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2004-07-23 | Stability of Fe-based alloys with structure type C6Cr23 | Bulk metallic glass forms when liquid metal alloys solidify without
crystalization. In the search for Iron-based bulk glass-forming alloys of the
metal-metalloid type (Fe-B- and Fe-C-based), crystals based on the structural
prototype C6Cr23 often preempt the amorphous phase. Destabilizing this
competing crystal structure could enhance glass-formability. We carry out
first-principles total energy calculations of enthalpy of formation to identify
third elements that can effectively destabilize C6Cr23. Yttrium appears optimal
among transition metals, and rare earths also are suitable. Atomic size is the
dominant factor. | 0407633v1 |
2004-11-10 | Optical properties of random alloys : Application to Cu_{50}Au_{50} and Ni_{50}Pt_{50} | In an earlier paper [K. K. Saha and A. Mookerjee, Phys. Rev. B 70 (2004) (in
press) or, cond-mat/0403456] we had presented a formulation for the calculation
of the configuration-averaged optical conductivity in random alloys. Our
formulation is based on the augmented-space theorem introduced by one of us [A.
Mookerjee, J. Phys. C: Solid State Phys. 6, 1340 (1973)]. In this communication
we shall combine our formulation with the tight-binding linear muffin-tin
orbitals (TB-LMTO) technique to study the optical conductivities of two alloys
Cu_{50}Au_{50} and Ni_{50}Pt_{50}. | 0411251v1 |
2004-11-19 | Local distribution approach to disordered binary alloys | We study the electronic structure of the binary alloy and (quantum)
percolation model. Our study is based on a self-consistent scheme for the
distribution of local Green functions. We obtain detailed results for the
density of states, from which the phase diagram of the binary alloy model is
constructed, and discuss the existence of a quantum percolation threshold. | 0411516v3 |
2004-12-05 | Surface Induced Order in Liquid Metals and Binary Alloys | Measurements of the surface x-ray scattering from several pure liquid metals
(Hg, Ga, and In) and from three alloys (Ga-Bi, Bi-In, and K-Na) with different
heteroatomic chemical interactions in the bulk phase are reviewed.
Surface-induced layering is found for each elemental liquid metal. The surface
structure of the K-Na alloy resembles that of an elemental liquid metal. Bi-In
displays pair formation at the surface. Surface segregation and a wetting film
are found for Ga-Bi. | 0412110v1 |
2005-01-20 | Dendritic to globular morphology transition in ternary alloy solidification | The evolution of solidification microstructures in ternary metallic alloys is
investigated by adaptive finite element simulations of a general multicomponent
phase-field model. A morphological transition from dendritic to globular growth
is found by varying the alloy composition at a fixed undercooling. The
dependence of the growth velocity and of the impurity segregation in the solid
phase on the composition is analyzed and indicates a smooth type of transition
between the dendritic and globular growth structures. | 0501494v1 |
2005-05-03 | Two-band second moment model for transition metals and alloys | A semi-empirical formalism based on the second moment tight binding approach,
considering two bands is presented for deriving interatomic potentials for
magnetic d-band materials and transition metal alloys. It incorporates an
empirical local exchange interaction, which accounts for magnetic effects
without increasing the computing time required for force evaluation. The
consequences of applying a two-band picture to transition metal alloys and
transition metal impurities is examined, which combined with evidence from {\it
ab initio calculations} leads to some surprisingly simplifying conclusions. | 0505060v1 |
2006-01-26 | Nanowires and Suspended Atom Chains from Metal alloys | We present a study of the elongation and rupture of gold-silver alloy
nanowires. Atomistic details of the evolution were derived from time-resolved
atomic resolution transmission electron microscopy and molecular dynamics
simulations. The results show the occurrence of gold enrichment at the
nanojunction region, leading to a gold-like structural behavior even for alloys
with minor gold content. Our observations have also revealed the formation of
mixed (Au and Ag) linear atomic chains. | 0601617v1 |
2006-02-13 | Portevin-Le Chatelier Effect: a Poisson Process | The Portevin-Le Chatelier (PLC) effect is a kind of plastic instability
observed in many dilute alloys when deformed at certain ranges of strain rate
and temperature. In this letter we establish that the dynamical process
responsible for the PLC effect is Poisson in nature by applying statistical
analysis to the observed experimental data obtained during the PLC effect in a
substitutional alloy, Al-2.5%Mg and in an interstitial alloy, low carbon steel
subjected to uniaxial tensile test at constant imposed strain rate. | 0602313v1 |
2006-11-14 | Probing the Structure and Energetics of Dislocation Cores in SiGe Alloys through Monte Carlo Simulations | We present a methodology for the investigation of dislocation energetics in
segregated alloys based on Monte Carlo simulations which equilibrate the
topology and composition of the dislocation core and its surroundings. An
environment-dependent partitioning of the system total energy into atomic
contributions allows us to link the atomistic picture to continuum elasticity
theory. The method is applied to extract core energies and radii of 60 degrees
glide dislocations in segregated SiGe alloys which are inaccessible by other
methods. | 0611366v1 |
2006-12-30 | Low relaxation rate in a low-Z alloy of iron | The longest relaxation time and sharpest frequency content in ferromagnetic
precession is determined by the intrinsic (Gilbert) relaxation rate \emph{$G$}.
For many years, pure iron (Fe) has had the lowest known value of $G=\textrm{57
Mhz}$ for all pure ferromagnetic metals or binary alloys. We show that an
epitaxial iron alloy with vanadium (V) possesses values of $G$ which are
significantly reduced, to 35$\pm$5 Mhz at 27% V. The result can be understood
as the role of spin-orbit coupling in generating relaxation, reduced through
the atomic number $Z$. | 0701004v1 |
2007-05-04 | Broadening effects due to alloy scattering in Quantum Cascade Lasers | We report on calculations of broadening effects in QCL due to alloy
scattering. The output of numerical calculations of alloy broadened Landau
levels compare favorably with calculations performed at the self-consistent
Born approximation. Results for Landau level width and optical absorption are
presented. A disorder activated forbidden transition becomes significant in the
vicinity of crossings of Landau levels which belong to different subbands. A
study of the time dependent survival probability in the lowest Landau level of
the excited subband is performed. It is shown that at resonance the population
relaxation occurs in a subpicosecond scale. | 0705.0649v1 |
2007-08-14 | Formation of bulk ferromagnetic nanostructured Fe40Ni40P14B6 alloys by metastable liquid spinodal decomposition | Nanostructured Fe40Ni40P14B6 alloys ingots of diameter 3~5 mm could be
synthesised by a metastable liquid state spinodal decomposition method. The
molten Fe40Ni40P14B6 alloy was purified by means of the fluxing technique and
thus a large undercooling could be achieved. For undercooling Delta T > 260 K,
the microstructure of the undercooled specimen had exhibited liquid state
spinodal decomposition in the undercooled liquid state. The microstructure
could be described as two intertwining networks with small grains dispersed in
them. For undercooling Delta T > 290 K, the overall microstructure of the
specimen changed into a granular morphology. The average grain sizes of the
small and large grains are ~ 30 nm and ~ 80 nm, respectively. These prepared
samples are soft magnets with saturation magnetization Bs ~0.744 T. | 0708.1809v1 |
2008-01-15 | A Simple Spin Glass Perspective on Martensitic Shape-memory Alloys | A brief qualitative mapping is given between austenite, tweed and twinned
phases of martensite alloys and corresponding paramagnetic, spin glass and
periodic phases in spin glass alloys. | 0801.2188v2 |
2008-02-15 | Using Alloy to model-check visual design notations | This paper explores the process of validation for the abstract syntax of a
graphical notation. We define an unified specification for five of the UML
diagrams used by the Discovery Method and, in this document, we illustrate how
diagrams can be represented in Alloy and checked against our specification in
order to know if these are valid under the Discovery notation. | 0802.2258v1 |
2008-04-25 | Spectral extrema and Lifshitz tails for non monotonous alloy type models | In the present note, we determine the ground state energy and study the
existence of Lifshitz tails near this energy for some non monotonous alloy type
models. Here, non monotonous means that the single site potential coming into
the alloy random potential changes sign. In particular, the random operator is
not a monotonous function of the random variables. | 0804.4079v2 |
2008-12-03 | Correlation between magnetism and spin-dependent transport in CoFeB alloys | We report a correlation between the spin polarization of the tunneling
electrons (TSP) and the magnetic moment of amorphous CoFeB alloys. Such a
correlation is surprising since the TSP involves s-like electrons close to the
Fermi level (EF), while the magnetic moment mainly arises due to all
d-electrons below EF. We show that probing the s and d-bands individually
provides clear and crucial evidence for such a correlation to exist through s-d
hybridization, and demonstrate the tuneability of the electronic and magnetic
properties of CoFeB alloys. | 0812.0679v1 |
2009-03-23 | Spin-polarized electronic structures and transport properties of Fe-Co alloys | The electrical resistivities of Fe-Co alloys owing to random alloy disorder
