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2018-08-30 | Modeling Selective Intergranular Oxidation of Binary Alloys | Intergranular attack of alloys under hydrothermal conditions is a complex
problem that depends on metal and oxygen transport kinetics via solid-state and
channel-like pathways to an advancing oxidation front. Experiments reveal very
different rates of intergranular attack and minor element depletion distances
ahead of the oxidation front for nickel-based binary alloys depending on the
minor element. For example, a significant Cr depletion up to 9 microns ahead of
grain boundary crack tips were documented for Ni-5Cr binary alloy, in contrast
to relatively moderate Al depletion for Ni-5Al (~100s of nm). We present a
mathematical kinetics model that adapts Wagner's model for thick film growth to
intergranular attack of binary alloys. The transport coefficients of elements
O, Ni, Cr, and Al in bulk alloys and along grain boundaries were estimated from
the literature. For planar surface oxidation, a critical concentration of the
minor element can be determined from the model where the oxide of minor element
becomes dominant over the major element. This generic model for simple grain
boundary oxidation can predict oxidation penetration velocities and minor
element depletion distances ahead of the advancing front that are comparable to
experimental data. The significant distance of depletion of Cr in Ni-5Cr in
contrast to the localized Al depletion in Ni-5Al can be explained by the model
due to the combination of the relatively faster diffusion of Cr along the grain
boundary and slower diffusion in bulk grains, relative to Al. | 1808.10397v1 |
2019-02-15 | Enhancing CdTe Solar Cell Performance by Reducing the "Ideal" Bandgap of CdTe through CdTe1-xSex Alloying | CdTe is one of the leading materials for low cost, high efficiency thin-film
solar cells, because it has a high absorption coefficient and a nearly ideal
band gap of 1.48 eV for solar cell according to the Shockley-Queisser limit.
However, its solar to electricity power conversion efficiency (PCE) is hindered
by the relatively low open circuit voltage (VOC) due to intrinsic defect
related issues. Here, we propose the strategy of improving CdTe solar cell
performance byr reducing the "ideal" band gap of CdTe to gain more
short-circuit current from long-wavelength absorption without sacrificing much
VOC. Alloying CdTe with CdSe seems to be the most appropriate approach to
reduce the band gap because of the large optical bowing and relatively small
lattice mismatch in this system, even though CdSe has larger band gap than
CdTe. Using the first principle hybrid functional calculation, we find that the
minimum band gap of the CdTe1-xSex alloy can be reduced from 1.48 eV at x=0 to
1.39 eV at x=0.32. We also show that the formation of the alloy can improve the
defect property, for example, p-type doping of CdTe by CuCd can be greatly
enhanced by the alloying effects. | 1902.05700v1 |
2020-04-12 | Investigation on the Mechanical Properties of Functionally Graded Nickel and Aluminium Alloy by Molecular Dynamics Study | Functionally graded materials (FGMs), have drawn considerable attention of
the worldwide researchers and scientific community because of its unique
mechanical, thermal and electrical properties which may be exploited by varying
the compositions gradually over volume. This makes FGM multifunctional material
(properties changing continuously in a certain direction) for specific purpose
without creating any phase interface thus making it superior to its composite
counterparts. In this paper, we applied Molecular Dynamics (MD) approach to
investigate the mechanical properties of functional graded Ni-Al alloy with Ni
coating by applying uniaxial tension. Nickel-Aluminum (Ni-Al) alloy has been
used extensively in the industry due to its remarkable mechanical and thermal
properties. Our aim is to find the difference in material behavior when we
change the grading function (linear, elliptical and parabolic), temperature and
crystallographic direction. We also observe distinct type of failure mechanism
for different grading function at different temperature. Close observation
reveals that elliptically graded Ni-Al alloy has high tensile strength at low
temperature whereas at high temperature, the highest tensile strength is found
for parabolic grading. Besides, at any temperature, the parabolically graded
Ni-Al alloy shows superior elasticity than its elliptical and linear
counterpart. Moreover, it is also observed that [111] crystallographic
direction for this alloy demonstrates more resistivity towards failure than any
