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2020-09-16 | Heat treatment - microstructure - hardness relationships of new nickel-rich nickel-titanium-hafnium alloys developed for tribological applications | The effects of various heat treatments on the microstructure and hardness of
new Ni56Ti41Hf3 and Ni56Ti36Hf8 (atomic %) alloys were studied to evaluate the
suitability of these materials for tribological applications. A solid-solution
strengthening effect due to Hf atoms was observed for the solution annealed
(SA) Ni56Ti36Hf8 alloy (716 HV), resulting in a comparable hardness to the
Ni56Ti41Hf3 alloy containing 54 vol.% of Ni4Ti3 precipitates (707 HV). In the
Ni56Ti41Hf3 alloy, the maximum hardness (752 HV), achieved after aging at 300C
for 12 h, was attributed to dense, semi-coherent precipitation of the Ni4Ti3
phase. Unlike the lenticular morphology usually observed within binary NiTi
alloys, a blocky Ni4Ti3 morphology formed within Ni56Ti36Hf3 due to a smaller
lattice mismatch in the direction normal to the habit plane at the
precipitate/matrix interface. The maximum hardness for Ni56Ti36Hf8 (769 HV) was
obtained after applying an intermediate aging step (300C for 12 h) followed by
normal aging (550C for 4 h). This two-step aging treatment induces dense
nanoscale precipitation of two interspersed precipitate phases, namely H-phase
and a new cubic Ni-rich precipitate phase, resulting in the highest hardness
exhibited yet by this family alloys. The composition of cubic Ni-rich
precipitates was measured using atom probe tomography to be approximately
Ni61.5Ti31Hf7.5, while HAADF-STEM revealed a 54 atom motif cubic structure (a=
8.87 Angstroms), and electron diffraction showed that the structure belongs to
the pm-3m (No. 221) space group. | 2009.07962v1 |
2020-12-21 | Impact of stoichiometry and strain on Ge$_{1-x}$Sn$_{x}$ alloys from first principles calculations | We calculate the electronic structure of germanium-tin (Ge$_{1-x}$Sn$_{x}$)
binary alloys for $0 \leq x \leq 1$ using density functional theory (DFT).
Relaxed alloys with semiconducting or semimetallic behaviour as a function of
Sn composition $x$ are identified, and the impact of epitaxial strain is
included by constraining supercell lattice constants perpendicular to the [001]
growth direction to the lattice constants of Ge, zinc telluride (ZnTe), or
cadmium telluride (CdTe) substrates. It is found that application of 1% tensile
strain reduces the Sn composition required to bring the (positive) direct band
gap to zero by approximately 5% compared to a relaxed Ge$_{1-x}$Sn$_{x}$ alloy
having the same gap at $\Gamma$. On the other hand, compressive strain has
comparatively less impact on the alloy band gap at $\Gamma$. Using DFT
calculated alloy lattice and elastic constants, the critical thickness for
Ge$_{1-x}$Sn$_{x}$ thin films as a function of $x$ and substrate lattice
constant is estimated, and validated against supercell DFT calculations. The
analysis correctly predicts the Sn composition range at which it becomes
energetically favourable for Ge$_{1-x}$Sn$_{x}$/Ge to become amorphous. The
influence of stoichiometry and strain is examined in relation to reducing the
magnitude of the inverted (``negative'') $\Gamma_{7}^{-}$-$\Gamma_{8}^{+}$ band
gap, which is characteristic of semimetallic alloy electronic structure. Based
on our findings, strategies for engineering the semimetal-to-semiconductor
transition via strain and quantum confinement in Ge$_{1-x}$Sn$_{x}$
nanostructures are proposed. | 2012.11311v1 |
2021-01-19 | Multiscale prediction of microstructure length scales in metallic alloy casting | In this article, we combine casting experiments and quantitative simulations
to present a novel multiscale modeling approach to predict local primary
dendritic spacings in metallic alloys solidified in conditions relevant to
industrial casting processes. To this end, primary dendritic spacings were
measured in instrumented casting experiments in Al-Cu alloys containing 1\,wt\%
and 4\,wt\% of Cu, and they were compared to spacing stability ranges and
average spacings in dendritic arrays simulated using phase-field (PF) and
dendritic needle network (DNN) models. It is first shown that PF and DNN lead
to similar results for the Al-1\,wt\%Cu alloy, using a dendrite tip selection
constant calculated with PF in the DNN simulations. PF simulations cannot
achieve quantitative predictions for the Al-4\,wt\%Cu alloy because they are
too computationally demanding due to the large separation of scale between tip
radius and diffusion length, a characteristic feature of non-dilute alloys.
Nevertheless, the results of DNN simulations for non-dilute Al-Cu alloys are in
overall good agreement with our experimental results as well as with those of
an extensive literature review. Simulations consistently suggest a widening of
the PDAS stability range with a decrease of the temperature gradient as the
microstructure goes from cellular-dendrites to well-developed hierarchical
dendrites. | 2101.07780v1 |
2021-02-10 | Nanocrystalline equiatomic CoCrFeNi alloy thin films: Are they single phase fcc? | The bulk quaternary equiatomic CoCrFeNi alloy is studied extensively in
literature. Under experimental conditions, it shows a single-phase fcc
structure and its physical and mechanical properties are similar to those of
the quinary equiatomic CoCrFeMnNi alloy. Many studies in literature have
focused on the mechanical properties of bulk nanocrystalline high entropy
alloys or compositionally complex alloys, and their microstructure evolution
upon annealing. The thin film processing route offers an excellent alternative
to form nanocrystalline alloys. Due to the high nucleation rate and high
density of defects in thin films synthesized by sputtering, the kinetics of
microstructure evolution is often accelerated compared to those taking place in
the bulk. Here, thin films are used to study the phase evolution in
nanocrystalline CoCrFeNi deposited on Si/SiO 2 and c-sapphire substrates by
magnetron cosputtering from elemental sources. The phases and microstructure of
the films are discussed in comparison to the bulk alloy. The main conclusion is
that second phases can form even at room temperature provided there are
sufficient nucleation sites. | 2102.05325v1 |
2021-05-11 | Strengthening magnesium by design: integrating alloying and dynamic processing | Magnesium (Mg) has the lowest density of all structural metals and has
excellent potential for wide use in structural applications. While pure Mg has
inferior mechanical properties; the addition of further elements at various
concentrations has produced alloys with enhanced mechanical performance and
corrosion resistance. An important consequence of adding such elements is that
the saturated Mg matrix can locally decompose to form solute clusters and
intermetallic particles, often referred to as precipitates. Controlling the
shape, number density, volume fraction, and spatial distribution of solute
clusters and precipitates significantly impacts the alloy's plastic response.
Conversely, plastic deformation during thermomechanical processing can
dramatically impact solute clustering and precipitation. In this paper, we
first discuss how solute atoms, solute clusters, and precipitates can improve
the mechanical properties of Mg alloys. We do so by primarily comparing three
alloy systems: Mg-Al, Mg-Zn, and Mg-Y-based alloys. In the second part, we
provide strategies for optimizing such microstructures by controlling
nucleation and growth of solute clusters and precipitates during
thermomechanical processing. In the third part, we briefly highlight how one
can enable inverse design of Mg alloys by a more robust Integrated
Computational Materials Design (ICMD) approach. | 2105.05354v1 |
2021-09-05 | Bulk nanocrystalline Al alloys with hierarchical reinforcement structures via grain boundary segregation and complexion formation | Grain size engineering, particularly reducing grain size into the
nanocrystalline regime, offers a promising pathway to further improve the
strength-to-weight ratio of Al alloys. Unfortunately, the fabrication of
nanocrystalline metals often requires non-equilibrium processing routes, which
typically limit the specimen size and require large energy budgets. In this
study, multiple dopant atoms in ternary Al alloys are deliberately selected to
enable segregation to the grain boundary region and promote the formation of
amorphous complexions. Three different fully dense bulk nanocrystalline Al
alloys (Al-Mg-Y, Al-Fe-Y, and Al-Ni-Y) with small grain sizes were successfully
fabricated using a simple powder metallurgy approach, with full densification
connected directly to the onset of amorphous complexion formation. All the
compositions demonstrate densities above 99% with grain sizes of <60 nm
following consolidation via hot pressing at 585 oC. The very fine grain
structure results in excellent mechanical properties, with nanoindentation
hardness values in the range of 2.2-2.8 GPa. Detailed microstructural
characterization verifies the segregation of all dopant species to grain
boundaries as well as the formation of amorphous complexions, which suggests
their influential role in aiding effective consolidation and endowing thermal
stability in the alloys. Moreover, nanorods with a core-shell structure are
also observed at the grain boundaries, which likely contribute to the
stabilization of the grain structure and high strength. Finally, intermetallic
particles with a sizes of hundreds of nanometers form. As a whole, the results
presented here demonstrate a general alloy design strategy of segregation and
boundary evolution pathway that enables the fabrication of multiple
nanocrystalline Al alloys with hierarchical microstructures and improved
performance. | 2109.02133v2 |
2022-04-12 | Macroscopic analysis of time dependent plasticity in Ti alloys | Component failure due to cold dwell fatigue of titanium and its alloys is a
long-standing problem which has significant safety and economic implications to
the aviation industry. This can be addressed by understanding the governing
mechanisms of time dependent plasticity behaviour of Ti at low temperatures.
Here, stress relaxation tests were performed at four different temperatures on
three major alloy systems: commercially pure titanium (two alloys with
different oxygen content), Ti-6Al-4V (two microstructures with differing beta
phase fractions) and Ti-6Al-2Sn-4Zr-xMo (two alloys with different Mo content
x= 2 or 6, and portion of beta phase). Key parameters controlling the time
dependent plasticity were determined as a function of temperature. Both
activation volume and energy were found to increase with temperature in all six
alloys. It was found that the dwell fatigue effect is more significant by
oxygen alloying but is suppressed by the addition of Mo. The presence of the
beta phase did not strongly affect the dwell fatigue, however, it was
suppressed at high temperature due to the low strain rate and strain rate
sensitivity. | 2204.05721v1 |
2022-05-03 | Mapping Thermoelectric Transport in a Multicomponent Alloy Space | Interest in high entropy alloy thermoelectric materials is predicated on
achieving ultralow lattice thermal conductivity $\kappa\sub{L}$ through large
compositional disorder. However, here we show that for a given mechanism, such
as mass contrast phonon scattering, $\kappa\sub{L}$ will be minimized along the
binary alloy with the highest mass contrast, such that adding an
intermediate-mass atom to increase atomic disorder can increase thermal
conductivity. Only when each component adds an independent scattering mechanism
(such as adding strain fluctuation to an existing mass fluctuation) is there a
benefit. In addition, both charge carriers and heat-carrying phonons are known
to experience scattering due to alloying effects, leading to a trade-off in
thermoelectric performance. We apply analytic transport models, based on
perturbation and effective medium theories, to predict how alloy scattering
will affect the thermal and electronic transport across the full compositional
range of several pseudo-ternary and pseudo-quaternary alloy systems. To do so,
we demonstrate a multicomponent extension to both thermal and electronic binary
alloy scattering models based on the virtual crystal approximation. Finally, we
show that common functional forms used in computational thermodynamics can be
applied to this problem to further generalize the scattering behavior that is
modeled. | 2205.01520v1 |
2022-06-21 | A method to computationally screen for tunable properties of crystalline alloys | Conventionally, high-throughput computational materials searches start from
an input set of bulk compounds extracted from material databases, and this set
is screened for candidate materials for specific applications. In contrast,
many functional materials, and especially semiconductors, are heavily
engineered alloys or solid solutions of multiple compounds rather than a single
bulk compound. To improve our ability to design functional materials, in this
work we propose a framework and open-source code to automatically construct
possible "alloy pairs" and "alloy systems" and detect "alloy members" from a
set of existing, experimental or calculated ordered compounds, without
requiring any additional metadata beyond their crystal structure. We provide
analysis tools to estimate stability across each alloy. As a demonstration, we
apply this framework to all inorganic materials in the Materials Project
database to create a new database of over 600,000 unique alloy pair entries
that can then be used in materials discovery studies to search for materials
with tunable properties. This new database has been incorporated into the
Materials Project website and linked with corresponding material identifiers
for any user to query and explore. Using an example of screening for p-type
transparent conducting materials, we demonstrate how using this methodology
reveals candidate material systems that might otherwise have been excluded by a
traditional screening. This work lays a foundation from which materials
databases can go beyond stoichiometric compounds, and approach a more realistic
description of compositionally tunable materials. | 2206.10715v3 |
2022-12-26 | Modeling high-entropy transition-metal alloys with alchemical compression | Alloys composed of several elements in roughly equimolar composition, often
referred to as high-entropy alloys, have long been of interest for their
thermodynamics and peculiar mechanical properties, and more recently for their
potential application in catalysis. They are a considerable challenge to
traditional atomistic modeling, and also to data-driven potentials that for the
most part have memory footprint, computational effort and data requirements
which scale poorly with the number of elements included. We apply a recently
proposed scheme to compress chemical information in a lower-dimensional space,
which reduces dramatically the cost of the model with negligible loss of
accuracy, to build a potential that can describe 25 d-block transition metals.