are calculated using the Kubo-Greenwood formula. The obtained electrical
esistivities agree well with experimental data quantitatively at low
temperature. The spin-polarization of Fe50Co50 estimated from the conductivity
(86%) has opposite sign to that from the densities of the states at the Fermi
level (-73%). It is found that the conductivity is governed mainly by
s-electrons, and the s-electrons in the minority spin states are less
conductive due to strong scattering by the large densities of the states of
d-electrons than the majority spin electrons. | 0903.3842v1 |
2009-04-04 | An Atom-Pair Bond Theory for the Alloying of Metals | We present an Atom-Pair Bond (APB) theory for the energy of a metallic bond
based on the ideas of covalent bonding proposed by Pauling. An expression is
derived which accurately predicts the signs of the heats of formation of binary
alloys. It also explains the characteristics of the Rajasekharan-Girgis lines
and their ability to predict accurately concomitant and mutually exclusive
structure types in phase diagrams. Evidence is provided to show that the charge
transfer on the atom-pair bond which is central to the present theory decides
the experimentally observed volume changes on alloying. | 0904.0697v1 |
2009-04-10 | Properties of KCo$_2$As$_2$ and Alloys with Fe and Ru: Density Functional Calculations | Electronic structure calculations are presented for KCo$_2$As$_2$ and alloys
with KFe$_2$As$_2$ and KRu$_2$As$_2$. These materials show electronic
structures characteristic of coherent alloys, with a similar Fermi surface
structure to that of the Fe-based superconductors, when the $d$ electron count
is near six per transition metal. However, they are less magnetic than the
corresponding Fe compounds. These results are discussed in relation to
superconductivity. | 0904.1607v1 |
2009-05-12 | Trends in Ferromagnetism in Mn doped dilute III-V alloys from a density functional perspective | Mn doping in dilute III-V alloys has been examined as a route to enhance
ferromagnetic stability. Strong valence band bowing is expected at the dilute
limit, implying a strong modification of the ferromagnetic stability upon
alloying, with even an increase in some cases. Using first principle electronic
structure calculations we show that while codoping with a group V anion
enhances the ferromagnetic stability in some cases when the effects of
relaxation of the lattice are not considered, strong impurity scattering in the
relaxed structure result in a reduction of the ferromagnetic stability. | 0905.1762v1 |
2009-07-01 | Stress orientation of second-phase in alloys: Hydrides in zirconium alloys | A model for precipitation of the plate-shaped second-phase under applied
stress is presented. The precipitates in the matrix-precipitate system are
represented by their local volume fraction and an orientation parameter that
defines the alignment of a precipitate platelet in a given direction. Kinetic
equations, based on diffusion theory and classical nucleation theory, are used
to describe the time evolution of these two parameters. The model is used to
describe the stress orientation of hydrides in Zr-alloys in light of
experiments. | 0907.0131v1 |
2009-08-17 | Origin of the anomalous Slater-Pauling curve in cobalt-manganese alloy clusters | Surprising enhancement of the magnetic moments recently observed in dilute
Co-Mn alloy clusters is explained using ab initio electronic structure
calculations. The calculated magnetic moments generally agree with the reported
experimental data. An equation for calculating the magnetic moments of the
Co-Mn alloy clusters has been derived to correct the deviations predicted by
the rigid-band model and the virtual bound states approximation. A strategy is
proposed to obtain the ground-state structures of the Co-Mn clusters and it was
also put to the test of the experiment. | 0908.2358v1 |
2010-01-27 | A review of the optical properties of alloys and intermetallics for plasmonics | Alternative materials are required to enhance the efficacy of plasmonic
devices. We discuss the optical properties of a number of alloys, doped metals,
intermetallics, silicides, metallic glasses and high pressure materials. We
conclude that due to the probability of low frequency interband transitions,
materials with partially occupied d-states perform poorly as plasmonic
materials, ruling out many alloys, intermetallics and silicides as viable. The
increased probability of electron-electron and electron-phonon scattering rules
out many doped and glassy metals. | 1001.4867v1 |
2010-05-18 | Accurate Ab-initio Predictions of III-V Direct-Indirect Band Gap Crossovers | We report the compositional dependence of the electronic band structure for a
range of III-V alloys. Density functional theory with the PBE functional is
insufficient to mimic the electronic gap energies at different symmetry points
of the Brillouin zone. The HSE hybrid functional with screened exchange
accurately reproduces the experimental band gaps and, more importantly, the
alloy concentration of the direct-indirect gap crossovers for the III-V alloys
studied here: AlGaAs, InAlAs, AlInP, InGaP, and GaAsP. | 1005.3322v1 |
2010-06-19 | Kondo effect in metallic glasses with non-Fermi liquid behavior | By microalloying of Gd atoms with 4f electrons into CuZrAl or MgCuY glassy
alloys, they display a variety of puzzling behaviors such as the characteristic
of Kondo effect. The ground states of the Gd-alloyed systems are determined to
have non-Fermi-liquid characteristics which derive from the strong structural
disorder. The Kondo effect in these glassy alloys is attributed to the strong
structural disorder. The coexistence of the Kondo effect and strong structural
disorder has implications for the understanding the origin of the puzzling
non-Fermi-liquid behavior. | 1006.3826v1 |
2011-02-04 | Initial oxidation of Fe-Al and Fe-Cr-Al alloys: Cr as an alumina booster | The boosting effect of Cr on the growth of the protective alumina scale on
Fe-Al alloys is investigated by x-ray photoelectron spectroscopy. Using low
oxygen pressure the surface chemistry of the alloys is monitored starting from
the first moments of oxidation. Chromium effect on the Fe-Al surface-bulk
exchange is clearly detected by analyzing the measured surface concentrations
within the atomic concentration models. Previous ab initio calculations agree
well with the present experiments. | 1102.0912v1 |
2011-04-02 | Role of Ni-Mn hybridization in magnetism of martensitic state of Ni-Mn-In shape memory alloys | Extended X-ray Absorption Fine Structure (EXAFS) studies on
Ni$_{50}$Mn$_{25+x}$In$_{25-x}$ have been carried out at Ni and Mn K edge as a
function of temperature. Thermal evolution of nearest neighbor Ni-Mn and Mn-Mn
bond distances in the martensitic phase give a clear evidence of a close
relation between structural and magnetic degrees of freedom in these alloys. In
particular, the study highlights the role of Ni 3d - Mn 3d hybridization in the
magnetism of martensitic phase of these alloys. | 1104.0297v1 |
2011-07-11 | Exchange bias effect in alloys and compounds | The phenomenology of exchange bias effects observed in structurally
single-phase alloys and compounds but composed of a variety of coexisting
magnetic phases such as ferromagnetic, antiferromagnetic, ferrimagnetic,
spin-glass, cluster-glass and disordered magnetic states are reviewed. The
investigations on exchange bias effects are discussed in diverse types of
alloys and compounds where qualitative and quantitative aspects of magnetism
are focused based on macroscopic experimental tools such as magnetization and
magnetoresistance measurements. Here, we focus on improvement of fundamental
issues of the exchange bias effects rather than on their technological
importance. | 1107.1978v1 |
2011-12-02 | Mooij Rule and Weak Localization | It has been shown that the observed correlation between the resistivity
$\rho$ of high-resistive metallic alloys and the sign of the temperature
derivative $d\rho/dT$ can be explained by taking into account the weak
localization. This correlation is known as Mooij rule: the derivative
$d\rho/dT$ is negative for alloys with resistivity in the range of
$300\div150\,\mu\Omega\cdot$cm, which corresponds to the electron mean free
path about the interatomic distance; however, this derivative is positive for
alloys with lower resistivity. | 1112.0429v1 |
2012-06-29 | Ab-initio calculations for structural properties of Zr-Nb alloys | Ab-initio calculations for the structural properties of Zr-Nb alloys at
different values of the niobium concentration are done at zero temperature.
Different cases for Zr-Nb alloys with unit cells having BCC and HCP structures
are considered. Optimal values of the lattice constants are obtained. Critical
value for the niobium concentration corresponding to the structural
transformation HCP \rightarrow BCC at zero temperature is determined.
Electronic densities of states for two different structures with niobium
concentrations 12.5% and 25% having HCP and BCC structures, accordingly, are
studied. | 1206.7035v2 |
2012-07-07 | Bounded Model Checking of Temporal Formulas with Alloy | Alloy is formal modeling language based on first-order relational logic, with
no specific support for specifying reactive systems. We propose the usage of
temporal logic to specify such systems, and show how bounded model checking can
be performed with the Alloy Analyzer. | 1207.2746v3 |
2012-07-31 | Ge condensation under SiGe oxidization: from Molecular Dynamics simulation to one-dimensional analytic modeling | Oxidization of a dilute Si(Ge) alloy is modeled using an original protocol
based on molecular dynamics simulation and rules for the oxygen insertions.
These rules, deduced from ab-initio calculations, favor the formation of SiO_2
against GeO_2 oxide which leads to segregation of Ge atoms into the alloy
during the oxidization front advance. Ge condensation is then observed close to
the SiO_2/Ge interface due to the strain induced by oxydization in this region.