other crystallographic direction. It is found that lattice disorder plays a
significant role on the mechanical properties of Functionally Graded Materials
(FGMs). This paper details a pathway to tune the mechanical properties like
Young's Modulus, plasticity and yield strength at molecular level by varying
the composition of materials along different grading functions. | 2004.05651v1 |
2020-04-14 | Forming a highly active, homogeneously alloyed AuPt co-catalyst decoration on O2 nanotubes directly during anodic growth | Au and Pt do not form homogeneous bulk alloys as they are thermodynamically
not miscible. However, we show that anodic TiO$_2$ nanotubes (NTs) can in-situ
be uniformly decorated with homogeneous AuPt alloy nanoparticles (NPs) during
their anodic growth. For this, a metallic Ti substrate containing low amounts
of dissolved Au (0.1 at%) and Pt (0.1 at%) is used for anodizing. The matrix
metal (Ti) is converted to oxide while at the oxide/metal interface direct
noble metal particle formation and alloying of Au and Pt takes place;
continuously these particles are then picked up by the growing nanotube wall.
In our experiments the AuPt alloy NPs have an average size of 4.2 nm and, at
the end of the anodic process, are regularly dispersed over the TiO$_2$
nanotubes. These alloyed AuPt particles act as excellent co-catalyst in
photocatalytic H2 generation - with a H2 production of 12.04 {\mu}L h-1 under
solar light. This represents a strongly enhanced activity as compared with
TiO$_2$ NTs decorated with monometallic particles of Au (7 {\mu}L h-1) or Pt
(9.96 {\mu}L h-1). | 2004.06566v1 |
2020-05-08 | Early stage phase separation of AlCoCr0.75Cu0.5FeNi high-entropy powder at the nanoscale | High entropy alloys are generally considered to be single phase material.
This state is, however, typically a non-equilibrium state after fabrication at
high cooling rates. Phase constitution after fabrication or heat treatment is
mostly known for isothermal annealing only and for casts as well as rapidly
quenched alloys. Knowledge on early phase separation stages of high entropy
alloys and their mechanisms are missing so far. Here, we present results on
phase separation at intermediate cooling rates, by characterization of gas
atomized powder of the AlCoCr0.75Cu0.5FeNi alloy. Although investigation by
X-ray diffraction and Electron Backscatter Diffraction indicates a single-phase
nature of the powder particles, aberration-corrected scanning transmission
electron microscopy and atom probe tomography reveal a nanoscale phase
separation into Ni-Al-rich B2 and Fe-Cr-rich A2 regions as well as a high
number density of 3.1x1024 Cu-rich clusters per m3 in the B2 matrix. The
observed phase separation and cluster formation are linked to spinodal
decomposition and nucleation processes, respectively. The study highlights that
adequate characterization techniques need to be chosen when making statements
about phase stability and structural evolution in compositionally complex
alloys. | 2005.04039v1 |
2020-08-14 | Density Functional Theory Study of Solute Cluster Growth Processes in Mg-Y-Zn LPSO Alloys | Solute clusters in long period stacking order (LPSO) alloys play a key role
in their idiosyncratic plastic behavior, for example kink formation and kink
strengthening. Identifying atomistic details of cluster structures is a
prerequisite for atomistic modeling of LPSO alloys and is crucial for improving
their strength and ductility; however, there is much uncertainty regarding
interstitial atoms in the cluster. Although density functional theory
calculations have shown that the inclusion of Mg interstitial atoms is
energetically most favorable in majority of LPSO alloys, solute elements have
also been experimentally observed at interstitial sites. To predict the
distributions of interstitial atoms in the cluster and to determine the kind of
elements present, it is necessary to identify mechanisms by which interstitial
atoms are created. In the present work, we use density functional theory
calculations to investigate growth processes of solute clusters, specifically
the Mg-Y-Zn LPSO alloy, in order to determine the precise atomistic structure
of its solute clusters. We show that a pair of an interstitial atom and a
vacancy are spontaneously created when a certain number of solute atoms are
absorbed into the cluster, and that all full-grown clusters should include
interstitial atoms. We also demonstrate that interstitial atoms are mostly Mg,
while the rest are Y; interstitial Zn atoms are negligible. This knowledge
greatly simplifies the atomistic modeling of solute clusters in Mg-Y-Zn alloys.
