first
principles-based molecular dynamics simulations were performed for various Si
and Mg content for Al-rich compositions, including the binary alloy
counterparts. Results indicate that Mg and/or Si in alloys create a more
compact ordering around Al than in pure Al, lowering diffusion. Mg promotes
icosahedral short-range order, while Si displays a preference towards cubic
local ordering, impacting diffusion based on their respective content. It
suggests a mechanism whereby an increase in Mg content generally lowers the
diffusion of each species, whereas an increase in Si content enhances their
diffusion, providing insights for future alloy design. | 2306.17264v1 |
2023-12-07 | Integrated Design of Aluminum-Containing High-entropy Refractory B2 Alloys with Synergy of High Strength and Ductility | Refractory high-entropy alloys, RHEAs, are promising high-temperature
structural materials. Their large compositional space poses great design
challenges for phase control and high strength-ductility synergy. The present
research pioneers using integrated high-throughput machine learning with Monte
Carlo simulations to effectively navigate phase-selection and
mechanical-properties predictions, developing aluminum-containing RHEAs in
single-phase ordered B2 alloys demonstrating both high strength and ductility.
These aluminum-containing RHEAs achieve remarkable mechanical properties,
including compressive yield strengths up to 1.6 GPa, fracture strains exceeding
50 percent, and significant high-temperature strength retention. They also
demonstrate a tensile yield strength of 1.1 GPa with a tension ductility of 6.3
percent. Besides, we identify a valence-electron-count domain for alloy
brittleness with the explanation from density-functional theory and provide
crucial insights into elements' influence on atomic ordering and mechanical
performance. The work sets forth a strategic blueprint for high-throughput
alloy design and reveals fundamental principles that govern the mechanical
properties of advanced structural alloys. | 2312.04708v1 |
2024-01-30 | Empirical tight-binding method for large-supercell simulations of disordered semiconductor alloys | We analyze and present applications of a recently proposed empirical
tight-binding scheme for investigating the effects of alloy disorder on various
electronic and optical properties of semiconductor alloys, such as the band gap
variation, the localization of charge carriers, and the optical transitions.
The results for a typical antimony-containing III-V alloy, GaAsSb, show that
the new scheme greatly improves the accuracy in reproducing the experimental
alloy band gaps compared to other widely used schemes. The atomistic nature of
the empirical tight-binding approach paired with a reliable parameterization
enables more detailed physical insights into the effects of disorder in alloyed
materials. | 2401.16951v1 |
2024-01-31 | Investigation of Microstructure and Corrosion Resistance of Ti-Al-V Titanium Alloys Obtained by Spark Plasma Sintering | The research results of the microstructure and corrosion resistance of Ti and
Ti-Al-V Russian industrial titanium alloys obtained by spark plasma sintering
(SPS) are described. Investigations of the microstructure, phase composition,
hardness, tensile strength, electrochemical corrosion resistance and hot salt
corrosion of Ti-Al-V titanium alloy specimens were carried out. It was shown
that the alloy specimens have a uniform highly dense microstructure and high
hardness values. The studied alloys also have high resistance to
electrochemical corrosion during tests in acidic aqueous solution causing the
intergranular corrosion as well as high resistance to the hot salt corrosion.