From the analysis of the simulation process, we propose a one-dimensional
description of Ge condensation wich perfectly reproduces the evolution of the
Ge concentration during oxidization of the SiGe alloy. | 1207.7238v1 |
2012-10-16 | Degree of order dependence on magnetocrystalline anisotropy in bct FeCo alloys | We investigate the magnetocrystalline anisotropy (MCA) energy of tetragonal
distorted FeCo alloys depending on the degree of order by first-principles
electronic structure calculation combined with the coherent potential
approximation. The obtained results indicate that the MCA energy of FeCo alloys
strongly depends on the degree of order under optimal conditions, where the
axial ratio of the bct structure is 1.25 and the composition is Fe0.5Co0.5. We
find that the modification of the electronic structure resulting from electron
scattering by chemical disorder has a considerable influence on the MCA under
these conditions. | 1210.4386v1 |
2012-10-18 | Lifshitz transition in Kondo alloys | We study the low energy states of Kondo alloys as function of the magnetic
impurity concentration per site, x, and the conduction electron average site
occupation, nc. Using two complementary approaches, the mean-field coherent
potential approximation and the strong coupling limit, we identify and
characterize two different Fermi liquid regimes. We propose that both regimes
are separated by a Lifshitz transition at x = nc. Indeed, we predict a
discontinuity of the number of quasiparticles which are enclosed in the Fermi
surface. This feature could provide a scenario for the non-Fermi liquid
properties that were recently observed in Kondo alloy systems around x = nc. | 1210.5105v2 |
2013-09-19 | On the Effect of Micro-alloying on the Mechanical Properties of Metallic Glasses | "Micro-alloying", referring to the addition of small concentration of a
foreign metal to a given metallic glass, was used extensively in recent years
to attempt to improve the mechanical properties of the latter. The results are
haphazard and nonsystematic. In this paper we provide a microscopic theory of
the effect of micro-alloying, exposing the delicate consequences of this
procedure and the large parameter space which needs to be controlled. In
particular we consider two very similar models which exhibit opposite trends
for the change of the shear modulus, and explain the origins of the difference
as displayed in the different microscopic structure and properties. | 1309.4998v1 |
2014-02-07 | An Extended Hückel Study of the Electronic Properties of III-V Compounds and Their Alloys | In this work, we performed tight binding calculations of the electronic
structure of III-V semiconductors compounds and their alloys based on the
Extended H\"uckel Theory (EHT), where the H\"uckel parameters for the binary
compounds were generated following a simulated annealing procedure. In
particular, this article is focused on the dependency between band gap and the
applied pressure and also the alloy composition. | 1402.1610v1 |
2014-12-05 | Viscosity and relaxation processes of the liquid become amorphous Al-Ni-REM alloys | The temperature and time dependencies of viscosity of the liquid alloys,
Al87Ni8Y5, Al86Ni8La6, Al86Ni8Ce6, and the binary Al-Ni and Al-Y melts with Al
concentration over 90 at.% have been studied. Non-monotonic relaxation
processes caused by destruction of nonequilibrium state inherited from the
basic-heterogeneous alloy have been found to take place in Al-Y, Al-Ni-REM
melts after the phase solid-liquid transition. The mechanism of nonmonotonic
relaxation in non-equilibrium melts has been suggested. | 1412.2010v1 |
2015-02-16 | Superconductivity in the elements, alloys and simple compounds | We give a brief review of superconductivity at ambient pressure in elements,
alloys, and simple three-dimensional compounds. Historically these were the
first superconducting materials studied, and based on the experimental
knowledge gained from them the BCS theory of superconductivity was developed in
1957. Extended to include the effect of phonon retardation, the theory is
believed to describe the subset of superconducting materials known as
`conventional superconductors', where superconductivity is caused by the
electron-phonon interaction. These include the elements, alloys and simple
compounds discussed in this article and several other classes of materials
discussed in other articles in this Special Issue. | 1502.04724v1 |
2015-05-01 | Entropy and diffuse scattering: comparison of NbTiVZr and CrMoNbV | The chemical disorder intrinsic to high entropy alloys inevitably creates
diffuse scattering in their x-ray or neutron diffraction patterns. Through
first principles hybrid Monte Carlo/molecular dynamics simulations of two BCC
high entropy alloy forming compounds, CrMoNbV and NbTiVZr, we identify the
contributions of chemical disorder, atomic size and thermal fluctuations to the
diffuse scattering. As a side benefit, we evaluate the reduction in entropy due
to pair correlations within the framework of the cluster variation method.
Finally, we note that the preference of Ti and Zr for HCP structures at low
temperature leads to a mechanical instability reducing the local BCC character
of NbTiVZr, while preserving global BCC symmetry. | 1505.00243v1 |
2015-06-08 | Impact of micro-alloying on the plasticity of Pd-based Bulk Metallic Glasses | Micro-alloying was performed using additions of Co and Fe to monolithic
Pd40Ni40P20 bulk metallic glass to study selectively the influence on the
plastic behavior in uniaxial compression and three-point bending tests. The
corresponding Poisson's ratios were determined by ultrasonic measurements. The
microstructure of the individual bulk metallic glasses was characterized by
electron microscopy, X-ray diffraction and calorimetry. A plastic strain of 13%
was found for the Co addition (1 at.%), whereas the Fe addition (0.6 at.%) led
to immediate failure after reaching the elastic limit. Surprisingly, the
plasticity is not reflected by the high Poisson's ratio of 0.4 since it
remained unaffected by the minor alloying. | 1506.02413v1 |
2015-06-18 | An alloy calculation of pure antiferromagnetic NiO | We use the many techniques of alloy theory to study antiferromagnetic NiO,
considered as an alloy of spin-up and spin-down Ni atoms. The questions are:
the true antiferromagnetic ground state and the possibility of obtaining
ferrimagnetic configurations. Further we use the GGA/LDA- 1/2 technique to
investigate the electronic excitation spectrum. We found two valence bands and
band gaps, of 4.0eV consistent with bremsstrahlung-isochromat-spectroscopy
(BIS) result, and 1.2eV consistent with the known 10Dq value for the Ni++ ion.
The features of a Mott insulator are presented without recurring to an
electron-pair correlation. | 1506.05778v2 |
2016-02-16 | Eutectic Growth in Two-Phase Multicomponent Alloys | A theory of two-phase eutectic growth for a multicomponent alloy is
presented. This theory employs the thermodynamic equilibrium at the
solid/liquid interface and thus makes it possible to use standard CALPHAD
databases to determine the effects of multicomponent phase equilibrium on
eutectic growth. Using the same hypotheses as the Jackson Hunt theory, we find
that the growth law determined for binary alloys in the Jackson Hunt theory can
be generalized to systems with N elements. In particular, a new model is
derived from this theory for ternary two-phase eutectics. The use of this model
to predict the eutectic microstructure of systems is discussed. | 1602.04931v1 |
2016-02-17 | Density and Glass Forming Ability in Amorphous Atomic Alloys: the Role of the Particle Softness | A key property of glass forming alloys, the anomalously small volume
difference with respect to the crystal, is shown to arise as a direct
consequence of the soft repulsive potentials between metals. This feature of
the inter-atomic potential is demonstrated to be responsible for a significant
component of the glass forming ability of alloys due to the decrease in the
enthalpy of fusion and the associated depression of the freezing point. | 1602.05294v1 |
2016-05-17 | Effects of spin fluctuation on the magnetic anisotropy constant of itinerant electron magnets | In the disordered local moment picture, we calculated the magnetization (M)
and magnetic anisotropy energy (MAE) of FePt, CoPt, and MnAl ordered alloys and
body-centered tegragonal FeCo (bct-FeCo) disordered alloy, assuming spatially
fluctuated spin configurations at finite temperatures. All alloys exhibit the
relation K1(T)/K1(0)=(M(T)/M(0))^n with the exponent (n) around 2. This is
consistent with the two-ion anisotropy model, in contrast to the usual
single-ion anisotropy model exhibiting n=3. Because these systems have
different mechanisms of MAE, we suggest that this relation is a general rule
for itinerant electron systems. | 1605.05058v1 |
2016-05-17 | Interfacial structure and electronic properties of AFM/FM magnetic multilayer revealed by Scanning Tunneling Microscope | Mixing of atoms at the interface was studied for Mn/Fe magnetic
hetero-epitaxial layers on Cu(001) by scanning tunneling
microscopy/spectroscopy. The formation of a surface alloy was observed when the
Mn layer was thinner than 3 atomic layers. From the fourth layer, Fe
segregation is suppressed, and a pure Mn surface appears. Accordingly,
spectroscopic measurements revealed the electronic difference between the
surface alloy and Mn layers. The surface electronic structure of the fourth Mn
layer is slightly different from that of the fifth layers, which is attributed
to the hybridization of the fourth layer with the underneath Fe-Mn alloy. | 1605.05086v1 |
2016-06-30 | Single-Crystal Growth of a FeCoCrMnAl High-Entropy Alloy | We have grown a single crystal of the equiatomic FeCoCrMnAl high-entropy
alloy. The crystal was grown by means of the Bridgman technique and is about 1
cm in diameter and 6.6 cm in length. X-ray Laue images taken at various
positions on the surface are sharp and mutually consistent, reflecting its
single-crystal nature. We thus report on the first successful growth of a
single crystal of a high-entropy alloy with a volume of the order of cubic
centimeters. The material has a microstructure consisting of B2 inclusions in a
body-centered cubic matrix on the 10 nm length scale. | 1606.09476v1 |
2016-07-27 | Alloy-like behaviour of the thermal conductivity of non-symmetric superlattices | In this work, we show a phenomenological alloy-like fit of the thermal
conductivity of (A)d1:(B)d2 superlattices with d1 /= d2, i.e. non-symmetric
structure. The presented method is a generalization of the Norbury rule of the
summation of thermal resistivities in alloy compounds. Namely, we show that
this approach can be also extended to describe the thermal properties of
crystalline and ordered-system composed by two or more elements, and, has a
potentially much wider application range. Using this approximation we estimate
that the interface thermal resistance depends on the period and the ratio of
materials that form the superlattice structure | 1607.08017v2 |
2016-08-09 | Formation of ordered and disordered interfacial films in immiscible metal alloys | Atomistic simulations are used to study segregation-induced intergranular
film formation in Cu-Zr and Cu-Nb alloys. While Cu-Zr forms structurally
disordered or amorphous films, ordered films comprised of a second phase
usually precipitate in Cu-Nb, with a critical nucleation size of ~1 nm below
which the ordered phase cannot form. While the ordered film is retained at high