Owing to the vacancies emitted from the cluster, vacancy density should be
super-saturated in regions where solute clusters are growing, and increased
vacancy density accelerates cluster growth. | 2008.06230v2 |
2021-03-09 | Yield strength insensitivity in a dual-phase high entropy alloy after prolonged high temperature annealing | Recent studies of FeMnCoCr-based high entropy alloys have demonstrated
uncommon deformation behaviors such as transformation-induced plasticity, which
were largely believed to be restricted to select families of steels. Coupled
with the potential for entropy stabilization of high symmetry phases at high
temperatures, this system represents a promising class of materials for
structural applications in extreme environments. Yet, transformation-induced
plasticity mechanisms are notably sensitive to microstructure parameters and
the literature offers examples of deleterious decomposition of high entropy
alloys under heat treatment, which raises concerns of resiliency in mechanical
performance. Here, we evaluate the evolution of microstructure and mechanical
properties of a FeMnCoCr high entropy alloy after prolonged heat treatment at
high temperature. Microstructures are found to retain their characteristic
austenite/martensite features, with parent face-centered cubic grains
partitioned by hexagonal close-packed laths after heat treatment at 1200 C for
up to 48 hours. Results of mechanical testing reveal an unusual insensitivity
of this alloy to grain growth-induced weakening effects. Namely, the yield
strengths of FeMnCoCr samples are observed to remain constant across all heat
treatment conditions, despite a near four-fold increase in the grain size.
Close examination of post-heat treatment microstructures reveals a dramatic
decrease in the inter-lath spacing at longer durations, which segments parent
austenite grains. This crystal partitioning counteracts conventional grain
growth-induced weakening by introducing additional barriers for dislocation
pile-up. These results offer new insights into the mechanical resiliency of
this transformation-induced plasticity high entropy alloy under prolonged high
temperature heat treatment. | 2103.05567v2 |
2022-02-02 | High pressure induced precipitation in Al7075 alloy | Precipitate-matrix interactions govern the mechanical behavior of precipitate
strengthened Al-based alloys. These alloys find a wide range of applications
ranging from aerospace to automobile and naval industries due to their low cost
and high strength to weight ratio. Structures made from Al-based alloys undergo
complex loading conditions such as high strain rate impact, which involves high
pressures. Here we use diamond anvil cells to study the behavior of Al-based
Al7075 alloy under quasi-hydrostatic and non-hydrostatic pressure up to ~53
GPa. In situ X-ray diffraction (XRD) and pre- and post-compression transmission
electron microscopy (TEM) imaging are used to analyze microstructural changes
and estimate high pressure strength. We find a bulk modulus of 75.2 +- 1.9 GPa
using quasi-hydrostatic pressure XRD measurements. XRD showed that
non-hydrostatic pressure leads to a significant increase in defect density and
peak broadening with pressure cycling. XRD mapping under non-hydrostatic
pressure revealed that the region with the highest local pressure had the
greatest increase in defect nucleation, whereas the region with the largest
local pressure gradient underwent texturing and had larger grains. TEM analysis
showed that pressure cycling led to the nucleation and growth of many
precipitates. The significant increase in defect and precipitate density leads
to an increase in strength for Al7075 alloy at high pressures. | 2202.01203v1 |