The assumption that the high hardness of the alloys as well as the differences
in the corrosion resistance of the central and lateral parts of the specimens
are due to the diffusion of carbon from the graphite mold into the specimen
surface was suggested. | 2401.17941v1 |
2024-02-09 | Right or Wrong -- Understanding How Novice Users Write Software Models | Writing declarative models has numerous benefits, ranging from automated
reasoning and correction of design-level properties before systems are built,
to automated testing and debugging of their implementations after they are
built. Alloy is a declarative modeling language that is well-suited for
verifying system designs. A key strength of Alloy is its scenario-finding
toolset, the Analyzer, which allows users to explore all valid scenarios that
adhere to the model's constraints up to a user-provided scope. However, even
with visualized scenarios, it is difficult to write correct Alloy models. To
address this, a growing body of work explores different techniques for
debugging Alloy models. In order to develop and evaluate these techniques in an
effective manor, this paper presents an empirical study of over 97,000 models
written by novice users trying to learn Alloy. We investigate how users write
both correct and incorrect models in order to produce a comprehensive benchmark
for future use as well as a series of observations to guide debugging and
educational efforts for Alloy model development. | 2402.06624v3 |
2024-02-29 | Searching for magnetically hard monoborides (and finding a few): A first-principles investigation | New hard magnetic materials with zero or low rare earth content are in demand
due to the high prices of the rare earth metals. Among the candidates for such
materials, we consider MnB, FeB and their alloys, because previous experiments
suggest that FeB has a relatively high magnetic hardness of about 0.83 at room
temperature. Using first-principles calculations, we examine the full range of
alloys from CrB, through MnB, FeB, to CoB. Furthrmore, we consider alloys of
MnB and FeB with substitutions of 3$d$, 4$d$ and 5$d$ transition metals. For
the above ninety compositions, we determine magnetic moment, magnetocrystalline
anisotropy energy and magnetic hardness. For (Fe-Co)B alloys, the calculated
values of magnetic hardness exceed five, which is an exceptionally high. While
these values are inflated by the virtual crystal approximation used, we still
expect actual magnetic hardnesses well above unity. Furthermore, we classify
considered MnB alloys substituted with transition metals as magnetically soft
or semi-hard and FeB alloys with Sc, Ti, V, Zr, Nb, Mo, Hf, Ta or W as
magnetically hard (with magnetic hardness exceeding unity). | 2403.00138v1 |
2024-03-25 | Enhanced mobility of ternary InGaAs quantum wells through digital alloying | High In content InGaAs quantum wells (In $\geq$ 75%) are potentially useful
for topological quantum computing and spintronics applications. In high
mobility InGaAs quantum wells, alloy disorder scattering is a limiting factor.
In this report, we demonstrate that by growing the InGaAs quantum wells as a
digital alloy, or a short period superlattice, we can reduce the alloy disorder
scattering within the quantum well and increase the peak 2 K electron mobility
to 545,000 cm^2/V s, which is the highest reported mobility for high In content
InGaAs quantum wells to the best of the authors' knowledge. Our results
demonstrate that the digital alloy approach can be used to increase the
mobility of quantum wells in random alloy ternary materials. | 2403.17166v2 |
2024-03-31 | First Principles Studies of Stacking Fault Energies in Ternary Magnesium Alloys | Magnesium (Mg) alloys have emerged as promising materials due to their low
density and high strength-to-weight ratio, offering a wide range of
applications across multiple industries. Nevertheless, the inherent brittleness
of Mg alloys poses a significant hurdle, necessitating innovative approaches to
enhance their mechanical performance. Among the various strategies,
manipulating stacking fault energy (SFE) has been a key focus, although
primarily within the realm of binary alloys. This study investigates SFE in Mg
alloys, focusing on ternary compositions. Utilizing first-principles DFT
calculations, we analyze solute interactions and their influence on SFE,
particularly in Mg-Al-X and Mg-Zn-X configurations. Predictive models are
developed for estimating SFE effects, revealing solute pairs that mimic rare
earth elements and show potential for improved ductility. The findings
contribute to fundamental insights into Mg alloy behavior, offering practical
directions for designing advanced materials with superior mechanical
properties. | 2404.00564v1 |
2012-09-29 | The effect of precipitation on strength and ductility in a Mg-Zn-Y alloy | The effect of pre-ageing deformation on the size and distribution of