temperature for a low energy {\Sigma}11 (113) boundary, a disordering
transition is observed for a high energy {\Sigma}5 (310) boundary at low dopant
concentrations. Finally, the effect of free surfaces on dopant segregation and
intergranular film formation is investigated for both alloys. | 1608.02981v2 |
2017-01-10 | The giant effect of magnetic ordering on a sound velocity in a sigma-Fe55Cr45 alloy | We studied atomic dynamics of sigma-Fe(100-x)Cr(x) (x=45 and 49.5) alloys
using nuclear inelastic scattering of synchrotron radiation. For the
sigma-Fe55Cr45 alloy, the derived reduced iron-partial density of phonon states
reveal a huge difference in the low-energy region between magnetic and
paramagnetic states. The latter implies a ca.36% increase of the sound velocity
in the magnetic phase, which testifies to a magnetically-induced hardening of
the lattice. | 1701.02467v1 |
2017-01-31 | A tentative model for estimating the compressibility of rock-salt AgCl_{x}Br_{1-x} alloys | Ab initio detailed calculations of the elastic properties of AgCl_{x}Br_{1-x}
alloys recently appeared using density-functional perturbation theory and
employing the virtual crystal approximation or by means of the full potential
linearized augmented plane wave method. Here, we suggest a simple theoretical
model that enables the estimation of the isothermal compressibility of these
alloys in terms of the elastic data of end members alone. The calculated values
are in satisfactory agreement with the experimental ones. The present model
makes use of an early suggestion that interconnects the Gibbs energy for the
formation and/or migration of defects in solids with bulk properties. | 1701.08998v1 |
2017-07-15 | Monte Carlo Study of the Crystalline and Amorphous NaK Alloy | Metropolis Monte Carlo simulations of the eutectic NaK alloy are performed
using the Second Moment Approximation (SMA) model potential across a wide range
of temperatures at constant pressure. The alloy structure and thermodynamics
are analyzed along with the atomic level structures using a variety of
structure identification methods. Both enthalpy and density are followed along
an annealing process that reveals a clear melting point around 250 K. At lower
temperatures, two thermodynamic branches are identified as crystalline and
amorphous solids. | 1707.04799v1 |
2018-03-21 | A single-phase bcc high-entropy alloy in the refractory Zr-Nb-Ti-V-Hf system | We report on the production and characterization of a high-entropy alloy in
the refractory Zr-Nb-Ti-V-Hf system. Equiatomic ingots were produced by arc and
levitation melting, and were subsequently homogenized by high-temperature
annealing. We obtained a coarse-grained, single-phase high-entropy alloy, with
a homogeneous distribution of the constituting elements. The phase is a
chemically disordered solid solution, based on a bcc lattice with a lattice
parameter of 0.336(5) nm. | 1803.07798v1 |
2018-05-18 | Comments on 'Revisiting building block ordering of long-period stacking ordered structures in Mg-Y-Al alloys' | In a recent paper, Zhang et al. [Acta Materialia 152 (2018) 96] studied the
in-plane ordering of the long-period stacking ordered (LPSO) structure in
Mg-Al-Y alloys. In addition to the well-known L12 type building cluster, they
proposed three new types of metastable building clusters. However, we will show
that these new types of building clusters are caused by the superimposition of
L12 type clusters located in different domains. In addition, the experimental
evidence for domain structures in a similar alloy system Mg-Al-Gd is provided. | 1805.07110v1 |
2018-11-08 | The impact of nanoscale compositional variation on the properties of amorphous alloys | The atomic distribution in amorphous FeZr alloys is found to be close to
random, nevertheless, the composition can not be viewed as being homogenous at
the nm-scale. The spatial variation of the local composition is identified as
the root of the unusual magnetic properties in amorphous Fe$_{1-x}$Zr$_{x}$
alloys. The findings are discussed and generalised with respect to the physical
properties of amorphous and crystalline materials. | 1811.03354v2 |
2018-07-31 | Extrinsic-Intrinsic Crossover of the Spin Hall Effect Induced by Alloying | We report the observation of the crossover between the extrinsic and
intrinsic spin Hall effect induced by alloying. We found that the spin Hall
angle, the ratio of the spin Hall conductivity to the electric conductivity,
changes drastically by tuning the composition of Au-Cu alloy. The spin Hall
angle changes the sign only in a limited range of the Cu concentration due to
the extrinsic skew scattering, while the intrinsic contribution becomes
dominant with increasing the Cu concentration. This observation provides
essential information for fundamental understanding of spin-orbit physics. | 1807.11821v1 |
2018-12-13 | Tailoring the magnetic properties of nanocrystalline Cu-Co alloys prepared by high-pressure torsion and isothermal annealing | In this study, severe plastic deformation by high pressure torsion is used as
a fabrication method for nanocrystalline magnetic CuCo alloys in bulk
quantities. By subsequent isothermal annealing, phase separation of the
supersaturated solid solutions can be obtained. The magnetic properties of the
as-processed and annealed materials have been studied systemically and
correlated to the evolving nanostructures investigated in detail by
transmission electron microscopy and atom probe tomography. By additional high
pressure torsion deformation at liquid nitrogen temperature the magnetic
properties of the Cu74Co26 alloy can be further tuned. | 1812.05387v1 |
2018-12-16 | Dirac cone in a non-honeycomb surface alloy | We demonstrate unexpected occurrence of linear bands resembling Dirac cone at
the zone-center of Au$_2$Sn surface alloy with $\left( \begin{smallmatrix}
2&1\\ 1&3 \end{smallmatrix} \right)$ surface structure formed by deposition of
about 0.9 ML Sn on Au(111) at elevated temperature. The surface exhibits an
oblique symmetry with unequal lattice constants making it the first two
dimensional surface alloy to exhibit Dirac cone with a non-honeycomb lattice. | 1812.06508v1 |
2019-02-28 | Ab initio vibrational free energies including anharmonicity for multicomponent alloys | A density-functional-theory based approach to efficiently compute numerically
exact vibrational free energies - including anharmonicity - for chemically
complex multicomponent alloys is developed. It is based on a combination of
thermodynamic integration and a machine-learning potential. We demonstrate the
performance of the approach by computing the anharmonic free energy of the
prototypical five-component VNbMoTaW refractory high entropy alloy. | 1902.11230v1 |
2019-06-15 | Studying Internal Compositions of Binary Alloy Pd-Rh Nanoparticles Using Bragg Coherent Diffraction Imaging | Bragg coherent diffraction imaging (BCDI), the well-established technique for
imaging internal strain of nanoparticles, was used to image the internal
compositional distribution of binary alloys in thermal equilibrium. The images
experimentally obtained for Pd-Rh alloy nanoparticles are presented and
discussed. The direct correspondence between the lattice strain and the
compositional deviation is discussed in the derivation of the BCDI displacement
field aided by illustrations. | 1906.06487v2 |
2020-05-21 | Cluster Dynamics Modeling of Niobium and Titanium Carbide Precipitates | Kinetics of niobium and titanium carbide precipitates in iron has been
simulated with cluster dynamics. The simulations, carried out in austenite and
ferrite for niobium carbides, respectively in austenite for titanium carbide,
were analyzed for dependency on temperature, solute concentration, and initial
cluster distribution. The results are presented for different temperatures and
solute concentrations and compared to available experimental data. They show
little impact of initial cluster distribution beyond a certain relaxation time
and that highly dilute alloys with only monomers present a significantly
different behavior than less dilute alloys or alloys with different initial
cluster distribution. | 2005.10574v2 |
2017-02-16 | A simple descriptor and predictor for the stable structures of two-dimensional surface alloys | Predicting the ground state of alloy systems is challenging due to the large
number of possible configurations. We identify an easily computed descriptor
for the stability of binary surface alloys, the effective coordination number
$\mathscr{E}$. We show that $\mathscr{E}(M)$ correlates well with the enthalpy
of mixing, from density functional theory (DFT) calculations on
$M_x$Au$_{1-x}$/Ru [$M$ = Mn or Fe]. At each $x$, the most favored structure
has the highest [lowest] value of $\mathscr{E}(M)$ if the system is
non-magnetic [ferromagnetic]. Importantly, little accuracy is lost upon
replacing $\mathscr{E}(M)$ by $\mathscr{E}^*(M)$, which can be quickly computed
without performing a DFT calculation, possibly offering a simple alternative to
the frequently used cluster expansion method. | 1702.04968v1 |
2017-06-12 | Low temperature pressureless immediate sintering of novel nanostructured WC/Co/NiCrSiB-alloy cemented carbide | A novel nanostructured cemented carbide formed from WC-5%Co-20%BNi2 brazing
alloy is described. During sintering, the BNi2 alloy is infiltrated into a
green compact of WC-5%Co at 1050-1100 {\deg}C for 2-60 minutes. Perfect wetting
behavior and a zero contact angle are achieved after only 40 s. Relative
densities of 98.5% and 100% and microhardness values of above 1500HV1 and
1800HV1 are obtained after 2 and 30 minutes, respectively. A change in mean
particle size of about 600 nm in the precursor to a bimodal distribution of
350-400 nm and 10-20 nm is explained by a solution/reprecipitation mechanism. | 1706.03426v1 |
2017-06-27 | Influence of Heat Treatment on the Corrosion Behavior of Purified Magnesium and AZ31 Alloy | Magnesium and its alloys are ideal for biodegradable implants due to their
biocompatibility and their low-stress shielding. However, they can corrode too
rapidly in the biological environment. The objective of this research was to
develop heat treatments to slow the corrosion of high purified magnesium and
AZ31 alloy in simulated body fluid at 37{\deg}C. Heat treatments were performed
at different temperatures and times. Hydrogen evolution, weight loss, PDP, and
EIS methods were used to measure the corrosion rates. Results show that heat
treating can increase the corrosion resistance of HP-Mg by 2x and AZ31 by 10x. | 1706.08663v1 |
2019-08-28 | BAlGaN alloys nearly lattice-matched to AlN for efficient UV LEDs | The lattice mismatch between AlGaN and AlN substrates limits the design and
efficiency of UV LEDs, but it can be mitigated by the co-incorporation of
boron. We employ hybrid density functional theory to investigate the
thermodynamic, structural, and electronic properties of BAlGaN alloys. We show
that BAlGaN can lattice match AlN with band gaps that match AlGaN of the same
gallium content. We predict that BAlGaN emits transverse-electric polarized for
gallium content of ~45% or more. Our results indicate that BAlGaN alloys are
promising materials for higher efficiency UV optoelectronic devices on bulk AlN
substrates. | 1908.10814v1 |
2019-09-14 | Superconductivity in High-Entropy-Alloy Telluride AgInSnPbBiTe5 | A polycrystalline sample of the high-entropy-alloy-type telluride
AgInSnPbBiTe5 was synthesized using high-pressure synthesis. Superconductivity
with a transition temperature (Tc) of 2.6 K was observed in AgInSnPbBiTe5.