2022-02-18 | Microstructure and phase transformation of nickel-titanium shape memory alloy fabricated by directed energy deposition with in-situ heat treatment | Additive manufacturing has been vastly applied to fabricate various
structures of nickel-titanium (NiTi) shape memory alloys due to its flexibility
to create complex structures with minimal defects. However, the microstructure
heterogeneity and secondary phase formation are two main problems that impede
the further application of NiTi alloys. Although post-heat treatment is usually
adopted to improve or manipulate NiTi alloy properties, it cannot realize the
spatial control of thermal and/or mechanical properties of NiTi alloys. To
overcome the limitations of uniform post-heat treatment, this study proposes an
in-situ heat treatment strategy that is integrated into the directed energy
deposition of NiTi alloys. The proposed method will potentially lead to new
manufacturing capabilities to achieve location-dependent performance or
property manipulation. The influences of in-situ heat treatment on the thermal
and mechanical properties of printed NiTi structures were investigated. The
investigations were carried out in terms of thermal cycling, microstructure
evolution, and mechanical properties by 3D finite element simulations and
experimental characterizations. A low-power laser beam was adopted to localize
the in-situ heat treatment only to the current printed layer, facilitating a
reverse peritectic reaction and a transient high solution treatment
successively. The proposed in-situ heat treatment on the specimen results in a
more obvious phase transformation peak in the differential scanning calorimetry
curves, about 50%~70% volume reduction for the Ti2Ni phase, and approximately
35 HV reduction on microhardness. | 2202.09428v1 |
2022-02-16 | Electromechanical coupling in Yb-substituted III-V nitride alloys | Group-III nitride alloys are currently used in various microwave
communication applications because of the giant enhancement in
electromechanical coupling after alloying with rocksalt nitrides such as YbN or
ScN. Herein, the Yb-substitution induced enhancement for electromechanical
coupling in wurtzite III-V nitrides is studied via theoretical calculations and
experiments. The substitution induced mechanical softening and local strain can
enhance electromechanical coupling. The mechanical softening induced by Yb
substitution shows less dependence on the parent AlN or GaN, which is caused by
the Yb-Yb pair interaction in the c-axis direction, and the difference of
electromechanical coupling between the GaN- and AlN-based alloys mainly comes
from their enhancement effect of Yb substitution for piezoelectric response.
The largest change in piezoelectric response relative to the parent nitride is
observed in GaN-based alloy, which is mainly considered as a consequence of
small piezoelectric constant of the parent GaN. Our calculations also reveal
that the substitutional element with a closer ionic size to the host cation is
easier to substitute into the host nitride, and produces a larger internal
strain to partly contribute to the enhancement in piezoelectric response. This
can serve as a simple guideline to identify alloying components in a search for
a massive increase in electromechanical coupling. | 2202.10500v3 |
2017-06-11 | Electronic structure and glass forming ability in early and late transition metal alloys | A correlation between the change in magnetic susceptibility
({\Delta}\c{hi}exp) upon crystallization of Cu-Zr, Hf metallic glasses (MG)
with their glass forming ability (GFA) observed recently is found to apply to
Cu-Ti and Zr-Ni alloys, too. In particular, a small {\Delta}\c{hi}exp , which
reflects similar electronic structures, ES, of glassy and corresponding