beta-prime precipitates and subsequently on the resulting strength and
ductility have been measured in a Mg-3.0at.%Zn-0.5at.%Y alloy. The alloy was
extruded and then subjected to a T8 heat treatment comprised of a
solution-treatment, cold-work and artificial ageing. Extrusion was used to
introduce texture, ensuring that deformation occurred via slip rather than
twinning. Samples were subjected to controlled uniaxial deformation and then
isothermally aged to peak hardness. Precipitate length, diameter and number
density were measured and evaluated in terms of the strength and ductility of
the alloy. The nucleation of the beta-prime precipitates in peak-aged condition
without pre-ageing deformation (i.e.T6 treatment) was poor, with only 0.5%
volume fraction, compared to approximately 3.5% in T6 treated binary
Mg-3.0at.%Zn alloy. The microstructure of the Mg-Zn-Y alloy was less refined,
with larger diameter precipitates and lower beta-prime number densities
compared to a binary Mg-3.0at.%Zn alloy. Deformation to 5% plastic strain
increased the volume fraction of beta-prime precipitates to approximately 2.3%
and refined the beta-prime precipitate length and diameter. The combination of
these effects increased the yield strength after isothermal ageing from 217MPa
(0% cold-work) to 287 MPa (5% cold-work). The yield stress increased linearly
with reciprocal interparticle spacing on the basal and prismatic planes and the
alloy showed similar strengthening against basal slip to Mg-Zn. The elongation
increased linearly with particle spacing. The ductility of Mg-Zn-Y alloys was
similar to that of Mg-Zn for equivalently spaced particles. | 1210.0079v1 |
2013-04-18 | Alloying effect on the ideal tensile strength of ferromagnetic and paramagnetic bcc iron | Using \emph{ab initio} alloy theory formulated within the exact muffin-tin
orbitals theory in combination with the coherent potential approximation, we
investigate the ideal tensile strength (ITS) in the $[001]$ direction of bcc
ferro-/ferrimagnetic (FFM) and paramagnetic (PM) Fe$_{1-x}M_{x}$ ($M=$ Al, V,
Cr, Mn, Co, or Ni) random alloys. The ITS of ferromagnetic (FM) Fe is
calculated to be $12.6$\,GPa, in agreement with available data, while the PM
phase turns out to posses a significantly lower value of $0.7\,$GPa. Alloyed to
the FM matrix, we predict that V, Cr, and Co increase the ITS of Fe, while Al
and Ni decrease it. Manganese yields a weak non-monotonic alloying behavior. In
comparison to FM Fe, the alloying effect of Al and Co to PM Fe is reversed and
the relative magnitude of the ITS can be altered more strongly for any of the
solutes. All considered binaries are intrinsically brittle and fail by cleavage
of the $(001)$ planes under uniaxial tensile loading in both magnetic phases.
We show that the previously established ITS model based on structural energy
differences proves successful in the PM Fe-alloys but is of limited use in the
case of the FFM Fe-based alloys. The different performance is attributed to the
specific interplay between magnetism and volume change in response to uniaxial
tension. We establish a strong correlation between the compositional effect on
the ITS and the one on the shear elastic constant $C'$ for the PM alloys and
briefly discuss the relation between hardenability and the ITS. | 1304.5129v2 |
2020-11-12 | Alloying behavior of wide band gap alkaline-earth chalcogenides | Alloying is a powerful tool for tuning materials that facilitates the
targeted design of desirable properties for a variety of applications. In this
work, we provide a comprehensive investigation of the synthetic accessibility
and electronic properties of nine alkaline-earth chalcogenide anion alloys
(CaS$_{1-x}$O$_x$, CaS$_{1-x}$Se$_x$, CaS$_{1-x}$Te$_x$, SrS$_{1-x}$O$_x$,
SrS$_{1-x}$Se$_x$, SrS$_{1-x}$Te$_x$, MgS$_{1- x}$O$_x$, MgS$_{1-x}$Se$_x$, and
MgS$_{1-x}$Te$_x$). We show that isostructural alloying within the rock salt
structure is favored for all systems except MgS$_{1-x}$Te$_x$, which is
predicted to be a heterostructural alloy between the rock salt and wurtzite
structures. Alloys of S and Se are shown to be readily accessible for all
cations with low miscibility critical temperatures, enabling continuous tuning
of electronic properties across this composition space. Alloys of S and Te have
higher critical temperatures but may be accessible through non-equilibrium
synthesis strategies and are predicted here to have desirable electronic
properties for optoelectronics with wide band gaps and lower effective masses
than alloys of S and Se. Anion alloying in MgS$_{1-x}$Te$_x$ stabilizes the
wurtzite structure across a significant fraction of composition space, which
may make it of particular interest as a transparent conducting material due to
its lower effective masses and a higher band gap than the rock salt structure.