Elemental and structural analyses revealed that five metals are mixed in a
metal site of an NaCl-type structure. Since AgInSnPbBiTe5 has a cation site
(Ag, In, Sn, Pb, and Bi) and an anion site (Te), this is the first example of a
high-entropy-alloy (HEA) pseudo-binary superconductor. | 1909.06505v2 |
2020-02-25 | Electronic and vibrational spectroscopy of miscible MgO-ZnO ternary alloys | The ordered structure of MgO-ZnO alloy system is a versatile tunable optical
material promising for diverse optoelectronic applications. However, isovalent
and isostructural alloy compositions of MgO-ZnO are generally unstable at
ambient conditions. Using state-of-the-art \textit{ab initio} evolutionary
simulations, we predict and study the properties of stable phases of MgO-ZnO.
We establish the dynamical stability of the predicted crystal structures
through the phonon and Raman spectroscopy. Detailed analyses of two of the most
stable structures reveal highly tunable properties that could be explored for
photonic and optical applications. | 2002.10660v1 |
2020-07-14 | Dynamical stability of two-dimensional metals in the periodic table | We study the dynamical stability of elemental two-dimensional (2D) metals
from Li to Pb by calculating the phonon band structure from first principles,
where 2D structures are assumed to be planer hexagonal, buckled honeycomb, and
buckled square lattice structures. We show the relationship between the
stability of 2D structures and that of three-dimensional structures. This
provides a material design concept for alloys, where the similarity with regard
to the stable 2D structures, rather than the energetic stability of alloy, is
important to yield dynamically stable alloys. | 2007.06774v1 |
2020-10-13 | Modeling evolution of composition patterns in a binary surface alloy | Evolution of composition patterns in the annealed, single-crystal surface
alloy film is considered in the presence of the spinodal decomposition, the
compositional stress and the diffusion anisotropy. While the former two effects
contribute to overall phase separation, the anisotropy, correlated with the
surface crystallographic orientation, guides the in-plane formation and
orientation of a pattern. The impacts of the anisotropy parameters on patterns
are systematically computed for [110], [100], and [111]-oriented fcc cubic
alloy surfaces. | 2010.06838v1 |
2021-01-20 | Generalization properties of restricted Boltzmann machine for short-range order | The restricted Boltzmann machine (RBM) is used to investigate short-range
order in binary alloys. The network is trained on the data collected by Monte
Carlo simulations for a simple Ising-like binary alloy model and used to
calculate the Warren--Cowley short-range order parameter and other
thermodynamic properties. We demonstrate that RBM not only reproduces the order
parameters for the alloy concentration at which it was trained, but can also
predict them for any other concentrations. | 2101.08089v5 |
2021-02-19 | Spinodal decomposition stabilizes plastic flow in a nanocrystalline Cu-Ti alloy | A combination of high strength and reasonable ductility has been achieved in
a copper-1.7 at.%titanium alloy deformed by high-pressure torsion. Grain
refinement and a spinodal microstructure provided a hardness of 254 +/- 2 HV ,
yield strength of 800 MPa and elongation of 10%. The spinodal structure
persisted during isothermal ageing, further increasing the yield strength to
890MPa while retaining an elongation of 7%. This work demonstrates the
potential for spinodal microstructures to overcome the difficulties in
retaining ductility in ultra-fine grained or nanocrystalline alloys, especially
upon post-deformation heating where strain softening normally results in
brittle behavior. | 2102.09906v1 |
2021-02-23 | Structure and hardness of in situ synthesized nano-oxide strengthened CoCrFeNi high entropy alloy thin films | In this study, we report on face-centered cubic structured CoCrFeNi
high-entropy alloy thin films with finely dispersed nano-oxide particles which
are formed by internal oxidation. Analytical scanning transmission electron
microscopy imaging found that the particles are Cr2O3. The oxide particles
contribute to the hardening of the film increasing its hardness by 14% compared
to that of the film without precipitates, through the Orowan-type strengthening
mechanism. Our novel approach paves the way to design medium- and high-entropy
alloys with high strength by making use of oxide phases. | 2102.11950v1 |
2021-04-29 | Unit Cell Volume, and Lattice Parameter of Cubic High Entropy Alloys | An equation has been derived to predict unit cell volume of high entropy
alloys, HEA, by two different methods. Both treatments led to the same
equation. For cubic HEA lattice parameters were calculated. The predicted
lattice parameters were compared with those reported for 68 HEAs. Lattice
parameters were also calculated using the equivalent of Vegards law for these
alloys. Average errors were 0.52, and 0.42 when Vegards law, and the equation
derived in this work were used, respectively. | 2104.14541v1 |
2022-01-10 | Boron based new high entropy alloy superconductor Mo0.11W0.11V0.11Re0.34B0.33 | Superconducting high entropy alloys (HEAs) are new members of disordered
superconductors. We report the synthesis and investigation of a new
superconducting high entropy alloy
Mo$_{0.11}$W$_{0.11}$V$_{0.11}$Re$_{0.34}$B$_{0.33}$ (MWVRB). It crystallized
in the tetragonal CuAl$_2$ crystal structure with space group (I4/$mcm$).
Comprehensive transport, magnetization and heat capacity measurements confirmed
bulk type-II superconductivity having transition temperature T$_{C}$ = 4.0 K.
The low temperature electronic specific heat suggests a fully gapped
superconducting state in weak coupling limit. | 2201.03300v1 |
2022-07-12 | A new palladium alloy with near-ideal hydrogen storage performance | Hydrogen-based fuels demand high-density storage that can operate under
ambient temperatures. Pd and its alloys are the most investigated metal
hydrides for hydrogen fuel cell applications. This study presented an
alternative Pd alloy for hydrogen storage that can store and release hydrogen
at room temperature. The surface of the most studied Pd (110) was modified with
Au and Rh so that the hydrogen adsorption energy was 0.49 eV and the release
temperature was 365 K. Both values are quite near to the optimum values for the
adsorption energy and release temperature of a hydrogen fuel cell in real-world
usage. | 2207.05330v1 |
2022-09-01 | High-Entropy Grain Boundaries | Do high-entropy alloys and ceramics have their grain boundary (GB)
counterparts? As the concept of high-entropy grain boundaries (HEGBs) was
initially proposed in 2016, this article provides the first complete and
rigorous discussion of the underlying interfacial thermodynamics. A simplified
segregation model can illustrate both GB and bulk high-entropy effects, which
reduce GB energy with increasing temperature for saturated multicomponent
(conventional and high-entropy) alloys. HEGBs can be utilized to stabilize
nanocrystalline alloys at high temperatures via thermodynamic and kinetic
effects. GB structural disordering and transitions offer further opportunities
to attain higher effective GB entropies. Future perspective is discussed. | 2209.00756v1 |
2023-01-31 | On the miscibility gap in tungsten-based alloys | In this work we establish an approach to model miscibility gaps of alloys
using statistical physics, lattice dynamics from first-principles calculations.
We carefully calculate the entropy to include all processes introducing
disorder to the system, i.e., combining the electronic, phononic, and
configuration entropies. Furthermore we present our algorithm for generating
Special Quasirandom Structures (SQS). We model the miscibility gap in tungsten
- chromium and tungsten - molybdenum systems, obtaining the agreement with the
experimental data. Furthermore, we propose an enhancement for the
tungsten-chromium W$_{70}$Cr$_{30}$ alloy with tantalum and hafnium, leading to
the modified stabilization temperatures $T_S$, where the solid solution is
miscible. | 2301.13588v1 |
2023-02-23 | Formation enthalpies of Al-Mn-Pd and the structure of the $i$-AlMnPd quasicrystal | This paper reports formation enthalpies of phases in the Al-Mn-Pd ternary
alloy system as calculated from first principles using electronic density
functional theory. We consider all crystal structures as reported in the
assessed phase diagrams of the ternary and its binary alloy subsystems (Al-Mn,
Al-Pd, and Mn-Pd), as well as additional reported or hypothetical structures.
Icosahedral and decagonal quasicrystalline approximants are among the
structures that we predict to be stable, or nearly so. Our results suggest the
need for careful experimental reexamination of phase stability in each of the
alloy systems, in tandem with further efforts to refine crystallographic and
ab-initio structures. | 2302.12110v2 |
2023-05-30 | Superconductivity with high upper critical field in Ta-Hf Alloys | High upper-critical field superconducting alloys are required for
superconducting device applications. In this study, we extensively
characterized the structure and superconducting properties of alloys Ta$_{x}$
Hf$_{1-x}$ (x = 0.2, 0.4, 0.5, 0.6 and 0.8). The substitution of Hf (T$_{C}$ =
0.12 K, type-I superconductor) with Ta (T$_{C}$ = 4.4 K, type-I superconductor)
shows an anomalous enhancement of T$_{C}$ with variation of composition.