The model shows semi-quantitative accuracy for prototypical alloys, and is
remarkably stable when extrapolating to structures outside its training set. We
use this framework to study element segregation in a computational experiment
that simulates an equimolar alloy of all 25 elements, mimicking the seminal
experiments by Cantor et al., and use our observations on the short-range order
relations between the elements to define a data-driven set of Hume-Rothery
rules that can serve as guidance for alloy design. We conclude with a study of
three prototypical alloys, CoCrFeMnNi, CoCrFeMoNi and IrPdPtRhRu, determining
their stability and the short-range order behavior of their constituents. | 2212.13254v2 |
2023-01-18 | Positional and Rotational Molecular Degrees of Freedom in a Metal-Organic Surface Alloy: the Copper-Fullerene System and its Multiple Structural Phases | Mixing two chemical elements at the surface of a substrate is known to
produce rich phase diagrams of surface alloys. Here, we extend the concept of
surface alloying to the case where the two constituent elements are not both
atoms, but rather one atom (copper) and one molecule (fullerene). When
deposited at room temperature on a Cu(111) surface, fullerenes intermix with
the metal substrate. Surprisingly, 10 distinct copper-fullerene surface alloys
are found to coexist. The structure of these alloys, i.e. their composition and
commensurability relationship with the substrate, is resolved using scanning
tunneling microscopy and density functional theory calculations. This diversity
in the alloying process is associated to the multiple possibilities a fullerene
can bind to the Cu surface. The molecules are indeed found to have in-plane and
out-of-plane positional degree of freedom: the molecular alloys have elastic
in-plane properties and can buckle. In addition, the molecules can rotate on
their binding sites, conferring extra degrees of freedom to the system. We
introduce a competing-interaction energy model, parametrized against the
results of the \textit{ab initio} calculations, that describes well all the
phases we observe experimentally. | 2301.07348v1 |
2023-03-08 | Nanoscale perspective on the stress-corrosion cracking behavior of a peak-aged 7XXX-Al alloy | High strength 7xxx Al-alloys are currently commonly used in aerospace and are
expected to be increasingly employed in the automotive sector for weight
reduction purposes. These alloys can however be sensitive to stress-corrosion
cracking (SCC) depending on temper and loading conditions. Both the alloy's
grain structure and composition are believed to play a key role in determining
sensitivity to SCC. Here, we study at the nanometer scale the evolution of the
microstructure near stress corrosion cracks on two different model variants of
the 7140 aluminum alloy. We performed double cantilever beam (DCB) crack growth
tests in hot (70{\deg}C) humid air, on samples extracted at quarter-thickness
(T/4) and mid-thickness (T/2) and heat treated to a non-industrial, SCC
sensitive T6 condition. The sample at T/4 shows a lower KISCC along with
flatter grains and a higher solute content, whereas both samples exhibit
similar crack growth rates at higher stress intensities. We report on
precipitate dissolution and matrix solute enrichment near the crack tips, with
the T/4 position presenting the higher increase in solute levels. The near
grain boundary microstructure ahead of the crack is modified, with evidence of
precipitate dissolution and transport of solutes towards the stress-corrosion
crack tip. These results agree with a recent report on another 7xxx Al-alloy
after SCC in Cl-solution, supporting the possibility that these mechanisms are
generally occurring. We relate our findings with the measured SCC behavior and
provide an array of possible mechanisms that could be widely applicable in SCC
of high strength Al-alloys. | 2303.04625v1 |
2023-06-25 | Transferable and Robust Machine Learning Model for Predicting Stability of Si Anodes for Multivalent Cation Batteries | Data-driven methodology has become a key tool in computationally predicting
material properties. Currently, these techniques are priced high due to
computational requirements for generating sufficient training data for
high-precision machine learning models. In this study, we present a Support
Vector Regression (SVR)-based machine learning model to predict the stability
of silicon (Si) - alkaline metal alloys, with a strong emphasis on the
transferability of the model to new silicon alloys with different electronic
configurations and structures. We elaborate on the role of the structural
descriptor in imparting transferability to the model that is trained on limited
data (~750 Si alloys) derived from the Material Project database. Three popular
descriptors, namely X-Ray Diffraction (XRD), Sine Coulomb Matrix (SCM), and
Orbital Field Matrix (OFM), are evaluated for representing Si alloys. The
material structures are represented by descriptors in the SVR model, coupled
with hyperparameter tuning techniques like Grid Search CV and Bayesian
Optimization (BO), to find the best performing model for predicting total
energy, formation energy and packing fraction of the Si alloy systems. The
models are trained on Si alloys with lithium (Li), sodium (Na), potassium (K),
magnesium (Mg), calcium (Ca), and aluminum (Al) metals, where Si-Na and Si-Al
systems are used as test structures. Our results show that XRD, an
experimentally derived characterization of structures, performs most reliably
as a descriptor for total energy prediction of new Si alloys. The study
demonstrates that by qualitatively selection of training data, using
hyperparameter tuning methods, and employing appropriate structural
descriptors, the data requirements for robust and accurate ML models can be
reduced. | 2306.14285v1 |
2023-11-17 | Using multicomponent recycled electronic waste alloys to produce high entropy alloys | The amount of electronic waste (e-waste) recycled worldwide is less than 20%
of the total amount produced. In a world where the need for critical and
strategic metals is increasing almost exponentially, it is unacceptable that
tons of these elements remain unrecycled. One of the causes of this low level
of recycling is that recycling is based on an expensive and complex selective
sorting of metals. Extracting all metals simultaneously is much simpler and if
this were done, it would significantly increase the recycling rate. Meanwhile,
it was demonstrated that high entropy alloys (HEAs), which are in great demand
in applications where very high performance is required, can be made from
mixtures of complex alloys, hence reducing their dependence on pure critical
metals. Here, we show that it is possible to obtain competitive HEAs from
complex alloy mixtures corresponding to typical electronic waste compositions,
combining two needs of high interest in our society, namely: to increase the
level of recycling of electronic waste and the possibility of developing
high-performance HEAs without the need of using critical and/or strategic
metals. To validate our hypothesis that e-waste can be used to produce
competitive HEAs, we propose an alloy design strategy combining computational
thermodynamics (CalPhaD) exploration of phase diagrams and phenomenological
criteria for HEA design based on thermodynamic and structural parameters. A
shortlist of selected compositions are then fabricated by arc melting ensuring
compositional homogeneity of such complex alloys and, finally, characterised
microstructurally, using electron microscopy and diffraction analysis, and
mechanically, using hardness testing. | 2311.10404v1 |
2023-12-19 | Enhanced mechanical properties and microstructural stability of ultrafine-grained biodegradable Zn-Li-Mn-Mg-Cu alloys produced by rapid solidification and high-pressure torsion | Zinc alloys have emerged as promising candidates for biodegradable materials
due to their remarkable biocompatibility and favorable mechanical
characteristics. The incorporation of alloying elements plays an essential role
in advancing the tensile strength of Zn alloys. Nevertheless, achieving uniform
dispersion of these elements poses challenges due to chemical segregation
during solidification. In this study, rapid solidification followed by
high-pressure torsion was successfully employed to fabricate Zn-Li-Mn-Mg-Cu
alloys characterized by ultrafine-grained microstructures with evenly
distributed nanometric intermetallic phases. A comprehensive examination,
including phase composition, microstructural evolution, tensile properties and
deformation mechanisms, was conducted. The impact of varying annealing
temperatures on microstructural stability was systematically examined. The
combined implementation of rapid solidification and high-pressure torsion
yielded alloys with an average grain size below 360 nm, thereby demonstrating
exceptional mechanical properties including yield stress (YS), ultimate tensile
strength (UTS), and elongation to failure (Ef) equal to at least 325+-6 MPa,
350+-8 MPa and 40+-11 %, respectively. Heat treatment notably augmented the
mechanical properties, resulting in a YS = 440+-11 MPa and UTS = 491+-6 MPa,
while preserving plasticity (Ef = 23+-4 %) in the Zn-0.33Li-0.27Mn-0.14Mg-0.1Cu
alloy. Nanoindentation strain rate jump tests identified thermally activated
mechanisms and grain boundary sliding as dominant deformation mechanisms. | 2312.12140v2 |
2024-02-25 | Effective Phonon Dispersion and Low field transport in AlxGa1-xN alloys using supercells: An ab-initio approach | To investigate the transport properties in random alloys, it is important to
model the alloy disorder using supercells. Though traditional methods like
Virtual Crystal Approximation (VCA) are computationally efficient, the local
disorder in the system is not accurately captured as artificial translational
symmetry is imposed on the system. However, in the case of supercells, the
error introduced by self-image interaction between the impurities is reduced
and translational symmetry is explicitly imposed over larger length scales. In
this work, we have investigated the Effective Phonon Dispersion (EPD) and
transport properties, from first principle calculations using supercells in
AlxGa1-xN alloy systems. Using our in-house developed code, the EPD of AlGaN is
obtained and the individual modes are identified. Next, we discuss our in-house
developed method to calculate low-field transport properties in supercells.
First to validate our methods we have solved the Boltzmann Transport Equation
using Rode method to compare the phonon limited mobility in the 4 atom GaN
primitive cell and 12 atom GaN supercell. Using the same technique, we have
investigated the low field transport in random AlxGa1-xN alloy systems. Our
calculations show that along with alloy scattering, electron-phonon scattering
may also play an important role at room temperature and high-temperature device
operation. This technique opens up the path for calculating phonon-limited
transport properties in random alloy systems. | 2402.16203v1 |
2008-01-02 | Correlation between site preference of ternary Mn addition in LaAg and superconductivity | The results of an extensive investigation of structure, surface morphology,
composition and the superconducting-normal phase diagram of a new
unconventional superconductor LaAg1-cMnc with nominal composition c = 0.0,
0.025, 0.05, 0.1, 0.2 and 0.3, reveal the following. The alloys with c = 0,
0.025 and 0.05 are essentially single phase alloys with the actual Mn
concentration, x, same as the nominal one, i.e., c = x, whereas in the alloys
with c = 0.1, 0.2 and 0.3, the actual Mn concentration of the majority phase
(crystalline grains) is x = 0.050(1), 0.080(1) and 0.100(1), respectively. The
ternary Mn addition does not alter the CsCl structure of the parent compound
LaAg. Neither a structural phase transition occurs nor a long-range
antiferromagnetic order exists at any temperature within the range 1.8K < = T <
= 50K in any of the Mn containing alloys. Mn has exclusive La (Ag) site
preference in the alloy (alloys) with x = c = 0.025 (x < = 0.05 or c < = 0.1)
whereas in the alloy with x = c = 0.05, Mn has essentially no site preference
in that all the Mn atoms either occupy the La sites or the Ag sites. In the
alloys (alloy) with x < = 0.05 (x = c = 0.025), substitution of Ag (La) by Mn
at the Ag (La) sub-lattice sites in LaAg host gives rise to unconventional
superconductivity (destroys the conventional phonon-mediated superconductivity
prevalent in the parent LaAg compound). | 0801.0363v1 |
2015-11-08 | On the Role of Elastic Strain on Electrocatalysis of Oxygen Reduction Reaction on Pt | The effect of elastic strain on catalytic activity of platinum (Pt) towards
oxygen reduction reaction (ORR) is investigated through de-alloyed Pt-Cu thin
films; stress evolution in the de-alloyed layer and the mass of the Cu removed
are measured in real-time during electrochemical de-alloying of (111)-textured
thin-film PtCu (1:1, atom %) electrodes. In situ stress measurements are made
using the cantilever-deflection method and nano-gravimetric measurements are
made using an electrochemical quartz crystal nanobalance. Upon de-alloying via
successive voltammetric sweeps between -0.05 and 1.15 V vs. standard hydrogen
electrode, compressive stress develops in the de-alloyed Pt layer at the
surface of thin-film PtCu electrodes. The de-alloyed films also exhibit
enhanced catalytic activity towards ORR compared to polycrystalline Pt. In situ
nanogravimetric measurements reveal that the mass of de-alloyed Cu is
approximately 210 +/- 46 ng/cm2, which corresponds to a de-alloyed layer
thickness of 1.2 +/- 0.3 monolayers or 0.16 +/- 0.04 nm. The average biaxial
stress in the de-alloyed layer is estimated to be 4.95 +/- 1.3 GPa, which
corresponds to an elastic strain of 1.47 +/- 0.4%. In addition, density
functional theory calculations have been carried out on biaxially strained Pt
(111) surface to characterize the effect of strain on its ORR activity; the
predicted shift in the limiting potentials due to elastic strain is found to be
in good agreement with the experimental shift in the cyclic voltammograms for
the dealloyed PtCu thin film electrodes. | 1511.02448v1 |
2016-07-07 | Temperature dependence of the electrical resistivity and the anisotropic magnetoresistance (AMR) of electrodeposited Ni Co alloys | The electrical resistivity and the anisotropic magnetoresistance (AMR) was
investigated for Ni Co alloys at and below room temperature. The Ni Co alloy
layers having a thickness of about 2 um were prepared by electrodeposition on
Si wafers with evaporated Cr and Cu underlayers. The alloy composition was
varied in the whole concentration range by varying the ratio of Ni sulfate and
Co sulfate in the electrolyte. The Ni Co alloy deposits were investigated first
in the as deposited state on the substrates and then, by mechanically stripping
them from the substrates, as self supporting layers both without and after
annealing. According to an X ray diffraction study, a strongly textured face
centered cubic (fcc) structure was formed in the as deposited state with an
average grain size of about 10 nm. Upon annealing, the crystal structure was
retained whereas the grain size increased by a factor of 3 to 5, depending on
alloy composition. The zero field resistivity decreased strongly by annealing
due to the increased grain size. The annealing hardly changed the AMR below 50
at.% Co but strongly decreased it above this concentration. The composition