crystalline alloys, corresponds to high GFA. Here, we studied {\Delta}\c{hi}exp
in five Cu-Ti and four Cu-Zr and Ni-Zr MGs. The fully crystalline final state
of all alloys was verified from X-ray diffraction patterns. The variation of
GFA with composition in Cu-Ti, Cu-Zr and Cu-Hf MGs was established from the
variation of the corresponding critical casting thickness, dc. Due to the
absence of data for dc in Ni-Zr MGs their GFA was described by using empirical
criteria, such as the reduced glass transition temperature. A very good
correlation between {\Delta}\c{hi}exp and dc (and/or other criteria for GFA)
was observed for all alloys studied. The correlation between the ES and GFA
showed up best for Cu-Zr and NiZr2 alloys where direct data for the change in
ES ({\Delta}ES) upon crystallization are available. The applicability of the
{\Delta}\c{hi}exp ({\Delta}ES) criterion for high GFA (which provides a simple
way to select the compositions with high GFA) to other metal-metal MGs
(including ternary and multicomponent bulk MGs) is briefly discussed. | 1706.03332v2 |
2017-10-24 | Thermal conductivity of ternary III-V semiconductor alloys: The role of mass difference and long-range order | Thermal transport in bulk ternary III-V arsenide (III-As) semiconductor
alloys was investigated using equilibrium molecular dynamics with optimized
Albe-Tersoff empirical interatomic potentials. Existing potentials for binary
AlAs, GaAs, and InAs were optimized to obtain accurate phonon dispersions and
temperature-dependent thermal conductivity. Calculations of thermal transport
in ternary III-Vs commonly employ the virtual-crystal approximation (VCA),
where the structure is assumed to be a random alloy and all group-III atoms
(cations) are treated as if they have an effective weighted-average mass. Here,
we showed that is critical to treat atomic masses explicitly, and that the
thermal conductivity obtained with explicit atomic masses differs considerably
from the value obtained with the average VCA cation mass. The larger the
difference between the cation masses, the poorer the VCA prediction for thermal
conductivity. The random-alloy assumption in the VCA is also challenged,
because X-ray diffraction and transmission electron microscopy show order in
InGaAs, InAlAs, and GaAlAs epi-layers. We calculated thermal conductivity for
three common types of order [CuPt-B, CuAu-I, and triple-period-A (TPA)] and
showed that the experimental results for In$_{0.53}$Ga$_{0.47}$As and
In$_{0.52}$Al$_{0.48}$As, which are lattice matched to the InP substrate, can
be reproduced in molecular dynamics simulation with 2% and 8% of random
disorder, respectively. Based on our results, thermal transport in ternary
III-As alloys appears to be governed by the competition between mass-difference
scattering, which is much more pronounced than the VCA suggests, and the
long-range order that these alloys support. | 1710.08851v1 |
2019-08-16 | Optimizing phonon scattering by tuning surface-interdiffusion-driven intermixing to break the random-alloy limit of thermal conductivity | We investigate the evolution of the cross-plane thermal conductivity $\kappa$
of superlattices (SLs) as interfaces change from perfectly abrupt to totally
intermixed, by using non-equilibrium molecular dynamics simulations in
combination with the spectral heat current calculations. We highlight the role
of surface-interdiffusion-driven intermixing by calculating the $\kappa$ of SLs
with changing interface roughness, whose tuning allows for the $\kappa$ values
much lower than the "alloy limit" and the abrupt interface limit in same cases.
The interplay between alloy and interface scattering in different frequency
ranges provides a physical basis to predict a minimum of thermal conductivity.