Interestingly, all compositions exhibited strongly coupled bulk type-II
superconductivity with a high upper critical field. In particular, for
compositions x = 0.2, and 0.4, the upper critical field (H$_{C2}$) approached
the Pauli limiting field. | 2305.19253v1 |
2023-10-23 | Simulation of adaptive feedforward control for magnetic alloy cavity | The upgrade plan of the China Spallation Neutron Source aims to enhance the
beam power from 100 kW to 500 kW. To achieve this, the plan involves
incorporating three new magnetic alloy cavities while maintaining the existing
system to enable double harmonic acceleration. As a consequence of the
increased current intensity, the beam loading effect will be significantly
amplified, presenting a considerable challenge for the low-level RF control
system of the magnetic alloy cavity. To address this challenge, an adaptive
feedforward algorithm has been developed to enable optimal control. In
addition, comprehensive simulations of the algorithm have been successfully
conducted to validate its. | 2310.15208v1 |
2023-10-25 | AFLOW for alloys | Many different types of phases can form within alloys, from highly-ordered
intermetallic compounds, to structurally-ordered but chemically-disordered
solid solutions, and structurally-disordered (i.e. amorphous) metallic glasses.
The different types of phases display very different properties, so predicting
phase formation is important for understanding how materials will behave. Here,
we review how first-principles data from the AFLOW repository and the aflow++
software can be used to predict phase formation in alloys, and describe some
general trends that can be deduced from the data, particularly with respect to
the importance of disorder and entropy in multicomponent systems. | 2310.16769v1 |
2024-02-20 | Electron-phonon coupling in ferromagnetic Fe-Co alloys from first principles | We calculate from first principles the electron-phonon coupling strength in
ferromagnetic iron-cobalt Fe$_{1-x}$Co$_x$ alloys for compositions ranging from
$x=0$ to $x=0.75$. We find strong, spin-dependent variation of the
electron-phonon coupling strength with alloy composition. The minimum of the
electron-phonon interaction is found near the composition $x=0.25$. We analyze
the variation of the electron-phonon interaction with composition, as a
function of electron spin, density of states, electron-phonon matrix elements,
and phonon frequencies. Our results are in good qualitative agreement with
magnetization dynamics experiments. | 2402.12650v1 |
1997-11-24 | On the phonon-induced superconductivity of disordered alloys | A model of alloy is considered which includes both quenched disorder in the
electron subsystem (``alloy'' subsystem) and electron-phonon interaction. For
given approximate solution for the alloy part of the problem, which is assumed
to be conserving in Baym's sense, we construct the generating functional and
derive the Eliashberg-type equations which are valid to the lowest order in the
adiabatic parameter.The renormalization of bare electron-phonon interaction
vertices by disorder is taken into account consistently with the approximation
for the alloy self-energy. For the case of exact configurational averaging the
same set of equations is established within the usual T-matrix approach. We
demonstrate that for any conserving approximation for the alloy part of the
self-energy the Anderson's theorem holds in the case of isotropic singlet
pairing provided disorder renormalizations of the electron-phonon interaction
vertices are neglected. Taking account of the disorder renormalization of the
electron-phonon interaction we analyze general equations qualitatively and
present the expressions for $T_{c}$ for the case of weak and intermediate
electron-phonon coupling. Disorder renormalizations of the logarithmic
corrections to the effective coupling, which arise when the effective
interaction kernel for the Cooper channel has the second energy scale, as well
as the renormalization of the dilute paramagnetic impurity suppression are
discussed. | 9711244v1 |
2000-01-19 | Fermi surface origin of the interrelationship between magnetocrystalline anisotropy and compositional order in transitional metal alloys | Recently, we outlined a scheme to investigate the effects of compositional
order on the magnetocrystalline anisotropy of alloys from a first-principles
electronic structure point of view \{Phys. Rev. Lett. {\bf 83}, 5369 (1999)\}
and showed that compositional order enhances the magnitude of
magnetocrystalline anisotropy energy (MAE) of Co$_{0.5}$Pt$_{0.5}$ alloy by
some two orders of magnitude as well as affecting the equilibrium magnetization
direction. Here we describe our scheme in detail and present an in-depth study
of the effect by demonstrating its Fermi surface origin. In
Co$_{0.25}$Pt$_{0.75}$ alloy we find that the perfect $ L1_2 $ structure has a
very small MAE whereas imposition of directional order enhances the MAE by two
orders of magnitude. We also present the effect of lattice distortion
(tetragonalization) on the MAE on the same footing and find that in the
Co$_{0.5}$Pt$_{0.5}$ alloy it accounts for only about 20% of the observed MAE,
thus confirming that compositional order is the major player in the enhancement
of MAE. We also examine the directional chemical order that can be produced by
magnetic annealing within the same framework. We extract a Fermi surface
mechanism for the effect in an explicit study of permalloy. Finally, we propose
that the Fermi surface plays a major role in the strong coupling between
magnetocrystalline anisotropy and compositional order in many magnetic alloys. | 0001271v1 |
2003-04-29 | Raman Scattering, Thermal studies and Reverse Monte Carlo Simulations of an Amorphous Ge$_{30}$Se$_{70}$ Alloy Produced by Mechanical Alloying | The short and intermediate range order of an amorphous Ge$_{30}$Se$_{70}$
alloy produced by Mechanical Alloying were studied by Reverse Monte Carlo
simulations of its x-ray total structure factor, Raman scattering and
differential scanning calorimetry. 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. | 0304665v3 |
2006-12-11 | Chemical ordering and composition fluctuations at the (001) surface of the Fe-Ni Invar alloy | We report on a study of (001) oriented fcc Fe-Ni alloy surfaces which
combines first-principles calculations and low-temperature STM experiments.
Density functional theory calculations show that Fe-Ni alloy surfaces are
buckled with the Fe atoms slightly shifted outwards and the Ni atoms inwards.
This is consistent with the observation that the atoms in the surface layer can
be chemically distinguished in the STM image: brighter spots (corrugation
maxima with increased apparent height) indicate iron atoms, darker ones nickel
atoms. This chemical contrast reveals a c2x2 chemical order (50% Fe) with
frequent Fe-rich defects on Invar alloy surface. The calculations also indicate
that subsurface composition fluctuations may additionally modulate the apparent
height of the surface atoms. The STM images show that this effect is pronounced
compared to the surfaces of other disordered alloys, which suggests that some
chemical order and corresponding concentration fluctuations exist also in the
subsurface layers of Invar alloy. In addition, detailed electronic structure
calculations allow us to identify the nature of a distinct peak below the Fermi
level observed in the tunneling spectra. This peak corresponds to a surface
resonance band which is particularly pronounced in iron-rich surface regions
and provides a second type of chemical contrast with less spatial resolution
but one that is essentially independent of the subsurface composition. | 0612262v1 |
2011-11-02 | Ab-initio elastic tensor of cubic Ti$_{0.5}$Al$_{0.5}$N alloy: the dependence of the elastic constants on the size and shape of the supercell model | In this study we discuss the performance of approximate SQS supercell models
in describing the cubic elastic properties of B1 (rocksalt)
Ti$_{0.5}$Al$_{0.5}$N alloy by using a symmetry based projection technique. We
show on the example of Ti$_{0.5}$Al$_{0.5}$N alloy, that this projection
technique can be used to align the differently shaped and sized SQS structures
for a comparison in modeling elasticity. Moreover, we focus to accurately
determine the cubic elastic constants and Zener's type elastic anisotropy of
Ti$_{0.5}$Al$_{0.5}$N. Our best supercell model, that captures accurately both
the randomness and cubic elastic symmetry, results in $C_{11}=447$ GPa,
$C_{12}=158$ GPa and $C_{44}=203$ GPa with 3% of error and $A=1.40$ for Zener's
elastic anisotropy with 6% of error. In addition, we establish the general
importance of selecting proper approximate SQS supercells with symmetry
arguments to reliably model elasticity of alloys. In general, we suggest the
calculation of nine elastic tensor elements - $C_{11}$, $C_{22}$, $C_{33}$,
$C_{12}$, $C_{13}$, $C_{23}$, $C_{44}$, $C_{55}$ and $C_{66}$, to evaluate and
analyze the performance of SQS supercells in predicting elasticity of cubic
alloys via projecting out the closest cubic approximate of the elastic tensor.