dependence of the resistivity and the AMR of the annealed Ni Co alloy deposits
was in good quantitative agreement with the available literature data both at
13 K and at room temperature. Both transport parameters were found to exhibit a
pronounced maximum in the composition range between 20 and 30 at.% Co and the
data of the Ni Co alloys fit well to the limiting values of the pure component
metals (fcc Ni and fcc Co). The only theoretical calculation reported formerly
on fcc Ni Co alloys yielded at T=0K a resistivity value smaller by a factor of
5 and an AMR value larger by a factor of about 2 than the corresponding low
temperature experimental data, although the theoretical results properly
reproduced the composition dependence of both quantities. | 1607.01960v1 |
2017-03-24 | Low temperature synthesis of heterostructures of transition metal dichalcogenide alloys (WxMo1-xS2) and graphene with superior catalytic performance for hydrogen evolution | Large-area ($\sim$cm$^2$) films of vertical heterostructures formed by
alternating graphene and transition-metal dichalcogenide(TMD) alloys are
obtained by wet chemical routes followed by a thermal treatment at low
temperature (300 $^\circ$C). In particular, we synthesized stacked graphene and
W$_x$Mo$_{1-x}$S$_2$ alloy phases that were used as hydrogen evolution
catalysts. We observed a Tafel slope of 38.7 mV dec$^{-1}$ and 96 mV onset
potential (at current density of 10 mA cm$^{-2}$) when the heterostructure
alloy is annealed at 300 $^o$C. These results indicate that heterostructure
formed by graphene and W$_{0.4}$Mo$_{0.6}$S$_2$ alloys are far more efficient
than WS$_2$ and MoS$_2$ by at least a factor of two, and it is superior than
any other reported TMD system. This strategy offers a cheap and low temperature
synthesis alternative able to replace Pt in the hydrogen evolution reaction
(HER). Furthermore, the catalytic activity of the alloy is stable over time,
i.e. the catalytic activity does not experience a significant change even after
1000 cycles. Using density functional theory calculations, we found that this
enhanced hydrogen evolution in the W$_x$Mo$_{1-x}$S$_2$ alloys is mainly due to
the lower energy barrier created by a favorable overlap of the d-orbitals from
the transition metals and the s-orbitals of H$_2$, with the lowest energy
barrier occurring for W$_{0.4}$Mo$_{0.6}$S$_2$ alloy. Thus, it is now possible
to further improve the performance of the "inert" TMD basal plane via metal
alloying, in addition to the previously reported strategies of creation of
point defects, vacancies and edges. The synthesis of
graphene/W$_{0.4}$Mo$_{0.6}$S$_2$ produced at relatively low temperatures is
scalable and could be used as an effective low cost Pt-free catalyst. | 1703.08597v2 |
2018-10-22 | Incorporation of random alloy GaBi$_x$As$_{1-x}$ barriers in InAs quantum dot molecules (I): energy levels and confined hole states | Self-assembled InAs quantum dots (QDs), which have long hole-spin coherence
times and are amenable to optical control schemes, have long been explored as
building blocks for qubit architectures. One such design consists of vertically
stacking two QDs to create a quantum dot molecule (QDM) and using the
spin-mixing properties of "molecule-like" coupled hole states for all-optical
qubit manipulation. In this article, the first of two papers, we introduce the
incorporation of dilute GaBiAs alloys in the barrier region between the two
dots. GaBiAs is expected to increase the spin-mixing of the molecular states
needed for qubit operations by raising the barrier valence band edge and
spin-orbit splitting. Using an atomistic tight-binding model, we compute the
properties of GaBiAs and the modification of hole states that arise when the
alloy is used in the barrier of an InAs QDM. An atomistic treatment is
necessary to correctly capture non-traditional alloy effects such as the
band-anticrossing valence band. It also allows for the study of configurational
variances and clustering effects of the alloy. We find that in InAs QDMs with a
GaBiAs interdot barrier, electron states are not strongly affected by the
inclusion of Bi. However, hole states are much more sensitive to the presence
and configuration of Bi in the barriers. By independently studying the
alloy-induced strain and electronic scattering off Bi and As orbitals, we
conclude that an initial increase in QDM hole state energy at low Bi
concentration is caused by the alloy-induced strain. We further find that the
decrease in QDM hole energy at higher Bi concentrations can only be explained
when both alloy strain and orbital effects are considered. In our second
article, we use the understanding developed here to discuss how the alloyed
barriers contribute to enhancement in hole spin-mixing and the implications for
QDM qubit architectures. | 1810.09483v2 |
2020-02-05 | Vertical bonding distances and interfacial band structure of PTCDA on a Sn-Ag surface alloy | Molecular materials enable a vast variety of functionalities for novel
electronic and spintronic devices. The unique possibility to alter or
substitute organic molecules or metallic substrates offers the opportunity to
modify and optimize interfacial properties for almost any desired field of
application. For this reason, we extend the successful approach to control
molecular interfaces by surface alloying. We present a comprehensive
characterization of the structural and electronic properties of the interface
formed between the prototypical molecule PTCDA and a Sn-Ag surface alloy grown
on an Ag(111) single crystal surface. We monitor the changes of adsorption
height of the surface alloy atoms and electronic valence band structure upon
adsorption of one layer of PTCDA using the normal incidence x-ray standing wave
technique in combination with momentum-resolved photoelectron spectroscopy. We
find that the vertical buckling and the surface band structure of the SnAg$_2$
surface alloy is not altered by the adsorption of one layer of PTCDA, in
contrast to our recent study of PTCDA on a PbAg$_2$ surface alloy [Phys. Rev.
Lett. 117, 096805 (2016)] . In addition, the vertical adsorption geometry of
PTCDA and the interfacial energy level alignment indicate the absence of any
chemical interaction between the molecule and the surface alloy. We attribute
the different interactions at these PTCDA/surface alloy interfaces to the
presence or absence of local $\sigma$-bonds between the PTCDA oxygen atoms and
the surface atoms. Combining our findings with results from literature, we are
able to propose an empiric rule for engineering the surface band structure of
alloys by adsorption of organic molecules. | 2002.01831v1 |
2023-01-23 | Study on the composition optimization method for improving the fluidity of cast Ti$_2$AlNb alloy and its mechanism | In this paper, the effects of Al, Nb main elements, Fe, Mo, W, Co, B, Si and
their contents on the fluidity of Ti-22Al-25Nb alloy were investigated. The
composition that was beneficial to improve the fluidity was screened through
the thermodynamic software calculating thermophysical parameters affecting the
fluidity of Ti$_2$AlNb alloy, the numerical simulation test of its fluidity and
the verification test of the fluidity of optimized alloys. Finally, the
improvement mechanism of the alloy fluidity was discussed. Results showed that
the appropriate reduction of Nb element was better than Al element for the
improvement of fluidity. The addition of trace Fe, B and Si elements were
beneficial to the improvement of fluidity, the improvement effect of B element
was best, while the addition of trace Mo, W, Co were not conducive to the
improvement of fluidity. The cessation mechanism of Ti$_2$AlNb alloy is the
cessation mechanism of the alloy with a wide crystallization temperature range.
The composition which was most beneficial to improve the fluidity was
Ti-22Al-24Nb-0.1B. The main reasons for the improvement of the fluidity had two
sides: on the one hand, the reduction of 1at% Nb and the addition of 0.1at% B
not only increased the superheat and crystallization latent heat of the alloy,
but also reduced the melt viscosity and thermal conductivity, thus improving
the fluidity. On the other hand, the TiB phase refined the grains, the fine
grains prevented the dendrite from growing into developed dendrite networks,
inhibited the adverse effect of the increase in the width of the solidification
zone on the fluidity, reduced the flow resistance of the molten metal, and
further improved the fluidity of the alloy. | 2301.09499v1 |
1998-08-24 | Heterovalent and A-atom effects in A(B'B'')O3 perovskite alloys | Using first-principles supercell calculations, we have investigated
energetic, structural and dielectric properties of three different A(B'B'')O_3
perovskite alloys: Ba(Zn_{1/3}Nb_{2/3})O_3 (BZN), Pb(Zn_{1/3}Nb_{2/3})O_3
(PZN), and Pb(Zr_{1/3}Ti_{2/3})O_3 (PZT). In the homovalent alloy PZT, the
energetics are found to be mainly driven by atomic relaxations. In the
heterovalent alloys BZN and PZN, however, electrostatic interactions among B'
and B'' atoms are found to be very important. These electrostatic interactions
are responsible for the stabilization of the observed compositional long-range
order in BZN. On the other hand, cell relaxations and the formation of short
Pb--O bonds could lead to a destabilization of the same ordered structure in
PZN. Finally, comparing the dielectric properties of homovalent and
heterovalent alloys, the most dramatic difference arises in connection with the
effective charges of the B' atom. We find that the effective charge of Zr in
PZT is anomalous, while in BZN and PZN the effective charge of Zn is close to
its nominal ionic value. | 9808268v1 |
1998-11-20 | Effects of disorder on the optical properties of the (Zn,Mg)(S,Se) quaternary alloy | The electronic and optical properties of (Zn,Mg)(S,Se) wide-gap solid
solutions are studied using ab initio techniques and starting from the
previously determined atomistic structure of the alloy. Compositional disorder
is shown to close substantially the gap with respect to the predictions of the
virtual-crystal approximation. The bowing of the fundamental gap vs.
composition predicted by our calculations is in very good agreement with
experiments available for the Zn(S,Se) pseudo-binary alloy. At temperatures
typical for MBE growth, the quaternary alloy displays a rather large amount of
short-range order whose effect is to slightly but unmistakably open the gap.
Our results agree well with recent experimental data for the quaternary alloy. | 9811306v1 |
2000-05-24 | Magnetic-Field-Controlled Twin Boundaries Motion and Giant Magneto-Mechanical Effects in Ni-Mn-Ga Shape Memory Alloy | Recently, several research groups have reported on the observation of
super-large more than 5% magneto-strain effect in some non-stoichiometric
Ni-Mn-Ga alloys close to 5.78% value expected from the tetragonality aspect
ratio of the martensite crystal lattice. New Ni-Mn-Ga alloys showing giant
magneto-strain effect display simultaneously few interesting physical effects
and new behavior for some magnetic and mechanical properties which is very
different from that earlier observed in Ni-Mn-Ga showing lower magneto-strain
effect. This report represents some new experimental results and the
quantitative model describing large magneto-strain effect and main mechanical
and magnetic properties observed in several ferromagnetic shape-memory alloys.
The model application to giant magneto-strain effect recently found in some
non-stoichiometric Ni-Mn-Ga alloys is discussed. | 0005425v1 |
2002-06-28 | A differential cluster variation method for analysis of spiniodal decomposition in alloys | A differential cluster variation method (DCVM) is proposed for analysis of
spinoidal decomposition in alloys. In this method, lattice symmetry operations
in the presence of an infinitesimal composition gradient are utilized to deduce
the connection equations for the correlation functions and to reduce the number
of independent variables in the cluster variation analysis.
Application of the method is made to calculate the gradient energy
coefficient in the Cahn-Hilliard free energy function and the fastest growing
wavelength for spinodal decomposition in Al-Li alloys. It is shown that the
gradient coefficient of congruently ordered Al-Li alloys is much larger than
that of the disordered system. In such an alloy system, the calculated fastest
growing wavelength is approximately 10 nm, which is an order of magnitude
larger than the experimentally observed domain size. This may provide a
theoretical explanation why spinodal decomposition after a congruent ordering
is dominated by the antiphase boundaries. | 0206578v1 |
2002-12-17 | Local Charge distributions in Metallic Alloys: a Local Field Coherent Potential Approximation Theory | Electronic structure calculations performed on very large supercells have
shown that the local charge excesses in metallic alloys are related through
simple linear relations to the local electrostatic field resulting from
distribution of charges in the whole crystal. By including local external
fields in the single site Coherent Potential Approximation theory, we develop a
novel theoretical scheme in which the local charge excesses for random alloys
can be obtained as the responses to local external fields. Our model maintains
all the computational advantages of a single site theory but allows for full
charge relaxation at the impurity sites. Through applications to CuPd and CuZn
alloys, we find that, as a general rule, non linear charge rearrangements occur
at the impurity site as a consequence of the complex phenomena related with the
electronic screening of the external potential. This nothwithstanding, we
observe that linear relations hold between charge excesses and external
potentials, in quantitative agreement with the mentioned supercell
calculations, and well beyond the limits of linearity for any other site
property. | 0212405v1 |
2003-12-13 | Pathways of structural and magnetic transitions in ferromagnetic shape memory alloys | A fundamental question in the study of ferromagnetic shap ememory alloys is
the nature of magnetoelastic coupling and the extent it drives the structural
transformation. This question also holds the key to developing new and
optimized alloys that combine high strains at low switching field. In the
present study it is shown that the reconfiguration of the micromagnetic
structure is enslaved to and follows the martensitic transformation in these
alloys using the NiMnGa and FePd alloy systems. This is determined by
developing a new, high speed electronic method to study the temperature
dependence of domain dynamics, called magnetic transition spectra. The sequence
of magnetic and structural transition was found to be as follows. For cooling,
structural transition followed by magnetic transition and for heating, magnetic
transition followed by structural transition. | 0312343v1 |
2004-04-05 | Phase Stability in 3d-5d (NiPt and CuAu) and 3d-4d (NiPd and CuAg) Systems | We show the differences in the stability of 3d-5d (NiPt and CuAu) and 3d-4d
(NiPd and CuAg) alloys arise mainly due to relativistic corrections. The
magnetic properties of disordered NiPd and NiPt alloys also differ due to these
corrections which lead to increase in the separation between s-d bands of 5d
elements in these alloys. For the magnetic case we analyze the results in terms
of splitting of majority and minority spin d-band centers of the 3d elements.