Zero-point corrected random phase approximation (RPA) energies were computed to
resolve the small polymorph energy differences of the Mg compounds and are
shown to be critical for accurately describing the thermodynamic properties of
the corresponding alloys. | 2011.06628v1 |
2023-08-15 | Electronic and optical properties of boron containing GaN alloys: The role boron atom clustering | Boron (B) containing III-nitride materials, such as wurtzite (B,Ga)N alloys,
have recently attracted significant interest to tailor the electronic and
optical properties of optoelectronic devices operating in the visible and
ultraviolet spectral range. However, the growth of high quality samples is
challenging and B atom clustering is often observed in (B,Ga)N alloys. To date,
fundamental understanding of the impact of such clustering on electronic and
optical properties of these alloys is sparse. In this work we employ density
functional theory (DFT) in the framework of the meta generalized gradient
approximation (modified Becke Johnson (mBJ) functional) to provide insight into
this question. We use mBJ DFT calculations, benchmarked against
state-of-the-art hybrid functional DFT, on (B,Ga)N alloys in the experimentally
relevant B content range of up to 7.4%. Our results reveal that B atom
clustering can lead to a strong reduction in the bandgap of such an alloy, in
contrast to alloy configurations where B atoms are not forming clusters, thus
not sharing nitrogen (N) atoms. We find that the reduction in bandgap is linked
mainly to carrier localization effects in the valence band, which stem from
local strain and polarization field effects. However, our study also reveals
that the alloy microstructure of a B atom cluster plays an important role: B
atom chains along the wurtzite c-axis impact the electronic structure far less
strongly when compared to a chain formed within the c-plane. This effect is
again linked to local polarization field effects and the orbital character of
the involved valence states in wurtzite BN and GaN. Overall, our calculations
show that controlling the alloy microstructure of (B,Ga)N alloys is of central
importance when it comes to utilizing these systems in future optoelectronic
devices with improved efficiencies. | 2308.07759v1 |
2002-09-11 | Exchange bias in [Co2MnGe/Au],[Co2MnGe/Cr] and [Co2MnGe/Cu2MnAl] multilayers | We report structural and magnetic properties of multilayers composed of thin
layers of the half metallic ferromagnetic Heusler compound Co2MnGe and layers
of Au, Cr and the Heusler compound Cu2MnAl. The hysteresis loops measured at
low temperatures reveal the existence of an exchange bias field HEB in all of
these multilayers. For the [Co2MnGe/Au] multilayer system HEB is largest
reaching up to 1 kOe at a temperature of 2 K. We characterize the exchange bias
phenomenon in detail and show that it originates from a spin glass type of
magnetic order for a thin interlayer at the interfaces. We discuss the results
in the light of different models proposed for the explanation of the exchange
bias effect. | 0209259v1 |
2003-04-15 | First-principles study of lattice instabilities in the ferromagnetic martensite Ni$_2$MnGa | The phonon dispersion relations and elastic constants for ferromagnetic
Ni$_2$MnGa in the cubic and tetragonally distorted Heusler structures are
computed using density-functional and density-functional perturbation theory
within the spin-polarized generalized-gradient approximation. For
$0.9<c/a<1.06$, the TA$_2$ tranverse acoustic branch along $[110]$ and
symmetry-related directions displays a dynamical instability at a wavevector
that depends on $c/a$. Through examination of the Fermi-surface nesting and
electron-phonon coupling, this is identified as a Kohn anomaly. In the parent
cubic phase the computed tetragonal shear elastic constant,