More specifically, we also explore how the interface roughness affects the
thermal conductivities for SLs materials with a broad span of atomic mass and
bond strength. In particular, we find that (i) only when the "spacer" thickness
of SLs increases up to a critical value the $\kappa$ of rough SLs can break the
corresponding "alloy limit". (ii) Whether the $\kappa$ changes monotonically as
interface roughness strongly depends on the period length and intrinsic
behavior of phonon transport for SLs materials. Especially, for the SL with
large period length, there exists an optimal interface roughness which can
minimize the thermal conductivity. (iii) Surface-interdiffusion-driven
intermixing is more effective in achieving the low $\kappa$ below the alloy
limit for SL materials with large mass mismatch than with small one. (iv) It's
possible for SLs materials with large lattice mismatch (i.e., bond strength) to
design an ideally abrupt interface structure with $\kappa$ much below the
"alloy limit". These results have a clear implications for optimization of
thermal transport for heat management and for the development of thermoelectric
materials. | 1908.05830v1 |
2019-08-20 | Superconductivity in V$_{1-x}$Zr$_x$ alloys]{Evolution of high field superconductivity and high critical current density in the as-cast V$_{1-x}$Zr$_x$ alloys | We report here the structural, electrical and magnetic properties of as-cast
V$_{1-x}$Zr$_x$ alloys ($x$ =0 - 0.4) at low temperatures. We observe that all
the alloys undergo successive peritectic and eutectic reactions during cooling
from the melt which leads to the formation of five phases, namely, a body
centred cubic $\beta$-V phase, two phases with slightly different compositions
having face centred cubic ZrV$_2$ structure, a hexagonal closed packed
$\alpha$-Zr phase, and the $\beta$-Zr precipitates. The amount of each phase is
found to be dependent on the concentration of zirconium in vanadium. The
$\beta$-V and ZrV$_2$ phases show superconductivity below 5.3~K and 8.5~K
respectively. We show that the critical current density is large for V-rich
V$_{1-x}$Zr$_x$ alloys with $x >$ 0.1. The grain boundaries generated from the
eutectic reaction, and the point defects formed due to the variation in the
composition are found to be responsible for the pinning of flux lines in low
and high magnetic fields respectively. Our studies reveal that the choice of
the composition and the heat treatment which leads to eutectic reaction are
important in improving the critical current density in this alloy system. | 1908.07288v1 |
2019-08-22 | Synthesis of strain-relaxed Ge-Sn alloys using ion implantation and pulsed laser melting | Ge-Sn alloys with a sufficiently high concentration of Sn is a direct bandgap
group IV material. Recently, ion implantation followed by pulsed laser melting
has been shown to be a promising method to realize this material due to its
high reproducibility and precursor-free process. A Ge-Sn alloy with ~9 at.% Sn
was shown to be feasible by this technique. However, the compressive strain,
inherently occurring in heterogeneous epitaxy of the film, evidently delays the
material from the direct bandgap transition. In this report, an attempt to
synthesize a highly-relaxed Ge-Sn alloy will be presented. The idea is to
produce a significantly thicker film with a higher implant energy and doses.
X-ray reciprocal space mapping confirms that the material is largely-relaxed.
The peak Sn concentration of the highest dose sample is 6 at.% as determined by
Rutherford backscattering spectrometry. Cross-sectional transmission electron
microscopy shows unconventional defects in the film as the mechanism for the
strain relaxation. Finally, a photoluminescence (PL) study of the
strain-relaxed alloys shows photon emission at a wavelength of 2045 nm,
suggesting an active incorporation of Sn concentration of ~6 at.%. The results
of this study pave way to produce high quality relaxed GeSn alloy using an
industrially scalable method. | 1908.08241v1 |
2019-08-23 | Compressive performance and crack propagation in Al alloy/Ti2AlC composites | Composite materials comprising a porous Ti2AlC matrix and Al 6061 alloy were
fabricated by a current-activated pressure assisted melt infiltration process.