We further examine the effect of relativistic corrections to the pair energies
and order disorder transition temperatures in these alloys. The magnetic
moments and Curie temperatures have also been studied along with the short
range ordering/segregation effects in NiPt/NiPd alloys. | 0404087v1 |
2004-12-06 | Surface Phases in Binary Liquid Metal Alloys | Surface sensitive x-ray scattering techniques with atomic scale resolution
are employed to investigate the microscopic structure of the surface of three
classes of liquid binary alloys: (i) Surface segregation in partly miscible
binary alloys as predicted by the Gibbs adsorption rule is investigated for
Ga-In. The first layer consists of a supercooled In monolayer and the bulk
composition is reached after about two atomic diameters. (ii) The Ga-Bi system
displays a wetting transition at a characteristic temperature T_w~220 C. The
transition from a Bi monolayer on Ga below T_w to a thick Bi-rich wetting film
above T_w is studied. (iii) The effect of attractive interactions between the
two components of a binary alloy on the surface structure is investigated for
two Hg-Au alloys. | 0412112v1 |
2005-05-10 | On the propensity of magnetism in 3d transition-metal-MgCNi_3 alloys | The changes in the electronic properties of the substitutionally disordered
MgC(Ni_{1-x}T_{x})_{3} (T=Fe, Co or Cu) alloys are studied using the atomic
sphere formulation of the Korringa-Kohn-Rostoker coherent-potential
approximation method (KKR-ASA CPA), while the effects of incipient magnetism in
these alloys are studied phenomenologically using Ginzburg-Landau coefficients
in conjunction with fixed-spin moment method. We find that the disordered
MgC(Ni_{1-x}T_{x})_{3} alloys have a small magnetic moment localized at Fe and
Co sites for low concentrations. The overestimation of the calculated magnetic
moment is likely to be due to the limitations of the local-density
approximation used in the present study. However, the calculated
Ginzburg-Landau coefficients clearly show that the disordered
MgC(Ni_{1-x}T_{x})_{3} alloys remain paramagnetic. At expanded volumes, we also
find the possibility of a ferromagnetic state for MgC(Ni_{0.95}Fe_{0.05})_{3}
and MgC(Ni_{0.90}Co_{0.10})_{3}. | 0505235v1 |
2005-10-07 | A Practical Guide for X-Ray Diffraction Characterization of Ga(Al, In)N Alloys | Ga(In, Al)N alloys are used as an active layer or cladding layer in light
emitting diodes and laser diodes. x-ray diffraction is extensively used to
evaluate the crystalline quality, the chemical composition and the residual
strain in Ga(Al,In)N thin films, which directly determine the emission
wavelength and the device performance. Due to the minor mismatch in lattice
parameters between Ga(Al, In)N alloy and a GaN virtual substrate, x-ray
diffraction comes to a problem to separate the signal from Ga(Al,In)N alloy and
GaN. We give a detailed comparison on different diffraction planes. In order to
balance the intensity and peak separation between Ga(Al,In)N alloy and GaN,
(0004) and (1015) planes make the best choice for symmetric scan and asymmetric
scan, respectively. | 0510174v1 |
2005-10-17 | Optimizing Tc in the (Mn,Cr,Ga)As and (Mn,Ga)(As,P) Ternary Alloys | We explore two possible ways to enhance the critical temperature $T_c$ in the
dilute magnetic semiconductor Mn$_{0.08}$Ga$_{0.92}$As. Within the context of
the double-exchange and RKKY pictures, the ternary alloys
Mn$_{x}$Cr$_{0.08-x}$Ga$_{0.92}$As and Mn$_{0.08}$Ga$_{0.92}$As$_y$P$_{1-y}$
might be expected to have $T_c$ higher than the pseudobinary
Mn$_{0.08}$Ga$_{0.92}$As. To test whether the expectations from model pictures
are confirmed, we employ linear response theory within the local-density
approximation to search for theoretically higher critical temperatures in these
ternary alloys. Our results show that neither co-doping Mn with Cr, nor
alloying As with P improves $T_c$. Alloying with Cr is found to be deleterious
to the $T_c$. Mn$_{0.08}$Ga$_{0.92}$As$_y$P$_{1-y}$ shows almost linear
dependence of $T_c$ on $y$. | 0510440v1 |
2007-06-26 | Effects of solute concentrations on kinetic pathways in Ni-Al-Cr alloys | The kinetic pathways resulting from the formation of coherent L12-ordered
y'-precipitates in the g-matrix (f.c.c.) of Ni-7.5 Al-8.5 Cr at.% and Ni-5.2
Al-14.2 Cr at.% alloys, aged at 873 K, are investigated by atom-probe
tomography (APT) over a range of aging times from 1/6 to 1024 hours; these
alloys have approximately the same volume fraction of the y'-precipitate phase.
Quantification of the phase decomposition within the framework of classical
nucleation theory reveals that the y-matrix solid-solution solute
supersaturations of both alloys provide the chemical driving force, which acts
as the primary determinant of the nucleation behavior. In the coarsening
regime, the temporal evolution of the y'-precipitate average radii and the
y-matrix supersaturations follow the predictions of classical coarsening
models, while the temporal evolution of the y'-precipitate number densities of
both alloys do not. APT results are compared to equilibrium calculations of the
pertinent solvus lines determined by employing both Thermo-Calc and
Grand-Canonical Monte Carlo simulation. | 0706.3916v1 |
2007-12-21 | First-principles investigation of Ag-Cu alloy surfaces in an oxidizing environment | In this paper we investigate by means of first-principles density functional
theory calculations the (111) surface of the Ag-Cu alloy under varying
conditions of pressure of the surrounding oxygen atmosphere and temperature.
This alloy has been recently proposed as a catalyst with improved selectivity
for ethylene epoxidation with respect to pure silver, the catalyst commonly
used in industrial applications. Here we show that the presence of oxygen leads
to copper segregation to the surface. Considering the surface free energy as a
function of the surface composition, we construct the convex hull to
investigate the stability of various surface structures. By including the
dependence of the free surface energy on the oxygen chemical potential, we are
able compute the phase diagram of the alloy as a function of temperature,
pressure and surface composition. We find that, at temperature and pressure
typically used in ethylene epoxidation, a number of structures can be present
on the surface of the alloy, including clean Ag(111), thin layers of copper
oxide and thick oxide-like structures. These results are consistent with, and
help explain, recent experimental results. | 0712.3652v1 |
2008-08-27 | Nanostructure and related mechanical properties of an Al-Mg-Si alloy processed by severe plastic deformation | Microstructural features and mechanical properties of an Al-Mg-Si alloy
processed by high-pressure torsion have been investigated using transmission
electron microscopy, X-ray diffraction, three-dimensional atom probe, tensile
tests and microhardness measurements. It is shown that HPT processing of the
Al-Mg-Si alloy leads to a much stronger grain size refinement than of pure
aluminium (down to 100 nm). Moreover, massive segregation of alloying elements
along grain boundaries is observed. This nanostructure exhibits a yield stress
even two times higher than that after a standard T6 heat treatment of the
coarse grained alloy | 0808.3715v1 |
2008-10-02 | Magnetic Modulation in Mechanical Alloyed Cr1.4fe0.6o3 Oxide | We have synthesized Cr1.4Fe0.6O3 compound through mechanical alloying of
Cr2O3 and Fe2O3 powders and subsequent thermal annealing. The XRD spectrum, SEM
picture and microanalysis of EDAX spectrum have been used to understand the
structural evolution in the alloyed compound. The alloyed samples are matching
to rhombohedral structure with R3C space group. The observation of a modulated
magnetic order confirmed a systematic diffusion of Fe atoms into the Cr sites
of lattice structure. A field induced magnetic behaviour is seen in the field
dependence of magnetization data of the annealed samples. The behaviour is
significantly different from the mechanical alloyed samples. The experimental
results provided the indications of considering the present material as a
potential candidate for opto-electronic applications. | 0810.0439v1 |
2008-11-06 | Ag-Cu alloy surfaces in an oxidizing environment: a first-principles study | Recent experiments on model catalysts have shown that Ag-Cu alloys have
improved selectivity with respect to pure silver for ethylene epoxidation. In
this paper we review our first-principles investigations on the (111) surface
of this alloy and present new findings on other low index surfaces. We find
that, for every surface orientation, the presence of oxygen leads to copper
segregation to the surface. Considering the alloy to be in equilibrium with an
oxygen atmosphere and accounting for the effect of temperature and pressure, we
compute the surface free energy and study the stability of several surface
structures. Investigating the dependence of the surface free energy on the
surface composition, we construct the phase diagram of the alloy for every
surface orientation. Around the temperature, pressure and composition of
interest for practical applications, we find that a limited number of
structures can be present, including a thin layer of copper oxide on top of the
silver surface and copper-free structures. Different surface orientations show
a very similar behavior and in particular a single layer with CuO
stoichiometry, significantly distorted when compared to a layer of bulk CuO,
has a wide range of stability for all orientations. Our results are consistent
with, and help explain, recent experimental measurements. | 0811.0864v1 |
2009-01-01 | Calculation of solubility in titanium alloys from first-principles | We present an approach to calculate the atomic bulk solubility in binary
alloys based on the statistical-thermodynamic theory of dilute lattice gas. The
model considers all the appropriate ground states of the alloy and results in a
simple Arrhenius-type temperature dependence determined by a {\it
"low-solubility formation enthalpy"}. This quantity, directly obtainable from
first-principle calculations, is defined as the composition derivative of the
compound formation enthalpy with respect to nearby ground states. We apply the
framework and calculate the solubility of the A specie in A-Ti alloys
(A=Ag,Au,Cd,Co,Cr,Ir,W,Zn). In addition to determining unknown low-temperature
ground states for the eight alloys, we find qualitative agreements with
solubility experimental results. The presented formalism, correct in the
low-solubility limit, should be considered as an appropriate starting point for
determining if more computationally expensive formalisms are otherwise needed. | 0901.0200v1 |
2009-01-09 | Decomposition process in a FeAuPd alloy nanostructured by severe plastic deformation | The decomposition process mechanisms have been investigated in a Fe50Au25Pd25
(at.%) alloy processed by severe plastic deformation. Phases were characterized
by X-ray diffraction and microstructures were observed using transmission
electron microscopy. In the coarse grain alloy homogenized and aged at $450
^{circ}\mathrm{C}$, the bcc \alpha-Fe and fcc AuPd phases nucleate in the fcc
supersaturated solid solution and grow by a discontinuous precipitation process
resulting in a typical lamellar structure. The grain size of the homogenized
FeAuPd alloy was reduced in a range of 50 to 100nm by high pressure torsion.
Aging at $450 ^{circ}\mathrm{C}$ this nanostructure leads to the decomposition
of the solid solution into an equi-axed microstructure. The grain growth is
very limited during aging and the grain size remains under 100nm. The
combination of two phases with different crystallographic structures (bcc
\alpha-Fe and fcc AuPd) and of the nanoscaled grain size gives rise to a
significant hardening of the alloy | 0901.1191v1 |
2009-08-01 | Phosphorous alloying: controlling the magnetic anisotropy in ferromagnetic (Ga,Mn)(As,P) Layers | Phosphorous alloying of GaMnAs thin films has been used for the manipulation
of the magnetic anisotropies in ferromagnetic Ga0.93Mn0.07As1-yPy layers. We
have determined the anisotropy constants as a function of temperature for
phosphorous alloying levels between 0 and 8.8 at % for a Mn doping level of ~
7at%. We show that it is possible to obtain layers with robust ferromagnetism
and either in-plane or out-of plane easy axes with small barriers for
magnetization reorientation by phosphorous alloying with y< 6at% or y> 6at%.
The critical temperatures are not significantly increased by the P alloying. | 0908.0063v2 |
2010-01-11 | Calculation of the P-T phase diagram and tendency toward decomposition in equiatomic TiZr alloy | Electronic, structural and thermodynamic properties of the equiatomic alloy
TiZr are calculated within the electron density functional theory and the
Debye-Gruneisen model. The calculated values of the lattice parameters a and
c/a agree well with the experimental data for the alpha, omega and beta phases.
The omega phase is shown to be stable at atmospheric pressure and low
temperatures; it remains energetically preferable up to T=600K. The alpha phase
of the TiZr alloy becomes stable in the range 600K<T<900K, and the beta phase
at temperatures above 900K. The constructed phase diagram qualitatively agrees
with the experimental data available. The tendency toward decomposition in the
equiatomic alloy omega-TiZr is studied. It is shown that in the ground state
the omega phase of the ordered equiatomic alloy TiZr has a tendency toward
ordering, rather than decomposition. | 1001.1700v1 |
2010-02-02 | Computational study of structural and elastic properties of random AlGaInN alloys | In this work we present a detailed computational study of structural and
elastic properties of cubic AlGaInN alloys in the framework of Keating valence
force field model, for which we perform accurate parametrization based on state
of the art DFT calculations. When analyzing structural properties, we focus on
concentration dependence of lattice constant, as well as on the distribution of
the nearest and the next nearest neighbour distances. Where possible, we
compare our results with experiment and calculations performed within other
computational schemes. We also present a detailed study of elastic constants
for AlGaInN alloy over the whole concentration range. Moreover, we include
there accurate quadratic parametrization for the dependence of the alloy
elastic constants on the composition. Finally, we examine the sensitivity of
obtained results to computational procedures commonly employed in the Keating
model for studies of alloys. | 1002.0437v1 |
2010-07-14 | Band gap bowing of binary alloys: Experimental results compared to theoretical tight-binding supercell calculations for CdZnSe | Compound semiconductor alloys of the type ABC find widespread applications as
their electronic bulk band gap varies continuously with x, and therefore a
tayloring of the energy gap is possible by variation of the concentration. We
model the electronic properties of such semiconductor alloys by a multiband
tight-binding model on a finite ensemble of supercells and determine the band
gap of the alloy. This treatment allows for an intrinsic reproduction of band
bowing effects as a function of the concentration x and is exact in the
alloy-induced disorder. In the present paper, we concentrate on bulk CdZnSe as
a well-defined model system and give a careful analysis on the proper choice of
the basis set and supercell size, as well as on the necessary number of
realizations. The results are compared to experimental results obtained from
ellipsometric measurements of CdZnSe layers prepared by molecular beam epitaxy
(MBE) and photoluminescence (PL) measurements on catalytically grown CdZnSe
nanowires reported in the literature. | 1007.2297v2 |
2010-10-24 | Crystallization of amorphous alloy of Fe-Cu-Nb-Si-B under the influence of high-power flashing optical radiation | Investigation of crystallization in an 5BDSR amorphous alloy by system
Fe-Cu-Nb-Si-B under the influence of an incoherent optical radiation generated
by gas discharge flash-lamp using X-ray diffraction scattering. It is shown
that depending on the magnitude of the input power to the flash-lamp of
structure of the alloy varies widely after radiation: a change in the
short-range order during retaining the amorphous structure or the formation of
several crystalline phases with different quantitative content in the alloy.