C$^\prime$=(C$_{11}-$C$_{12}$)/2, is close to zero, indicating a marginal
elastic instability towards a uniform tetragonal distortion. We conclude that
the cubic Heusler structure is unstable against a family of energy-lowering
distortions produced by the coupling between a uniform tetragonal distortion
and the corresponding $[110]$ modulation. The computed relation between the
$c/a$ ratio and the modulation wavevector is in excellent agreement with
structural data on the premartensitic ($c/a$ = 1) and martensitic ($c/a$ =
0.94) phases of Ni$_2$MnGa. | 0304349v1 |
2006-06-21 | Magnetic anisotropies and magnetization reversal of the Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al Heusler compound | Magnetic anisotropies and magnetization reversal properties of the epitaxial
Heusler compound Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al (CCFA) deposited on Fe and Cr
buffer layers are studied. Both samples exhibit a growth-induced fourfold
anisotropy, and magnetization reversal occurs through the formation of stripy
domains or 90 degree domains. During rotational magnetometric scans the sample
deposited on Cr exhibits about 2 degree sharp peaks in the angular dependence
of the coercive field, which are oriented along the hard axis directions. These
peaks are a consequence of the specific domain structure appearing in this
particular measurement geometry. A corresponding feature in the sample
deposited on Fe is not observed. | 0606542v1 |
2006-12-01 | Epitaxial Co2Cr0.6Fe0.4Al thin films and magnetic tunneling junctions | Epitaxial thin films of the theoretically predicted half metal
Co2Cr0.6Fe0.4Al were deposited by dc magnetron sputtering on different
substrates and buffer layers. The samples were characterized by x-ray and
electron beam diffraction (RHEED) demonstrating the B2 order of the Heusler
compound with only a small partition of disorder on the Co sites. Magnetic
tunneling junctions with Co2Cr0.6Fe0.4Al electrode, AlOx barrier and Co counter
electrode were prepared. From the Julliere model a spin polarisation of
Co2Cr0.6Fe0.4Al of 54% at T=4K is deduced. The relation between the annealing
temperature of the Heusler electrodes and the magnitude of the tunneling
magnetoresistance effect was investigated and the results are discussed in the
framework of morphology and surface order based of in situ STM and RHEED
investigations. | 0612022v1 |
1994-11-21 | A Mass Bound for Spherically Symmetric Black Hole Spacetimes | Requiring that the matter fields are subject to the dominant energy
condition, we establish the lower bound $(4\pi)^{-1} \kappa {\cal A}$ for the
total mass $M$ of a static, spherically symmetric black hole spacetime. (${\cal
A}$ and $\kappa$ denote the area and the surface gravity of the horizon,
respectively.) Together with the fact that the Komar integral provides a simple
relation between $M - (4\pi)^{-1} \kappa A$ and the strong energy condition,
this enables us to prove that the Schwarzschild metric represents the only
static, spherically symmetric black hole solution of a selfgravitating matter
model satisfying the dominant, but violating the strong energy condition for
the timelike Killing field $K$ at every point, that is, $R(K,K) \leq 0$.
Applying this result to scalar fields, we recover the fact that the only black
hole configuration of the spherically symmetric Einstein-Higgs model with
arbitrary non-negative potential is the Schwarzschild spacetime with constant
Higgs field. In the presence of electromagnetic fields, we also derive a
stronger bound for the total mass, involving the electromagnetic potentials and
charges. Again, this estimate provides a simple tool to prove a ``no-hair''
theorem for matter fields violating the strong energy condition. | 9411054v1 |
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