Coarse, medium and fine meso-structures were prepared with Al alloy filled
pores of differing sizes. Materials were subjected to uniaxial compressive
loading up to stresses of 668 MPa, leading to the failure of specimens through
crack propagation in both phases. As-fabricated and post-failure specimens were
analysed by X-ray microscopy and electron microscopy. Quasi-static mechanical
testing results revealed that compressive strength was the highest in the fine
structured composite materials. While the coarse structured specimens exhibited
a compressive strength of 80% relative to this. Reconstructed micro-scale X-ray
tomography data revealed different crack propagation mechanisms. Large planar
shear cracks propagated throughout the fine structured materials while the
coarser specimens exhibited networks of branching cracks propagating
preferentially along Al alloy-Ti2AlC phase interfaces and through shrinkage
pores in the Al alloy phase. Results suggest that control of porosity,
compensation for Al alloy shrinkage and enhancement of the Al alloy-Ti2AlC
phase interfaces are key considerations in the design of high performance
metal/Ti2AlC phase composites. | 1908.08757v1 |
2019-12-10 | Magnetic properties of thin epitaxial Pd$_{1-x}$Fe$_x$ alloy films | In the paper we present the results of extensive studies of palladium-rich
Pd1-xFex alloy films epitaxially grown on MgO single-crystal substrate. In a
composition range of x = 0.01-0.07 these materials are soft ferromagnets, the
saturation magnetization and magnetic anisotropy of which can be tuned by its
composition. Vibrating sample magnetometry was used to study temperature
dependences of spontaneous magnetic moment and to establish the temperature of
magnetic ordering (Curie temperature). Ferromagnetic resonance (FMR)
measurements at low temperatures in the in-plane and out-of-plane geometries
revealed the four-fold in-plane magnetic anisotropy with the easy directions
along the <110> axes of the substrate. The modelling of the angular dependence
of the field for resonance allowed to extract the cubic and tetragonal
contributions to the magnetic anisotropy of the films and establish their
dependence on the concentration of iron in the alloy. Experimental data are
discussed in the framework of existing theories of dilute magnetic alloys.
Using the anisotropy constants established from FMR, the magnetic hysteresis
loops are reproduced utilizing the Stoner-Wohlfarth model thus indicating the
predominant coherent magnetic moment rotation at low temperatures. The obtained
results compile a database of magnetic properties of a palladium-iron alloy
considered as a material for superconducting spintronics. | 1912.04852v1 |
2020-03-15 | Vacancy diffusion in multi-principal element alloys: the role of chemical disorder in the ordered lattice | Many of the purported virtues of Multi-Principal Element Alloys (MPEAs), such
as corrosion, high-temperature oxidation and irradiation resistance, are highly
sensitive to vacancy diffusivity. Similarly, solute interdiffusion is governed
by vacancy diffusion -- it is often unclear whether MPEAs are truly stable, or
effectively stabilized by slow interdiffusion. The considerable composition
space afforded to these alloys makes optimizing for desired properties a
daunting task; theoretical and computational tools are necessary to guide alloy
development. For diffusion, such tools depend on both a knowledge of the
vacancy migration barriers within a given alloy and an understanding of how
these barriers influence vacancy diffusivity. We present a generalized theory
of vacancy diffusion in rugged energy landscapes, paired with Kinetic Monte
Carlo simulations of MPEA vacancy diffusion. The barrier energy statistics are
informed by nudged elastic band calculations in the equiatomic CoNiCrFeMn
alloy. Theory and simulations show that vacancy diffusion in solid-solution
MPEAs is not necessarily sluggish, but can potentially be tuned, and that trap
models are an insufficient explanation for sluggish diffusion in the CoNiCrFeMn
HEA. These results also show that any model that endeavors to faithfully
represent diffusion-related phenomena must account for the full nature of the
energy landscape, not just the migration barriers. | 2003.06900v2 |
2020-03-22 | Numerical Investigation of Mechanical Properties of Aluminum-Copper Alloys at Nanoscale | Nanoindentation is a powerful tool capable of providing fundamental insights
of material elastic and plastic response at the nanoscale. Alloys at nanoscale
are particularly interesting as the local heterogeneity and deformation
mechanism revealed by atomistic study offers a better way to understand
hardening mechanism to build a stronger material. In this work, nanoindentation
in Al-Cu alloys are studied using atomistic simulations to investigate the
effects of loading direction, alloying percentages of Cu via dislocation-driven
mechanisms. Also, a low-fidelity finite element (FE) model has been developed
for nanoindentation simulations where nanoscale materials properties are used
from atomistic simulations. Material properties, such as hardness and reduced
modulus, are computed from both the FE and MD simulations and then compared.