Conditions are found for the formation of the nanocrystalline structure.
Analysis in terms of the Kolmogorov-Johnson-Mehl-Avrami kinetics showed that
the mechanism of primary crystallization under flash irradiation is associated
with a well-known for finemet-type alloys: rapid nucleation, followed by growth
slowing of nanocrystalline phase alpha-Fe(Si) with the D03 structure. | 1010.5010v1 |
2011-02-23 | Atomistic approach to alloy scattering in $Si_{1-x}Ge_{x}$ | SiGe alloy scattering is of significant importance with the introduction of
strained layers and SiGe channels into CMOS technology. However, alloy
scattering has till now been treated in an empirical fashion with a fitting
parameter. We present a theoretical model within the atomistic tight-binding
representation for treating alloy scattering in SiGe. This approach puts the
scattering model on a solid atomistic footing with physical insights. The
approach is shown to inherently capture the bulk alloy scattering potential
parameters for both n-type and p-type carriers and matches experimental
mobility data. | 1102.4805v3 |
2011-04-22 | Carbon release by selective alloying of transition metal carbides | We have performed first principles density functional theory calculations on
TiC alloyed on the Ti sublattice with 3d transition metals ranging from Sc to
Zn. The theory is accompanied with experimental investigations, both as regards
materials synthesis as well as characterization. Our results show that by
dissolving a metal with a weak ability to form carbides, the stability of the
alloy is lowered and a driving force for the release of carbon from the carbide
is created. During thin film growth of a metal carbide this effect will favor
the formation of a nanocomposite with carbide grains in a carbon matrix. The
choice of alloying elements as well as their concentrations will affect the
relative amount of carbon in the carbide and in the carbon matrix. This can be
used to design the structure of nanocomposites and their physical and chemical
properties. One example of applications is as low-friction coatings. Of the
materials studied, we suggest the late 3d transition metals as the most
promising elements for this phenomenon, at least when alloying with TiC. | 1104.4469v1 |
2011-05-10 | Thermoelectric properties of Co, Ir, and Os-Doped FeSi Alloys: Evidence for Strong Electron-Phonon Coupling | The effects of various transition metal dopants on the electrical and thermal
transport properties of Fe1-xMxSi alloys (M= Co, Ir, Os) are reported. The
maximum thermoelectric figure of merit ZTmax is improved from 0.007 at 60 K for
pure FeSi to ZT = 0.08 at 100 K for 4% Ir doping. A comparison of the thermal
conductivity data among Os, Ir and Co doped alloys indicates strong
electron-phonon coupling in this compound. Because of this interaction, the
common approximation of dividing the total thermal conductivity into
independent electronic and lattice components ({\kappa}Total =
{\kappa}electronic + {\kappa}lattice) fails for these alloys. The effects of
grain size on thermoelectric properties of Fe0.96Ir0.04Si alloys are also
reported. The thermal conductivity can be lowered by about 50% with little or
no effect on the electrical resistivity or Seebeck coefficient. This results in
ZTmax = 0.125 at 100 K, still about a factor of five too low for solid-state
refrigeration applications. | 1105.2006v1 |
2011-06-13 | Short-range correlations in binary alloys: Spin model approach to Ag$_c$Pd$_{1-c}$ | Short-range correlations in Ag-Pd alloys are investigated by analyzing the
{\em ab initio} total energy of fcc based random Ag$_c$Pd$_{1-c}$. Since the
information on the atomic interactions is incorporated in the energetics of
alloys it is possible with a suitable model, Bethe-Peierls-Weiss model is used
in the present work, to invert the problem, i.e.\ to obtain information on the
short-range correlation from the total energy of a random system. As an example
we demonstrate how site correlations can be extracted from random alloy data.
Bethe-Peierls-Weiss model predicts positive first neighbor correlator and
mixing energy for substitutional face centered cubic (fcc) Ag-Pd alloys at low
temperature which can be related to the optimal structures of
Ag$_{0.5}$Pd$_{0.5}$. | 1106.2455v1 |
2011-08-15 | Role of Alloying-Atom Size Factor and System Shape Factor in Energetics of bcc Fe under Macroscopic Deformation | We present an \emph{ab initio} study of the effect of macroscopic deformation
on energetics of twelve alloying elements in bcc Fe under three specially
designed strain modes. We find that there exists a universal linear relation of
describing the volume dependence of substitutional energy of alloying elements
via introducing two factors --- the system shape factor ($f_{\scriptsize{ss}}$)
and the size factor of alloying element $M$ ($\Omega^{M}_{\scriptsize{sf}}$):
$E_{\scriptsize{sub}} \sim f_{\scriptsize{ss}}\Omega^{M}_{\scriptsize{sf}}V$.
$\Omega^{M}_{\scriptsize{sf}}$ well describes the effect of intrinsic
alloying-atom size and the influence of chemical interaction with matrix atom,
and $f_{\scriptsize{ss}}$ characterizes the degree of system lattice distortion
under deformation. This relation is further validated using the published data
of stained-modulated doping in GaP | 1108.2953v1 |
2011-08-17 | Role of Site-selective Doping on Melting Point of CuTi Alloys: A Classical Molecular Dynamics Simulation Study | Effect of site-selective substitution of Ti in Cu on the thermal stability of
CuTi alloy is investigated using classical molecular dynamics simulations with
Embedded Atom Method potentials. It has been observed experimentally that
melting point of all the naturally occurring stable phases of CuTi alloys do
not show a definite trend with gradual increase in Ti concentration. To
understand the phenomenon, super cells of CuTi alloy are constructed where Cu
atom is substituted by Ti randomly and at selective sites. For random
substitution, the melting point decreases linearly with increase in Ti
concentration. A non-monotonous dependence is seen when Cu atoms at selective
sites are replaced by Ti. For a particular doping concentration, the melting
point shows a wide range of variation depending on the order of atomic
arrangement, and can be fine tuned by selecting the sites for substitution. The
variations in melting points in different cases are explained in terms of the
peak height, width and position of the corresponding radial distribution
functions. Finally, it is verified that initial structures of the naturally
occurring CuTi alloys are responsible for the non-definite trend in their
melting points. | 1108.3396v1 |
2011-11-30 | First-principles calculations atomic structure and elastic properties of Ti-Nb alloys | Elastic properties of Ti based \beta-alloy were studied by the method of the
model structure first principle calculations. Concentrational dependence of
Young modulus for the binary \beta-alloy Ti-Nb was discovered. It is shown that
peculiarities visible at 15-18% concentrations can be related to the different
Nb atoms distribution. Detailed comparison of the calculation results with the
measurement results was done. Young modulus for the set of the ordered
structures with different Nb atoms location, which simulate triple \beta-alloys
Ti-29.7%Zr-18.5%Nb and Ti-51.8%Zr-18.5%Nb have been calculated. The results of
these calculations allowed us to suggest the concentration region for
single-phase ternary \beta-phase alloys possessing low values of Young's
modulus. | 1111.7182v1 |
2012-03-02 | Precipitate assemblies formed on dislocation loops in aluminium-silver-copper alloys | The precipitation microstructure of the \gamma' (AlAg2) intermetallic phase
has been examined in aluminium-silver-copper alloys. The microstructure
developed in an Al-0.90at.%Ag-0.90at.%Cu alloy was significantly different from
that reported for binary Al-Ag alloys. The orientation relationship between the
matrix and precipitate was unchanged; however, the \gamma' phase formed as
aggregates with a two-dimensional open assemblies. Each such assembly contained
two variants of the \gamma' phase alternately arranged to form a faceted
elliptical unit. The \theta' (Al2Cu) phase formed on these assemblies after
further ageing. The faceted elliptical assembly morphology has not been
previously reported for the \gamma' precipitate. The change in precipitation
behaviour was attributed to copper modifying the as-quenched defect structure
of the matrix. This precipitation morphology clarifies earlier observations on
the precipitate number density and mechanical properties of
aluminium-silver-copper alloys. | 1203.0406v1 |
2012-04-27 | Mössbauer spectroscopy of short-range ordered Fe-Cr alloys | Short-range ordering (SRO) in Fe-Cr has been the subject of a number of
recent experimental and theoretical investigations, as ordering effects are
significant for the phase stability of this technologically important alloy.
Recently, discrepancies regarding the SRO behavior of Fe-Cr have been reported
between results obtained with M\"ossbauer spectroscopy and diffuse neutron
scattering, respectively. As the methodology for reducing SRO parameters from
M\"ossbauer spectra is indirect and relies on the determination of a large
number of parameters, here a new method for directly connecting the M\"ossbauer
effect with SRO is presented. The method is verified using synthetic spectra
derived from Monte-Carlo simulated alloy structures with different SRO
parameters and subsequently it is applied to experimental data obtained from
Fe-Cr alloys. Agreement with diffuse neutron scattering is found at low Cr
concentrations; however, some discrepancies still remain for more concentrated
alloys and possible reasons for these are discussed. | 1204.6195v1 |
2012-10-02 | A new first principles approach to calculate phonon spectra of disordered alloys | The lattice dynamics in substitutional disordered alloys with constituents
having large size differences is driven by strong disorder in masses,
inter-atomic force constants and local environments. In this letter, a new
first-principles approach based on special quasi random structures and
itinerant coherent potential approximation to compute the phonon spectra of
such alloys is proposed and applied to Ni$_{0.5}$Pt$_{0.5}$ alloy. The
agreement between our results with the experiments is found to be much better
than for previous models of disorder due to an accurate treatment of the
interplay of inter-atomic forces among various pairs of chemical species. This
new formalism serves as a potential solution to the longstanding problem of a
proper microscopic understanding of lattice dynamical behavior of disordered
alloys. | 1210.0662v1 |
2012-10-17 | Simulation of Early-stage Clustering in Ternary Metal Alloys Using the Phase Field Crystal Method | Phase field crystal methodology is applied, for the first time, to study the
effect of alloy composition on the clustering behavior of a quenched/aged
supersaturated ternary Al alloy system. An analysis of the work of formation is
built upon the methodology developed in Fallah {\it et al.} to describe the
dislocation-mediated formation mechanisms of early clusters in binary alloys
[Phys. Rev. B., DOI: 10.1103/PhysRevB.00.004100]. Consistent with the
experiments, we demonstrate that the addition of Mg to an Al-1.1Cu alloy
increases the nucleation rate of clusters in the quenched/aged state by
increasing the effective driving force for nucleation, enhancing the
dislocation stress relaxation and decreasing the surface energy associated with
the Cu-rich Cu-Mg co-clusters. Furthermore, we show that it is
thermodynamically favourable for small sub-critical clusters to have higher
affinity for Mg than larger overcritical Cu-rich clusters, particularly
depicting a two-stage clustering phenomenon. | 1210.4977v1 |
2012-10-19 | Form-free size distributions from complementary stereological TEM/SAXS on precipitates in a Mg-Zn alloy | This work describes a multidisciplinary research methodology for quantifying
the size distribution of nanoscale precipitates in polycrystalline alloys.
Complementary transmission electron microscopy (TEM) and small-angle x-ray
scattering (SAXS) are employed in a study of precipitate growth in an
isothermally aged Mg-Zn alloy. TEM is used to identify the precipitate phases
as rod-shaped \beta' particles and to determine their radii and aspect ratio.
Subsequently, SAXS data obtained from bulk quantities of the alloy is
interpreted via a novel Monte-Carlo method to obtain accurate, form-free size
distributions. Good agreement was obtained between particle radii distributions
measured by both methods, exemplifying the applicability of this complementary
methodology to study precipitation in textured alloys containing particles
anisotropic with well-defined orientation-relationships to the matrix. | 1210.5366v4 |
2013-01-03 | Compositionally-modulated Si1-xGex multilayers with cross-plane thermal conductivity below the thin-film alloy limit | We describe epitaxial Ge/Si multilayers with cross-plane thermal
conductivities which can be systematically reduced to exceptionally low values,
as compared both with bulk and thin-film SiGe alloys of the same average
concentration, by simply changing the thicknesses of the constituent layers. Ab
initio calculations reveal that partial interdiffusion of Ge into the Si
spacers, which naturally results from Ge segregation during growth, plays a
determinant role, lowering the thermal conductivity below what could be
achieved without interdiffusion (perfect superlattice), or with total
interdiffusion (alloy limit). This phenomenon is similar to the one previously
observed in alloys with embedded nanoparticles, and it stresses the importance
of combining alloy and nanosized scatterers simultaneously to minimize thermal
conductivity. Our calculations thus suggest that superlattices with sharp
interfaces, which are commonly sought but difficult to realize, are worse than
compositionally-modulated Si1-xGex multilayers in the search for materials with
ultralow thermal conductivities. | 1301.0405v1 |
2013-08-02 | Control of Spin in La(Mn,Zn)AsO Alloy by Carrier Doping | The control of spin without magnetic field is one of challenges in developing
spintronic devices. In an attempt to solve this problem, we proposed a novel
hypothetic LaMn0.5Zn0.5AsO alloy from two experimentally synthesized rare earth
element transition metal arsenide oxides, i.e. LaMnAsO and LaZnAsO. On the
basis of the first-principles calculations with strong-correlated correction,
we found that the LaMn0.5Zn0.5AsO alloy is an antiferromagnetic semiconductor
at ground state, while bipolar magnetic semiconductor at ferromagnetic state.