The here described methodology is general enough to be applied in discussing
elasticity of substitutional alloys with any symmetry and at arbitrary
composition. | 1111.0548v1 |
2012-02-07 | Chemical order and crystallographic texture of FePd:Cu thin alloy films | FePd thin films have been recently considered as promising material for
high-density magnetic storage devices. However, it is necessary to find a
proper method of fabrication for the (001)-textured and chemically well-ordered
alloy. In this paper, we present the detailed investigations of lattice
parameters, chemical order degree, grain sizes and crystallographic texture,
carried out on FePd alloys with 10 at.% of Cu addition. The initial [Cu(0.2
nm)/Fe(0.9 nm)/Pd(1.1 nm)]x5 multilayers were thermally evaporated in an
ultra-high vacuum on MgO(100), Si(100), Si(111) and Si(100) covered by 100 nm
thick layer of amorphous SiO2. In order to obtain homogeneous FePd:Cu alloy,
the multilayers were annealed in two different ways. First, the samples were
rapidly annealed in nitrogen atmosphere at 600oC for 90 seconds. Next, the long
annealing in a high vacuum for 1 hour at 700oC was done. This paper focuses on
quantitative investigations of the chemical order degree and crystallographic
texture of ternary FePd:Cu alloys deposited on four different substrates. In
order to obtain both quantities we have taken a novel approach to consider the
problem of dopant atoms located in the FePd structure. The studies of the
structure were done using X-Ray Diffraction (XRD) performed with synchrotron
radiation and pole figures measurements. We have found that the addition of Cu
changes the FePd lattice parameters and lattice distortion. We have also shown,
that using different substrates it is possible to obtain a FePd:Cu alloy with
different chemical order and texture. Moreover, it was observed that texture
category is substrate dependent. | 1202.1419v1 |
2016-06-08 | Atomic Ordering in Cubic Bismuth Telluride Alloy Phases at High Pressure | Pressure-induced transitions from ordered intermetallic phases to
substitutional alloys to semi-ordered phases were studied in a series of
bismuth tellurides. Using angle-dispersive x-ray diffraction, the compounds
Bi4Te5, BiTe, and Bi2Te were observed to form alloys with the disordered
body-centered cubic (bcc) crystal structure upon compression to above 14--19
GPa at room temperature. The BiTe and Bi2Te alloys and the previously
discovered high-pressure alloys of Bi2Te3 and Bi4Te3 were all found to show
atomic ordering after gentle annealing at very moderate temperatures of
~100{\deg}C. Upon annealing, BiTe transforms from the bcc to the B2 (CsCl)
crystal structure type, and the other phases adopt semi-disordered variants
thereof, featuring substitutional disorder on one of the two crystallographic
sites. The transition pressures and atomic volumes of the alloy phases show
systematic variations across the Bi_mTe_n series including the end members Bi
and Te. First-principles calculations were performed to characterize the
electronic structure and chemical bonding properties of B2-type BiTe and to
identify the driving forces of the ordering transition. The calculated Fermi
surface of B2-type BiTe has an intricate structure and is predicted to undergo
three topological changes between 20 and 60 GPa. | 1606.02495v1 |
2016-06-29 | The role of interstitial binding in radiation induced segregation in W-Re alloys | Due to their high strength and advantageous high-temperature properties,
tungsten-based alloys are being considered as plasma-facing candidate materials
in fusion devices. Under neutron irradiation, rhenium, which is produced by
nuclear transmutation, has been found to precipitate in elongated precipitates
forming thermodynamic intermetallic phases at concentrations well below the
solubility limit. Recent measurements have shown that Re precipitation can lead
to substantial hardening, which may have a detrimental effect on the fracture
toughness of W alloys. This puzzle of sub-solubility precipitation points to
the role played by irradiation induced defects, specifically mixed solute-W
interstitials. Here, using first-principles calculations based on density
functional theory, we study the energetics of mixed interstitial defects in
W-Re, W-V, and W-Ti alloys, as well as the heat of mixing for each
substitutional solute. We find that mixed interstitials in all systems are
strongly attracted to each other with binding energies of -2.4 to -3.2 eV and
form interstitial pairs that are aligned along parallel first-neighbor <111>
strings. Low barriers for defect translation and rotation enable defect
agglomeration and alignment even at moderate temperatures. We propose that
these elongated agglomerates of mixed-interstitials may act as precursors for
the formation of needle-shaped intermetallic precipitates. This
interstitial-based mechanism is not limited to radiation induced segregation
and precipitation in W-Re alloys but is also applicable to other body-centered
cubic alloys. | 1607.00230v1 |
2017-08-08 | Isoelectronic Substitutions and Aluminium Alloying in the Ta-Nb-Hf-Zr-Ti High-Entropy Alloy Superconductor | High-entropy alloys (HEAs) are a new class of materials constructed from
multiple principal elements statistically arranged on simple crystallographic
lattices. Due to the large amount of disorder present, they are excellent model
systems for investigating the properties of materials intermediate between
crystalline and amorphous states. Here we report the effects of systematic
isoelectronic replacements, using Mo-Y, Mo-Sc, and Cr-Sc mixtures, for the
valence electron count 4 and 5 elements in the BCC Ta-Nb-Zr-Hf-Ti high entropy
alloy (HEA) superconductor. We find that the superconducting transition
temperature Tc strongly depends on the elemental make-up of the alloy, and not
exclusively its electron count. The replacement of niobium or tantalum by an
isoelectronic mixture lowers the transition temperature by more than 60 %,
while the isoelectronic replacement of hafnium, zirconium, or titanium has a
limited impact on Tc. We further explore the alloying of aluminium into the
nearly optimal electron count [TaNb]0.67(ZrHfTi)0.33 HEA superconductor. The
electron count dependence of the superconducting Tc for (HEA)Alx is found to be
more crystalline-like than for the [TaNb]1-x(ZrHfTi)x HEA solid solution. For
an aluminum content of x = 0.4 the high-entropy stabilization of the simple BCC
lattice breaks down. This material crystallizes in the tetragonal beta-uranium
structure type and superconductivity is not observed above 1.8 K. | 1708.02452v1 |
2017-12-05 | Lattice thermal transport in group II-alloyed PbTe | PbTe, one of the most promising thermoelectric materials, has recently
demonstrated thermoelectric figure of merit ($ZT$) of above 2.0 when alloyed
with group II elements. The improvements are due mainly to significant
reduction of lattice thermal conductivity ($\kappa_{l}$), which was in turn
attributed to nanoparticle precipitates. However, a fundamental understanding
of various phonon scattering mechanisms within the bulk alloy is still lacking.
In this work, we apply the newly-developed density-functional-theory
(DFT)-based compressive sensing lattice dynamics (CSLD) approach to model
lattice heat transport in PbTe, MTe, and Pb$_{0.94}$M$_{0.06}$Te (M=Mg, Ca, Sr
and Ba), compare our results with experimental measurements, with focus on
strain effect and mass disorder scattering. We find that (1) CaTe, SrTe and
BaTe in the rock-salt structure exhibit much higher $\kappa_{l}$ than PbTe,
while MgTe in the same structure shows anomalously low $\kappa_{l}$; (2)
lattice heat transport of PbTe is extremely sensitive to static strain induced
by alloying atoms in solid solution form; (3) mass disorder scattering plays a
major role in reducing $\kappa_{l}$ for Mg/Ca/Sr-alloyed PbTe through strongly
suppressing the lifetimes of intermediate- and high-frequency phonons, while
for Ba-alloyed PbTe, precipitated nanoparticles are also important. | 1712.01926v1 |
2017-12-29 | Origins and dissociation of pyramidal <c + a> dislocations in magnesium and its alloys | Alloying magnesium (Mg) with rare earth elements such as yttrium (Y) has been
reported to activate the pyramidal <c + a> slip systems and improve the
plasticity of Mg at room temperature. However, the origins of such dislocations
and their dissociation mechanisms remain poorly understood. Here, we
systematically investigate these mechanisms using dispersion-inclusive density
functional theory, in combination with molecular dynamics simulations. We find
that <c + a> dislocations form more readily on the pyramidal I plane than on
the pyramidal II plane in Mg. The addition of Y atoms in Mg facilitates the
dissociation of <c + a> dislocations on pyramidal II, leading to the easier
formation of the pyramidal II than pyramidal I in Mg-Y alloy. Importantly, in
pyramidal II slip plane, a flat potential-energy surface (PES) exists around
the position of stable stacking fault energy (SFE), which allows cooperative
movement of atoms within the slip plane. Alloying Mg with Y atoms increases the
range of the PES, and ultimately promotes different sliding pathways in the
Mg-Y alloy. These findings are consistent with experimentally observed
activation of the pyramidal II <c + a> slip system in Mg-Y alloys, and provide
important insight into the relationship between dislocation structure and
macroscopic enhancement of plasticity. | 1712.10098v1 |
2018-03-28 | Strong electron-phonon coupling and multiband effects in the superconducting $β$-phase Mo$_{1-x}$Re$_x$ alloys | Superconducting transition temperature $T_C$ of some of the cubic
$\beta$-phase Mo$_{1-x}$Re$_x$ alloys with x > 0.10 is an order of magnitude
higher than that in the elements Mo and Re. We investigate this rather
enigmatic issue of the enhanced superconductivity with the help of experimental
studies of the temperature dependent electrical resistivity ($\rho$(T)) and
heat capacity (C$_P$(T)), as well as the theoretical estimation of electronic
density of states (DOS) using band structure calculations. The $\rho$(T) in the
normal state of the Mo$_{1-x}$Re$_x$ alloys with x > 0.15 is distinctly
different from that of Mo and the alloys with x < 0.10. We have also observed
that the Sommerfeld coefficient of electronic heat capacity $\gamma$,
superconducting transition temperature $T_C$ and the DOS at the Fermi level
show an abrupt change above x > 0.10. The analysis of these results indicates
that the value of electron-phonon coupling constant {\lambda}ep required to
explain the $T_C$ of the alloys with x > 0.10 is much higher than that
estimated from $\gamma$. On the other hand the analysis of the results of the
$\rho$(T) reveals the presence of phonon assisted inter-band s-d scattering in
this composition range. We argue that a strong electron-phonon coupling arising
due to the multiband effects is responsible for the enhanced $T_C$ in the
$\beta$-phase Mo$_{1-x}$Re$_x$ alloys with x > 0.10. | 1803.10877v1 |
2019-11-05 | First-principles concentration-wave approach to predict incipient order in high-entropy alloys: case of Ti$_{0.25}$CrFeNiAl$_{x}$ | Multi-principal-element alloys, including high-entropy alloys, experience
segregation or partially-ordering as they are cooled to lower temperatures. For
Ti$_{0.25}$CrFeNiAl$_{x}$, experiments suggest a partially-ordered B2 phase,
whereas CALculation of PHAse Diagrams (CALPHAD) predicts a region of
L2$_{1}$+B2 coexistence. We employ first-principles density-functional theory
(DFT) based electronic-structure approach to assess stability of phases of
alloys with arbitrary compositions and Bravais lattices (A1/A2/A3). In
addition, DFT-based linear-response theory has been utilized to predict
Warren-Cowley short-range order (SRO) in these alloys, which reveals
potentially competing long-range ordered phases. The resulting SRO is uniquely
analyzed using concentration-waves analysis for occupation probabilities in
partially-ordered states, which is then be assessed for phase stability by
direct DFT calculations. Our results are in good agreement with experiments and
CALPHAD in Al-poor regions ($x \le 0.75$) and with CALPHAD in Al-rich region
($0.75 \le {x} \le 1$), and they suggest more careful experiments in Al-rich
region are needed. Our DFT-based electronic-structure and SRO predictions
supported by concentration-wave analysis are shown to be a powerful method for
fast assessment of competing phases and their stability in
multi-principal-element alloys. | 1911.01602v2 |
2018-10-30 | Hot embossing of Au- and Pb- based alloys for X ray grating fabrication | Grating-based X-ray phase-contrast interferometry has a high application
impact in material science and medicine for imaging of weakly absorbing (low Z)
materials and soft tissues. For the absorbing gratings, casting of highly x-ray
absorbing metals, such as Au and Pb alloys, has proven to be a viable way to
generate large area periodic high aspect ratio microstructures. In this paper,
we review the grating fabrication strategy with a special focus on a novel
approach of casting low temperature melting alloys (Au-Sn and Pbbased alloy)
into Si grating templates using hot embossing. The process, similar to
nanoimprint lithography, requires particular adjusting efforts of process
parameters as a function of the metal alloy and the grating feature size. The
transition between solid and liquid state depends on the alloy phase diagram,
the applied pressure can damage the high aspect ratio Si lamellas and the
microstructure of the solid metal can affect the grating structure. We
demonstrate that metal casting by hot embossing can be used to fabricate
gratings on large area (up to 70x70 mm2) with aspect ratio up to 50:1 and pitch
in the range of 1-20 {\mu}m. | 1810.12586v1 |
2019-01-24 | Structure and superconductivity in the binary Re$_{1-x}$Mo$_x$ alloys | The binary Re$_{1-x}$Mo$_x$ alloys, known to cover the full range of solid
solutions, were successfully synthesized and their crystal structures and
physical properties investigated via powder x-ray diffraction, electrical
resistivity, magnetic susceptibility, and heat capacity. By varying the Re/Mo
ratio we explore the full Re$_{1-x}$Mo$_x$ binary phase diagram, in all its
four different solid phases: hcp-Mg ($P6_3/mmc$), $\alpha$-Mn
($I\overline{4}3m$), $\beta$-CrFe ($P4_2/mnm$), and bcc-W ($Im\overline{3}m$),
of which the second is non-centrosymmetric with the rest being centrosymmetric.