Both electron and hole doping in the LaMn0.5Zn0.5AsO alloy induces the
transition from antiferromagnetic to ferromagnetic, as well as semiconductor to
half metal. In particular, the spin-polarization direction is switchable
depending on the doped carrier's type. As carrier doping can be realized easily
in experiment by applying a gate voltage, the LaMn0.5Zn0.5AsO alloy stands for
a promising spintronic material to generate and control the spin-polarized
carriers with electric field. | 1308.0439v1 |
2013-09-27 | Constrained non-collinear magnetism in disordered Fe and Fe-Cr alloys | The development of quantitative models for radiation damage effects in iron,
iron alloys and steels, particularly for the high temperature properties of the
alloys, requires understanding of magnetic interactions, which control the
phase stability of ferritic-martensitic, ferritic, and austenitic steels. In
this work, disordered magnetic configurations of pure iron and Fe-Cr alloys are
investigated using Density Functional Theory (DFT) formalism, in the form of
constrained non-collinear magnetic calculations, with the objective of creating
a database of atomic magnetic moments and forces acting between the atoms. From
a given disordered atomic configuration of either pure Fe or Fe-Cr alloy, a
penalty contribution to the usual spin-polarized DFT total energy has been
calculated by constraining the magnitude and direction of magnetic moments. An
extensive database of non-collinear magnetic moment and force components for
various atomic configurations has been generated and used for interpolating the
spatially-dependent magnetic interaction parameters, for applications in
large-scale spin-lattice dynamics and magnetic Monte-Carlo simulations. | 1309.7183v1 |
2013-10-24 | First Principles Calculation of Elastic Moduli of Early-Late Transition Metal Alloys | Motivated by interest in the elastic properties of high strength amorphous
metals, we examine the elastic properties of select crystalline phases. Using
first principles methods, we calculate elastic moduli in various chemical
systems containing transition metals, specifically early (Ta,W) and late
(Co,Ni). Theoretically predicted alloy elastic properties are verified for
Ni-Ta by comparison with experimental measurements using resonant ultrasound
spectroscopy. Comparison of our computed elastic moduli with effective medium
theories shows that alloying leads to enhancement of bulk moduli relative to
averages of the pure elements, and considerable deviation of predicted and
computed shear moduli. Specifically, we find an enhancement of bulk modulus
relative to effective medium theory and propose a candidate system for high
strength, ductile amorphous alloys. Trends in the elastic properties of
chemical systems are analyzed using force constants, electronic densities of
state and Crystal Overlap Hamilton Populations. We interpret our findings in
terms of the electronic structure of the alloys. | 1310.6709v2 |
2013-12-04 | Species Fractionation in Atomic Chains from Mechanically Stretched Alloys | Bettini et al. [Nature Nanotech 1, 182 (2006)] reported the first
experimental realization of linear atomic chains (LACs) composed of different
atoms (Au and Ag). Different contents of Au and Ag were observed in the chains
from what found in the bulk alloys, which rises the question of what is the
wire composition if in equilibrium with a bulk alloy. In this work we address
the thermodynamic driving force for species fractionation in LACs under
tension, and we present density-functional theory results for Ag-Au chain
alloys. A pronounced stabilization of wires with an alternating Ag-Au sequence
is observed, which could be behind the experimentally observed Au enrichment in
LACs from alloys of high Ag content. | 1312.1285v1 |
2014-07-05 | Irradiation effects in the Ni-17Mo-7Cr alloy bombarded with MeV Au ions | Irradiation effects in Ni-17Mo-7Cr alloy, which is an newly developed
structural material for molten salt reactor (MSR), have been systematically
investigated by using 3MeV Au ions at different fluences, corresponding to dpa
number (displacement per atom) of 1~ 30. GIXRD measurement indicates that the
microstrain of the irradiated samples increased from 0.14% to 0.22% as dpa
increased from 1 to 30. In the meanwhile, nanoindentation results reveal the
Ni-17Mo-7Cr alloy underwent radiation-induced hardening first and then
softening at dpa of 30. The swelling rate of Ni-17Mo-7Cr alloy was found around
1.3% at 30 dpa, which means only 0.04% per dpa. Besides, Raman spectra shows
that carbon segregation appeared after Au ions irradiation. Our results are
very helpful for understanding irradiation damages in Nickel-base alloys,
especially for those in purpose of being used in future MSR nuclear energy
system. | 1407.1398v1 |
2014-07-17 | Brillouin zone unfolding method for effective phonon spectra | Thermal properties are of great interest in modern electronic devices and
nanostructures. Calculating these properties is straightforward when the device
is made from a pure material, but problems arise when alloys are used.
Specifically, only approximate bandstructures can be computed for random alloys
and most often the Virtual Crystal Approximation (VCA) is used. Unfolding
methods [T. B. Boykin, N. Kharche, G. Klimeck, and M. Korkusinski, J. Phys.:
Condens. Matt. 19, 036203 (2007).] have proven very useful for tight-binding
calculations of alloy electronic structure without the problems in the VCA, and
the mathematical analogy between tight-binding and valence-force-field
approaches to the phonon problem suggest they be employed here as well.
However, there are some differences in the physics of the two problems
requiring modifications to the electronic structure approach. We therefore
derive a phonon alloy bandstructure (vibrational mode) approach based on our
tight-binding electronic structure method, modifying the band-determination
method to accommodate the different physical situation. Using the method, we
study In$_x$Ga$_{1-x}$As alloys and find very good agreement with available
experiments. | 1407.4598v1 |
2014-07-24 | Atomic-scale pathway of early-stage precipitation in Al-Mg-Si alloys | Strengthening in age-hardenable alloys is mainly achieved through nano-scale
precipitates whose formation paths from the atomic-scale, solute-enriched
entities are rarely analyzed and understood in a directly-verifiable way. Here,
we discover a pathway for the earliest-stage precipitation in Al-Mg-Si alloys:
solute clustering leading to three successive variants of FCC clusters,
followed by the formation of non-FCC $GP$-$zones$. The clusters, which
originally assume a spherical morphology ($C1$), evolve into elongated clusters
and orient themselves on $\{111\}_{Al}$ ($C2$) and subsequently on
$\{100\}_{Al}$ planes and $<$$100$$>_{Al}$ directions ($C3$). We also analyze
the association of quenched-in dislocations with clustering phenomena. The
results of this work can open a new frontier in advancing
alloy-process-property design for commercially-important age-hardenable Al
alloys. | 1407.6412v1 |
2014-07-31 | First principles prediction of structural and electronic properties of TlxIn(1-x)N alloy | Structural and electronic properties of zinc blende TlxIn(1-x)N alloy have
been evaluated from first principles. The band structures have been obtained
within the density functional theory (DFT), the modified Becke-Johnson (MBJLDA)
approach for the exchange-correlation potential, and fully relativistic
pseudopotentials. The calculated band-gap dependence on Tl content in this
hypothetical alloy exhibits a linear behaviour up to the 25 % of thalium
content where its values become close to zero. In turn, the split-off energy at
the Gamma point of the Brillouin zone, related to the spin-orbit coupling, is
predicted to be comparable in value with the band-gap for relatively low
thalium contents of about 5 %. These findings suggest TlxIn(1-x)N alloy as a
promising material for optoelectronic applications. Furthermore, the band
structure of TlN reveals some specific properties exhibited by topological
insulators. | 1407.8424v1 |
2015-02-24 | Accommodation of Tin in Tetragonal ZrO2 | Atomic scale computer simulations using density functional theory were used
to investigate the behaviour of tin in the tetragonal phase oxide layer on
Zr-based alloys. The $Sn_{Zr}^{\times}$ site defect was shown to be dominant
across most oxygen partial pressures, with $Sn_{Zr}^{"}$ charge compensated by
fully charged oxygen vacancies occurring at partial pressures below $10^{-31}$
atm. Insertion of additional positive charge into the system was shown to
significantly increase the critical partial pressure at which $Sn_{Zr}^{"}$ is
stable. Recently developed low-Sn nuclear fuel cladding alloys have
demonstrated an improved corrosion resistance and a delayed transition compared
to Sn-containing alloys, such as Zircaloy-4. The interaction between the
positive charge and the tin defect is discussed in the context of alloying
additions, such as niobium and their influence on corrosion of cladding alloys. | 1502.06883v1 |
2015-03-04 | Structural and magnetic properties of MnCo1-xFexSi alloys | The crystal structures, martensitic structural transitions and magnetic
properties of MnCo1-xFexSi (0 <= x <= 0.50) alloys were studied by differential
scanning calorimetry (DSC), x-ray powder diffraction (XRD) and magnetic
measurements. In high-temperature paramagnetic state, the alloys undergo a
martensitic structural transitions from the Ni2In-type hexagonal parent phase
to the TiNiSi-type orthorhombic martensite. Both the martensitic transition
temperature (TM) and Curie temperatures of martensite (T_C^M) decrease with
increasing Fe content. The introduced Fe atoms establish ferromagnetic (FM)
coupling between Fe-Mn atoms and destroy the double spiral antiferromagnetic
(AFM) coupling in MnCoSi compound, resulting in a magnetic change in the
martensite phase from a spiral AFM state to a FM state. For the alloys with x =
0.10, 0.15 and 0.20, a metamagnetic transition was observed in between the two
magnetic states. A magnetostructural phase diagram of MnCo1-xFexSi (0 <= x <=
0.50) alloys was proposed. | 1503.01226v1 |
2015-03-09 | Anharmonicity changes the solid solubility of an alloy at high temperatures | We have developed a method to accurately and efficiently determine the
vibrational free energy as a function of temperature and volume for
substitutional alloys from first principles. Taking Ti$_{1-x}$Al$_x$N alloy as
a model system, we calculate the isostructural phase diagram by finding the
global minimum of the free energy, corresponding to the true equilibrium state
of the system. We demonstrate that the anharmonic contribution and temperature
dependence of the mixing enthalpy have a decisive impact on the calculated
phase diagram of a Ti$_{1-x}$Al$_x$N alloy, lowering the maximum temperature
for the miscibility gap from 6560 K to 2860 K. Our local chemical composition
measurements on thermally aged Ti$_{0.5}$Al$_{0.5}$N alloys agree with the
calculated phase diagram. | 1503.02459v2 |
2015-04-14 | Electron mobility in few-layer MoxW1-xS2 | In this letter, we theoretically study the electron mobility in few-layer
MoxW1-xS2 as limited by various scattering mechanisms. The room temperature
energy-dependent scattering times corresponding to polar longitudinal optical
(LO) phonon, alloy and background impurity scattering mechanisms are estimated
based on the Born approximation to Fermi's Golden rule. The contribution of
individual scattering rates is analyzed as a function of 2D electron density as
well as of alloy composition in MoxW1-xS2. While impurity scattering limits the
mobility for low carrier density (<2x1012 cm-2), LO polar phonon scattering is
the dominant mechanism for high electron densities. Alloy scattering is found
to play a non-negligible role for 0.5 < x < 0.7 in MoxW1-xS2. The LO phonon
limited and impurity limited mobilities show opposing trends with respect to
alloy mole fraction. The understanding of electron mobility in MoxW1-xS2
presented here is expected to aid the design and realization of
hetero-structures and devices based on alloys of MoS2 and WS2. | 1504.03593v1 |
2015-05-23 | Debye temperature of nanocrystalline Fe-Cr alloys obtained by mechanical alloying | A series on nanocrystalline Fe100-xCrx alloys prepared by mechanical alloying
was investigated with X-ray diffraction (XRD), scanning electron microscopy
(SEM) and M\"ossbauer spectroscopy (MS) techniques. XRD and SEM were used to
structurally characterize the samples whereas MS permitted phase analysis as
well as determination of the Debye temperature, Theta_D. Concerning the latter,
an enhancement relative to bulk Theta_D-values was revealed in the range of ca.
40 < x <50. In a sample of Fe55.5Cr44.5 two phases were detected viz. (1)
crystalline and magnetic with Theta_D=572(56) K and (2) amorphous and
paramagnetic with Theta_D=405(26) K. | 1505.06374v1 |
2015-11-27 | First-Principles prediction of the deformation modes in austenitic Fe-Cr-Ni alloys | First-principles alloy theory is used to establish the $\gamma$-surface of
Fe-Cr-Ni alloys as function of chemical composition and temperature. The
theoretical stacking fault energy (SFE) versus chemistry and temperature trends
agree well with experiments. Combining our results with the recent plasticity
theory based on the $\gamma$-surface, the stacking fault formation is predicted
to be the leading deformation mechanism for alloys with effective stacking
fault energy below about 18 mJ m$^{-2}$. Alloys with SFE above this critical
value show both twinning and full slip at room temperature and twinning remains
a possible deformation mode even at elevated temperatures, in line with
observations. | 1511.08623v1 |
2015-12-20 | Thermal vacancies in random alloys in the single-site mean-field approximation | A formalism for the vacancy formation energies in random alloys within the
single-site mean-filed approximation, where vacancy-vacancy interaction is
neglected, is outlined. It is shown that the alloy configurational entropy can
substantially reduce the concentration of vacancies at high temperatures. The
energetics of vacancies in random Cu-0.5Ni-0.5 alloy is considered as a
numerical example illustrating the developed formalism. It is shown that the
effective formation energy is increases with temperature, however, in this
particular system it is still below the mean value of the vacancy formation
energy which would correspond to the vacancy formation energy in a homogeneous
model of a random alloy, such as given by the coherent potential approximation. | 1512.06379v3 |
2016-04-01 | Analysis of Feature Models Using Alloy: A Survey | Feature Models (FMs) are a mechanism to model variability among a family of
closely related software products, i.e. a software product line (SPL). Analysis
of FMs using formal methods can reveal defects in the specification such as
inconsistencies that cause the product line to have no valid products. A
popular framework used in research for FM analysis is Alloy, a light-weight
formal modeling notation equipped with an efficient model finder. Several works
in the literature have proposed different strategies to encode and analyze FMs
using Alloy. However, there is little discussion on the relative merits of each
proposal, making it difficult to select the most suitable encoding for a
specific analysis need. In this paper, we describe and compare those strategies
according to various criteria such as the expressivity of the FM notation or
the efficiency of the analysis. This survey is the first comparative study of
research targeted towards using Alloy for FM analysis. This review aims to
identify all the best practices on the use of Alloy, as a part of a framework
for the automated extraction and analysis of rich FMs from natural language
requirement specifications. | 1604.00349v1 |
2016-04-11 | Au-Ag-Cu nano-alloys: tailoring of permittivity | Precious metal alloys enables new possibilities to tailor materials for
specific optical functions. Here we present a systematic study of the effects
of a nanoscale alloying on the permittivity of Au-Ag-Cu metals at 38 different
atomic mixing ratios. The permittivity was measured and analyzed numerically by
applying the Drude model. X-ray diffraction (XRD) revealed the face centered
cubic lattice of the alloys. Both, optical spectra and XRD results point
towards an equivalent composition-dependent electron scattering behavior.
Correlation between the fundamental structural parameters of alloys and the
resulting optical properties is elucidated. Plasmonic properties of the
Au-Ag-Cu alloy nanoparticles were investigated by numerical simulations.
Guidelines for designing plasmonic response of nano- structures and their
patterns are presented from the material science perspective. | 1604.02944v1 |
2016-05-10 | Giant magnetocaloric effect near room temperature in the off-stoichiometric Mn-Co-Ge alloy | We report a giant magnetocaloric effect near room temperature in an
off-stoichiometric Mn-Co-Ge alloy, across the magnetostructural transition. The
isothermal entropy change accompanying this transition has a peak value of
nearly 40 J/kg-K near 297 K and a refrigerant capacity of 270 J/kg with the hot
end at 302.5 K and cold end at 293.5 K. We also present an experimental
protocol to avoid spurious peaks in the magnetocaloric effect across a sharp
first order magnetostructural transition, not confined to Mn-Co-Ge alone, where
metastability during the transition could influence the measured magnetization
and thus the estimated entropy change. The estimated entropy change in the
present off-stoichiometric Mn-Co-Ge alloy is possibly the highest reported
value near room temperature in undoped Mn-Co-Ge alloys and underlines the
potential of the alloy for technological applications in room temperature
magnetic refrigeration. | 1605.02902v1 |
2016-05-26 | Phonon transport in single-layer Mo1-xWxS2 alloy embedded with WS2 nanodomains | Two-dimensional (2-D) transition metal dichalcogenides (TMDs) have shown
numerous interesting physical and chemical properties, making them promising
materials for electronic, optoelectronic, and energy applications. Tuning
thermal conductivity of two-dimensional (2-D) materials could expand their
applicability in many of these fields. In this paper, we propose a strategy of
using alloying and nanodomains to suppress the thermal conductivity of 2-D
materials. To predict the thermal conductivity of 2-D alloy embedded with
nanodomains, we employ the Green's function approach to assess the phonon
scattering strength due to alloying and nanodomain embedding. Our
first-principles-driven phonon Boltzmann transport equation calculations show
that the thermal conductivity of single-layer MoS2 can be reduced to less than
one-tenth of its intrinsic thermal conductivity after alloying with W and
introducing nanodomains due to the strong scattering for both high- and
low-frequency phonons. The strategies to further reduce the thermal
conductivity are also discussed. | 1605.08468v2 |
2016-06-16 | Atomic configuration and properties of austenitic steels at finite temperature: The effect of longitudinal spin fluctuations | High temperature atomic configurations of fcc Fe-Cr-Ni alloys with alloy
composition close to austenitic steel are studied in statistical thermodynamic
simulations with effective interactions obtained in ab initio calculations. The
latter are done taking longitudinal spin fluctuations (LSF) into consideration
within a quasiclassical phenomenological model. It is demonstrated that
magnetic state affects greatly the alloy properties and in particular, it is
shown that the LSF substantially modify the bonding and interatomic
interactions of fcc Fe-Cr-Ni alloys even at ambient conditions. The calculated
atomic short-range order (SRO) is in reasonable agreement with existing
experimental data for Fe0.56}Cr0.21Ni0.23, which has strong preference for the
(001) type ordering between Ni and Cr atoms. A similar ordering tendency is
found for the Fe0.75Cr0.17Ni0.08 alloy composition, which approximately
corresponds to the widely used 304 and 316 austenitic steel grades. | 1606.05096v1 |
2016-07-09 | Theoretical and numerical investigation of diffusive instabilities in multi-component alloys | Mechanical properties of engineering alloys are strongly correlated to their
microstructural length scale. Diffusive insta- bilities of the Mullins-Sekerka
type is one of the principal mechanisms through which the scale of the
microstructural features are determined during solidification. In contrast to
binary systems, in multicomponent alloys with arbitrary interdiffusivities, the
growth rate as well as the maximally growing wavelengths characterizing these
instabilities depend on the the dynamically selected equilibrium tie-lines and
the steady state growth velocity. In this study, we derive analytical
expressions to characterize the dispersion behavior in isothermally solidified
multicomponent (quaternary) alloys for different choices of the
inter-diffusivity matrices and confirm our calculations using phase-field
simulations. Thereafter, we perform controlled studies to capture and isolate
the dependence of instability length scales on solute diffusivities and steady
state planar front velocities, which leads to an understanding of the process
of length scale selection during the onset of instability for any alloy
composition with arbitrary diffusivities, comprising of both independent and
coupled diffusion of solutes. | 1607.02570v1 |
2016-09-17 | A new Wang-Landau approach to obtain phase diagrams for multicomponent alloys | We develop an approach to apply Wang-Landau algorithm to multicomponent
alloys in semi-grand-canonical ensemble. Although the Wang-Landau algorithm has
great advantages over conventional sampling methods, there are few applications
to alloys. This is because calculating compositions in semi-grand-canonical
ensemble using the Wang-Landau algorithm requires a multi-dimensional density
of states in terms of total energy and compositions. However, constructing the
multi-dimensional density of states is difficult. In this study, we develop a
simple approach to calculate the alloy phase diagram using Wang-Landau
algorithm, and show that compositions in semi-grand-canonical ensemble require
just some one-dimensional densities of states. Finally, we applied the present
method to Cu-Au and Pd-Rh alloys and confirmed that the present method
successfully describes the phase diagram with high validity and accuracy. | 1609.05292v2 |
2017-07-14 | Alloying strategy for two-dimensional GaN optical emitters | The recent progress in formation of two-dimensional (2D) GaN by a
migration-enhanced encapsulated technique opens up new possibilities for group
III-V 2D semiconductors with a band gap within the visible energy spectrum.
Using first-principles calculations we explored alloying of 2D-GaN to achieve
an optically active material with a tuneable band gap. The effect of
isoelectronic III-V substitutional elements on the band gaps, band offsets, and
spatial electron localization is studied. In addition to optoelectronic
properties, the formability of alloys is evaluated using impurity formation
energies. A dilute highly-mismatched solid solution 2D-GaN$_{1-x}$P$_x$
features an efficient band gap reduction in combination with a moderate energy
penalty associated with incorporation of phosphorous in 2D-GaN, which is
substantially lower than in the case of the bulk GaN. The group-V alloying
elements also introduce significant disorder and localization at the valence
band edge that facilitates direct band gap optical transitions thus implying
the feasibility of using III-V alloys of 2D-GaN in light-emitting devices. | 1707.04625v4 |
2017-07-24 | Probing local lattice distortion in medium- and high-entropy alloys | The atomic-level tunability that results from alloying multiple transition
metals with d electrons in concentrated solid solution alloys (CSAs), including
high-entropy alloys (HEAs), has produced remarkable properties for advanced
energy applications, in particular, damage resistance in high-radiation
environments. The key to understanding CSAs radiation performance is
quantitatively characterizing their complex local physical and chemical
environments. In this study, the local structure of a FeCoNiCrPd HEA is
quantitatively analyzed with X-ray total scattering and extended X-ray
absorption fine structure methods. Compared to FeCoNiCr and FeCoNiCrMn,
FeCoNiCrPd with a quasi-random alloy structure has a strong local lattice
distortion, which effectively pins radiation-induced defects. Distinct from a
relaxation behavior in FeCoNiCr and FeCoNiCrMn, ion irradiation further
enhanced the local lattice distortion in FeCoNiCrPd due to a preference for
forming Pd-Pd atomic pairs. | 1707.07745v1 |
2017-08-04 | Tunable dimensional crossover and magnetocrystalline anisotropy in Fe$_2$P-based alloys | Electronic structure calculations are used to examine the magnetic properties
of Fe$_2$P-based alloys and the mechanisms through which the Curie temperature
and magnetocrystalline anisotropy can be optimized for specific applications.
It is found that at elevated temperatures the magnetic interaction in pure
Fe$_2$P develops a pronounced two-dimensional character due to the suppression
of the magnetization in one of the sublattices, but the interlayer coupling is
very sensitive to band filling and structural distortions. This feature
suggests a natural explanation of the observed sharp enhancement of the Curie
temperature by alloying with multiple elements, such as Co, Ni, Si, and B. The
magnetocrystalline anisotropy is also tunable by electron doping, reaching a
maximum near the electron count of pure Fe$_2$P. These findings enable the
optimization of the alloy content, suggesting co-alloying of Fe$_2$P with Co
(or Ni) and Si as a strategy for maximizing the magnetocrystalline anisotropy
at and above room temperature. | 1708.01683v2 |
2017-11-07 | Direct prediction of the solute softening-to-hardening transition in W-Re alloys using stochastic simulations of screw dislocation motion | Interactions among dislocations and solute atoms are the basis of several
important processes in metals plasticity. In body-centered cubic (bcc) metals
and alloys, low-temperature plastic flow is controlled by screw dislocation
glide, which is known to take place by the nucleation and sideward relaxation
of kink pairs across two consecutive \emph{Peierls} valleys. In alloys,
dislocations and solutes affect each other's kinetics via long-range stress
field coupling and short-range inelastic interactions. It is known that in
certain substitutional bcc alloys a transition from solute softening to solute
hardening is observed at a critical concentration. In this paper, we develop a
kinetic Monte Carlo model of screw dislocation glide and solute diffusion in
substitutional W-Re alloys. We find that dislocation kinetics is governed by
two competing mechanisms. At low solute concentrations, nucleation is enhanced
by the softening of the Peierls stress, which overcomes the elastic repulsion
of Re atoms on kinks. This trend is reversed at higher concentrations,
resulting in a minimum in the flow stress that is concentration and temperature
dependent. This minimum marks the transition from solute softening to
hardening, which is found to be in reasonable agreement with experiments. | 1711.02240v1 |
2017-11-28 | Ta-Nb-Mo-W refractory high-entropy alloys: anomalous ordering behavior and its intriguing electronic origin | From electronic-structure-based thermodynamic linear-response, we establish
chemical ordering behavior in complex solid solutions versus how Gibbs' space
is traversed -- applying it on prototype refractory A2 Ta-Nb-Mo-W high-entropy
alloys. Near ideal stoichiometry, this alloy has anomalous, intricate chemical
ordering tendencies, with long-ranged chemical interactions that produce
competing short-range order (SRO) with a crossover to spinodal segregation.