All Re$_{1-x}$Mo$_x$ alloys are superconductors, whose critical temperatures
exhibit a peculiar phase diagram, characterized by three different
superconducting regions. In most alloys the $T_c$ is almost an order of
magnitude higher than in pure Re and Mo. Low-temperature electronic
specific-heat data evidence a fully-gapped superconducting state, whose
enhanced gap magnitude and specific-heat discontinuity suggest a moderately
strong electron-phonon coupling across the series. Considering that several
$\alpha$-Mn-type Re$T$ alloys ($T$ = transition metal) show time-reversal
symmetry breaking (TRSB) in the superconducting state, while TRS is preserved
in the isostructural Mg$_{10}$Ir$_{19}$B$_{16}$ or Nb$_{0.5}$Os$_{0.5}$, the
Re$_{1-x}$Mo$_x$ alloys represent another suitable system for studying the
interplay of space-inversion, gauge, and time-reversal symmetries in future
experiments expected to probe TRSB in the Re$T$ family. | 1901.08633v1 |
2019-04-22 | Ab initio study of phosphorus effect on vacancy-mediated process in nickel alloys - an insight into Ni2Cr ordering | The development of long range order in nickel-chromium alloys is of great
technological interest but the kinetics and mechanisms of the transformation
are poorly understood. The present research utilizes a combined computational
and experimental approach to elucidate the mechanism by which phosphorus
accelerates the ordering rate of stoichiometric Ni_2Cr in Ni-Cr alloys. A
series of Ni-33%Cr-x%P samples (in atomic percent) were fabricated with
phosphorus concentrations, x = <0.005-0.1 at.% and aged between 373 and
470{\deg}C for times up to 3000 h. The first-principles modeling considers fcc
Ni with dilute P as a reasonable approximation for the complex Ni-Cr-P alloy
system. Calculation results show a pronounced enhancement of vacancy transport
by vacancy-solute pair diffusion via consecutive exchange and rotation jumps of
vacancies associated with the phosphorus atom. The energy barriers of these two
migration paths are at least 0.35 eV lower than that of vacancy-atom exchange
in pure Ni solvent. The analytical diffusion model predicts enhanced solvent
diffusion by 2 orders of magnitude for 0.1 at.% P at 400-500{\deg}C. The model
prediction is in good agreement with the evolution of micro-hardness. We
characterize the micro-hardness result by a kinetic ordering model, showing a
significant decrease of the activation energy of ordering transformation. These
results help gauge the risk of industrial alloys developing long range order
which increases strength but degrades ductility and toughness. Specifically,
minor alloying additions that bind with excess vacancies and lower the vacancy
migration barrier can greatly accelerate hardening via Ni_2Cr precipitation. | 1904.09684v1 |
2019-10-15 | Enhanced Figure of Merit in Bismuth-Antimony Fine-Grained Alloys at Cryogenic Temperatures | Thermoelectric (TE) materials research plays a vital role in
heat-to-electrical energy conversion and refrigeration applications.
Bismuth-antimony (Bi-Sb) alloy is a promising material for thermoelectric
cooling. Herein, a high figure of merit, ZT, near 0.6 at cryogenic temperatures
(100-150K) has been achieved in melt-spun n-type Bi85Sb15 bulk samples
consisting of micron-size grains. The achieved ZT is nearly 50 percents higher
than polycrystalline averaged single crystal ZT of ~0.4, and it is also
significantly higher than ZT of less than ~0.3 measured below 150K in Bi-Te
alloys commonly used for cryogenic cooling applications. The improved
thermoelectric properties can be attributed to the fine-grained microstructure
achieved from rapid solidification, which not only significantly reduced the
thermal conductivity but also mitigated a segregation effect. A record low
thermal conductivity of ~1.5 W m-1 K-1 near 100 K was measured using the hot
disk method. The thermoelectric properties for this intriguing
semimetal-semiconductor alloy system were analyzed within a two-band effective
mass model. The study revealed a gradual narrowing of the band gap at
increasing temperature in Bi-Sb alloy for the first time.
Magneto-thermoelectric effects of this Bi-Sb alloy further improved the TE
properties, leading to ZT of about 0.7. The magneto-TE effect was further
demonstrated in a combined NdFeB/BiSb/NdFeB system. The compactness of the
BiSb-magnet system with high ZT enables the utilization of magneto-TE effect in
thermoelectric cooling applications. | 1910.06648v1 |
2019-10-21 | Origin of the Phase Separation into B2 and L21 Ordered Phases in the X-Al-Ti (X: Fe, Co, and Ni) Alloys from the First-principles Cluster Variation Method | The phase separation behaviors from the single B2 ordered phase into the two
separate B2 and L2$_1$ ordered phases in the X-Al-Ti (X: Fe, Co, and Ni) alloys
are analyzed using the cluster variation method (CVM) with the interaction
energies evaluated from the electronic band structure calculations. The cubic
approximation of the CVM is employed for the X$_2$Al$_{2-x}$Ti$_x$ ($0 \leq x
\leq 2$) alloys limiting an interchange between Al and Ti atoms on the
$\alpha$- and $\beta$-sublattices of the L2$_1$ ordered structure with the X
atoms fixed on the $\gamma$-sublattice. The phase stabilities of the B2 and
L2$_1$ structures are examined, and the phase diagrams at the pseudo-binary
section, XAl-XTi, are determined. The two-phase regions of B2 and L2$_1$
phases, i.e., phase separation behavior, are successfully produced in both Co-
and Ni-Al-Ti alloy systems, whereas no phase separation is predicted in the
Fe-Al-Ti alloy. The origin of the phase separation in the Co- and Ni-Al-Ti
alloys is, respectively, attributed to the mechanical instability and the
combination of mechanical instability and chemical repulsions of unlike pairs. | 1910.09660v1 |
2019-10-23 | From single phase to dual-phase TRIP-TWIP titanium alloys for the improvement of the yield strength | Aiming at increasing the yield strength of transformation and twinning
induced plasticity (TRIP and TWIP) titanium alloys, a dual-phase
$\alpha$/$\beta$ alloy is designed and studied. The composition Ti 7Cr 1.5Sn
(wt.%) is proposed, based on an approach coupling Calphad calculations and
classical Bo-Md design tool used in Ti-alloys. Its microstructure is made of
20% of $\alpha$ precipitates in a $\beta$ matrix, the matrix having optimal Bo
and Md parameters for deformation twinning and martensitic transformation. The
alloy indeed displays a yield strength of 760 MPa, about 200 MPa above that of
a Ti 8.5Cr 1.5Sn (wt.%) single beta phase TRIP/TWIP alloy, combined with good
ductility and work-hardening. In situ synchrotron X ray diffraction and
post-mortem electron back-scattered analyses are performed to characterize the
deformation mechanisms. They evidence that the TRIP and TWIP mechanisms are
successfully obtained in the material, validating the design strategy. The
interaction of the precipitates with the {332}<113> $\beta$ twins is analyzed,
evidencing that the precipitates are sheared when hit by a twin, and therefore
do not hinder the propagation of the twins. The detailed nature of the
interaction is discussed, as well as the impact of the precipitates on the
mechanical properties. | 1910.10555v1 |
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