This atypical SRO arises from canonical band behavior that, with alloying,
create features near the Fermi-surface (well-defined even with disorder) that
change to simple commensurate SRO with (un)filling of these states. Our results
reveal how complexity and competing electronic effects control ordering in
these alloys. | 1711.10591v2 |
2017-12-07 | First-principles quantitative prediction of the lattice thermal conductivity in random semiconductor alloys: the role of force-constant disorder | The standard theoretical understanding of the lattice thermal conductivity,
$\kappa_{\ell}$, of semiconductor alloys assumes that mass disorder is the most
important source of phonon scattering. In contrast, we show that the hitherto
neglected contribution of force-constant (IFC) disorder is essential to
accurately predict the $\kappa_{\ell}$ of those polar compounds characterized
by a complex atomic-scale structure. We have developed an \emph{ab initio}
method based on special quasirandom structures and Green's functions, and
including the role of IFC disorder, and applied it in order to calculate the
$\kappa_{\ell}$ of $\mathrm{In_{1-x}Ga_xAs}$ and $\mathrm{Si_{1-x}Ge_x}$
alloys. We show that, while for $\mathrm{Si_{1-x}Ge_x}$, phonon-alloy
scattering is dominated by mass disorder, for $\mathrm{In_{1-x}Ga_xAs}$, the
inclusion of IFC disorder is fundamental to accurately reproduce the
experimentally observed $\kappa_{\ell}$. As the presence of a complex
atomic-scale structure is common to most III-V and II-VI random semiconductor
alloys, we expect our method to be suitable for a wide class of materials. | 1712.02577v1 |
2017-12-20 | Magnetic Compton profiles of disordered Fe$_{0.5}$Ni$_{0.5}$ and ordered FeNi alloys | We study the magnetic Compton profile (MCP) of the disordered
Fe$_{0.5}$Ni$_{0.5}$ and of the ordered FeNi alloys and discuss the interplay
between structural disorder and electronic correlations. The Coherent Potential
Approximation is employed to model the substitutional disorder within the
single-site approximation, while local electronic correlations are captured
with the Dynamical Mean Field Theory. Comparison with the experimental data
reveals the limitation of local spin-density approximation in low momentum
region, where we show that including local but dynamic correlations the
experimental spectra is excellently described. We further show that using local
spin-density approximation no significant difference is seen between the MCP
spectra of the disordered Fe$_{0.5}$Ni$_{0.5}$ and a hypothetical, ordered FeNi
alloy with a simple cubic unit cell. Only by including the electronic
correlations, the spectra significantly separate, from the second Brillouin
zone boundary down to zero momenta. The difference between the MCP spectra of
ordered and disordered alloys is discussed also in terms of the atomic-type
decompositions. Finally based on the presented calculations we predict the
shape of the MCP profile for the ordered FeNi alloy along the [111] direction. | 1712.07619v1 |
2017-12-26 | Effect of Si on Fe-rich intermetallic formation and mechanical properties of heattreated Al-Cu-Mn-Fe alloys | The effect of Si on Fe-rich intermetallics formation and mechanical
properties of heat-treated squeeze cast Al-5.0Cu-0.6Mn-0.7Fe alloy was
investigated. Our results show that increasing Si content promotes the
formation of Al15(FeMn)3(SiCu)2 (${\alpha}$-Fe), and varying the morphology of
T (Al20Cu3Mn2) where the size decreases and the amount increases. The major
reason is that Si promotes heterogeneous nucleation of the intermetallics
leading to finer precipitates. Si addition significantly enhances ultimate
tensile strength and yield strength of the alloys. The strengthening effect is
mainly owing to the dispersoid strengthening by increasing volume fraction of T
phase and less harmful ${\alpha}$-Fe with a compact structure, which make the
cracks more difficult to initiate and propagation during tensile test. The
squeeze cast Al-5.0Cu-0.6Mn-0.7Fe alloy with 1.1% Si shows significantly
improved mechanical properties than the alloy without Si addition, which has
tensile strength of 386 MPa, yield strength of 280 MPa and elongation of 8.6%. | 1712.09176v1 |
2018-01-02 | Modeling solid-state dewetting of a single-crystal binary alloy thin films | Dewetting of a binary alloy thin film is studied using a continuum
many-parameter model that accounts for the surface and bulk diffusion, the bulk
phase separation, the surface segregation and the particles formation.
Analytical solution is found for the quasistatic equilibrium concentration of a
surface-segregated atomic species. This solution is factored into the nonlinear
and coupled evolution PDEs for the bulk composition and surface morphology.
Stability of a planar film surface with respect to small perturbations of the
shape and composition is analyzed, revealing the dependence of the particles
size on major physical parameters. Computations show various scenarios of the
particles formation and the redistribution of the alloy components inside the
particles and on their surface. In most situations, for the alloy film composed
initially of 50% A and 50% B atoms, a core-shell particles are formed, and they
are located atop a wetting layer that is modestly rich in the B phase. Then the
particles shell is the nanometric segregated layer of the A phase, and the core
is the alloy that is modestly rich in the A phase. | 1801.00764v1 |
2018-03-11 | Characterizing solute hydrogen and hydrides in pure and alloyed titanium at the atomic scale | Ti has a high affinity for hydrogen and is a typical hydride formers.
Ti-hydride are brittle phases which probably cause premature failure of
Ti-alloys. Here, we used atom probe tomography and electron microscopy to
investigate the hydrogen distribution in a set of specimens of commercially
pure Ti, model and commercial Ti-alloys. Although likely partly introduced
during specimen preparation with the focused-ion beam, we show formation of
Ti-hydrides along {\alpha} grain boundaries and {\alpha}/\b{eta} phase
boundaries in commercial pure Ti and {\alpha}+\b{eta} binary model alloys. No
hydrides are observed in the {\alpha} phase in alloys with Al addition or
quenched-in Mo supersaturation. | 1803.04007v1 |
2018-03-19 | Tuning phase-stability and short-range order through Al-doping in (CoCrFeMn)100-xAlx high entropy alloys | For (CoCrFeMn)$_{100-x}$Al$_{x}$ high-entropy alloys, we investigate the
phase evolution with increasing Al-content (0 $\le$ x $\le$ 20 at.%). From
first-principles theory, the Al-doping drives the alloy structurally from FCC
to BCC separated by a narrow two-phase region (FCC+BCC), which is well
supported by our experiments. We highlight the effect of Al-doping on the
formation enthalpy and electronic structure of (CoCrFeMn)$_{100-x}$Al$_{x}$
alloys. As chemical short-range order (SRO) in multicomponent alloys indicates
the nascent local order (and entropy changes), as well as expected
low-temperature ordering behavior, we use thermodynamic linear-response within
density-functional theory to predict SRO and ordering transformation and
temperatures inherent in (CoCrFeMn)$_{100-x}$Al$_{x}$. The predictions agree
with our present experimental findings, and other reported ones. | 1803.06771v3 |
2018-03-30 | Phase field modelling voids nucleation and growth in binary systems | We present a comprehensive study of voids formation, nucleation and growth in
a prototype model of binary alloys subjected to irradiation by using a combined
approach based on phase field and rate theories. It is shown that voids
formation is caused by interaction of irradiation-produced vacancies through
elastic deformation of a lattice and vacancy coupling with composition field of
the alloy. Phase diagrams illustrating the formation of states related to solid
solution, phase decomposition, and patterning are obtained. Formation of voids
from supersaturated ensemble of vacancies is accompanied by composition
rearrangement of alloy components. It was found that elastic inhomogeneity
leading to the formation of anisotropic precipitates in an initially prepared
binary alloy results in the formation of a void super-lattice under
irradiation. It was shown that voids nucleate and grow with dose according to
diffusion controlled precipitation processes, where universal dynamics of voids
growth is revealed. Estimations of main quantitative and statistical
characteristics of voids by using material parameters relevant to most of
alloys and steels give good agreement with experimental observations. | 1803.11408v1 |
2018-05-12 | Impact of corrosion on the emissivity of advanced-reactor structural alloys | Under standard operating conditions, the emissivity of structural alloys used
for various components of nuclear reactors may evolve, affecting the heat
transfer of the systems. In this study, mid-infrared emissivities of several
reactor structural alloys were measured before and after exposure to
environments relevant to next-generation reactors. We evaluated nickel-based
alloys Haynes 230 and Inconel 617 exposed to helium gas at 1000 $^{\circ}$C,
nickel-based Hastelloy N and iron-based 316 stainless steel exposed to molten
salts at 750-850 $^{\circ}$C, 316 stainless steel exposed to liquid sodium at
650 $^{\circ}$C, and 316 stainless steel and Haynes 230 exposed to
supercritical CO2 at 650 $^{\circ}$C. Emissivity was measured via emissive and
reflective techniques using a Fourier transform infrared (FTIR) spectrometer.
Large increases in emissivity are observed for alloys exposed to oxidizing
environments, while only minor differences were observed in other exposure
conditions. | 1805.04631v1 |
2018-06-11 | Uncovering electron scattering mechanisms in NiFeCoCrMn derived concentrated solid solution and high entropy alloys | Whilst it has long been known that disorder profoundly affects transport
properties, recent measurements on a series of solid solution 3d-transition
metal alloys reveal two orders of magnitude variations in the residual
resistivity. Using ab-initio methods, we demonstrate that, while the carrier
density of all alloys is as high as in normal metals, the electron
mean-free-path can vary from ~10 {\AA} (strong scattering limit) to ~10$^3$
{\AA} (weak scattering limit). Here, we delineate the underlying electron
scattering mechanisms responsible for this disparate behavior. While spin
dependent site-diagonal disorder is always dominant, for alloys containing only
Fe, Co, and Ni the majority spin channel experiences negligible disorder
scattering, thereby providing a short circuit, while for Cr/Mn containing
alloys both spin channels experience strong disorder scattering due to an
electron filling effect. Unexpectedly, other scattering mechanisms (e.g.
displacement scattering) are found to be relatively weak in most cases. | 1806.03785v2 |
2021-07-19 | Superconductivity in Al-Nb-Ti-V-Zr multicomponent alloy | The superconducting high-entropy alloys (HEAs) recently attract considerable
attention due to their exciting properties, such as the robustness of
superconductivity against atomic disorder and extremely high-pressure. The
well-studied crystal structure of superconducting HEAs is body-centered-cubic
(bcc) containing Nb, Ti, and Zr atoms. The same elements are contained in
Al5Nb24Ti40V5Zr26, which is a recently discovered bcc HEA and shows a
gum-metal-like behavior after cold rolling. The gum metal is also an
interesting system, exhibiting superelasticity and low Young's modulus. If gum
metals show superconductivity and can be used as a superconducting wire, the
gum-metal HEA superconductors might be the next-generation superconducting wire
materials. Aiming at a fundamental assessment of as-cast Al-Nb-Ti-V-Zr
multicomponent alloys including Al5Nb24Ti40V5Zr26, we have investigated the
structural and superconducting properties of the alloys. All alloys
investigated show the superconductivity, and the valence electron concentration
dependence of the superconducting critical temperature is very close to those
of typical superconducting bcc HEAs. | 2107.09187v1 |
2017-04-10 | Local segregation versus irradiation effects in high-entropy alloys: Steady-state conditions in a driven system | We study order transitions and defect formation in a model high-entropy alloy
(CuNiCoFe) under ion irradiation by means of molecular dynamics simulations.
Using a hybrid Monte-Carlo/molecular dynamics scheme a model alloy is generated
which is thermodynamically stabilized by configurational entropy at elevated
temperatures, but partly decomposes at lower temperatures by copper
precipation. Both the high-entropy and the multiphase sample are then subjected
to simulated particle irradiation. The damage accumulation is analyzed and
compared to an elemental Ni reference system. The results reveal that the
high-entropy alloy---independent of the initial configuration---installs a
certain fraction of short-range order even under particle irradiation.
Moreover, the results provide evidence that defect accumulation is reduced in
the high-entropy alloy. This is because the reduced mobility of point defects
leads to a steady state of defect creation and annihilation. The lattice
defects generated by irradiation are shown to act as sinks for Cu segregation. | 1704.02812v2 |
2017-04-30 | Comparison of dynamic mechanical properties of non-superheated and superheated A357 alloys | The influence of superheat treatment on the microstructure and dynamic
mechanical properties of A357 alloys has been investigated. The study of
microstructure was performed by the optical microscope. Dynamic mechanical
properties (storage modulus, loss modulus, and damping capacity) were measured
by the dynamic mechanical analyzer (DMA). Microstructure showed coarser and
angular eutectic Si particles with larger {\alpha}-Al dendrites in the
non-superheated A357 alloy. In contrast, finer and rounded eutectic Si
particles together with smaller and preferred oriented {\alpha}-Al dendrites
have been observed in the superheated A357 alloy. Dynamic mechanical properties
showed an increasing trend of loss modulus and damping capacity meanwhile a
decreasing trend of storage modulus at elevated temperatures for superheated
and non-superheated A357 alloys. The high damping capacity of superheated A357
has been ascribed to the grain boundary damping at elevated temperatures. | 1705.00350v1 |
2018-07-17 | Alloy broadening of the transition to the non-trivial topological phase of Pb_{1-x}Sn_{x}Te | Transition between the topologically trivial and non-trivial phase of
Pb_{1-x}Sn_{x}Te alloy is driven by the increasing content $x$ of Sn, or by the
hydrostatic pressure for $x<0.3$. We show that a sharp border between these two
topologies exists in the Virtual Crystal Approximation only. In more realistic
models, the Special Quasirandom Structure method and the supercell method (with
averaging over various atomic configurations), the transitions are broadened.
We find a surprisingly large interval of alloy composition, $0.3<x<0.6$, in
which the energy gap is practically vanishing. A similar strong broadening is
also obtained for transitions driven by hydrostatic pressure. Analysis of the
band structure shows that the alloy broadening originates in splittings of the
energy bands caused by the different chemical nature of Pb and Sn, and by the
decreased crystal symmetry due to spatial disorder. Based on our results of ab
initio and tight binding calculations for Pb_{1-x}Sn_{x}Te we discuss different
criteria of discrimination between trivial and nontrivial topology of the band
structure of alloys. | 1807.06314v1 |
2018-10-10 | First-principles investigation of the effect of substitution and surface adsorption on magnetostrictive properties of Fe-Ga alloys | Materials with large magnetostriction are widely used in sensors, actuators,
micro electromechanical systems, and energy-harvesters. Binary Fe-Ga alloys
(Galfenol) are the most promising rare-earth-free candidates combining numerous
advantages such as low saturation magnetic field (~200 Oe), excellent ductility
and low cost, while further improving their performance is imperative for
practical applications. Using density functional theory calculation, we report
results of the effect of substituting small amount of additional elements X
(eg. X = Ag, Pd and Cu) on magnetostriction of Fe-Ga alloys, and find that it
may double the magnetostriction with a substitutional percentage of only 1.6%.
Moreover, adsorbents with high chemical activity (eg. O or Os atoms) may affect
the surface energy of different face-orientations of Fe-Ga alloys, indicating
proper surface treatments are necessary to tune the alignment of Fe-Ga grains
to achieve better performance. These results may be helpful to further optimize
the magnetostrictive properties of Fe-Ga alloys for device applications. | 1810.04708v1 |
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