Considering the fundamental difference between these two numerical approaches,
the FE results obtained from the present study conform fairly with those from
MD simulations. This paves a way into finding material properties of alloys
with reduced simulation time and cost by using FE where high-fidelity results
are not required. The results have been presented as load-displacement
analysis, dislocation density, dislocation loops nucleation and propagation,
von-Mises stress distribution and surface imprints. The techniques adopted in
this paper to incorporate atomistic data into FE simulations can be further
extended for finding other mechanical and fracture properties for complex alloy
materials. | 2003.09973v3 |
2020-06-02 | Phase Transformations During Continuous Cooling in Inconel 718 Alloys Manufactured by Laser Powder Bed Fusion and Suction Casting | Understanding alloy phase transformations during continuous cooling is
important for post-processing design and optimization. In this work,
continuous-cooling-transformation (CCT) diagrams of Inconel 718 alloys
manufactured by laser powder bed fusion (LPBF) and suction casting are
developed under different homogenization conditions. Unlike the available CCT
diagrams in the reported studies, no gamma double prime and gamma prime
precipitates can be observed. NbC and delta are determined to be the
precipitates after cooling from the gamma matrix. Importantly, homogenization
time and manufacturing methods are found to affect the Nb homogeneity in the
matrix near NbC particles and thus significantly influence the precipitation
process of the delta phase, which has a high content in Nb. In the alloys with
high Nb homogeneity, the nucleation process mainly contributes to the
precipitation, whereas in the alloys with low Nb homogeneity, the precipitation
is primarily associated with the growth process. Subgrains are found to form
after cooling at 0.1 K/s and can cause the highest hardness in samples. This
work provides a new viewpoint on the study of processing-structure-property
relationships during cooling in Inconel 718 and is beneficial to the
development of alloy post-processing strategies. | 2006.01737v2 |
2020-06-19 | High-Temperature Oxidation Kinetics of Additively Manufactured NiTiHf | NiTi-based high-temperature shape memory alloys (HTSMAs) such as NiTiHf have
been utilized in a broad range of applications due to their high strength and
work output, as well as, their ability to increase the transformation
temperatures (TTs). Recently, additive manufacturing techniques (AM) have been
widely used to fabricate complex shape memory alloy components without any
major modifications or tooling and has paved the way to tailor the
manufacturing and fabrications of microstructure and critical properties of
their final parts. NiTi alloys properties such as transformation temperatures
can be significantly altered due to oxidation, which can occur during the
manufacturing process or post-processing. In this work, the oxidation behavior
of Ni-rich NiTi20Hf shape memory alloys, which was fabricated by the selective
laser melting (SLM) method, is evaluated. Thermogravimetric analysis (TGA) is
used to assess the kinetic behavior of the oxidation at different temperature
ranges of 500, 700, and 900 C for 20 hours in the air. After oxidation, to
evaluate the microstructure and chemical composition X-ray diffraction (XRD),
scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy
(EDS) was conducted. The isothermal oxidation kinetics of conventional NiTi20Hf
alloys were studied, and the results were compared to AM samples. Results show
a two-stage oxidation rate at which oxidation increased with the high rate at
the initial stage. As the oxidation time increased, the oxidation rate
gradually decreased. The oxidation behavior of NiTiHf alloys initially obeyed
logarithmic rate law and then followed by parabolic rate law. SEM results
showed the formation of a multi-layered oxide scale, including TiO2, NiTiO3,
and Hf oxide. | 2006.11114v1 |
2020-07-22 | Formation energy puzzle in intermetallic alloys: Random phase approximation fails to predict accurate formation energies | We performed density functional calculations to estimate the formation
energies of intermetallic alloys. We used two semilocal approximations, the
generalized gradient approximation (GGA) by Perdew-Burke-Ernzerhof (PBE) and
the strongly constrained and appropriately normed (SCAN) meta-GGA. In addition,
we utilized two nonlocal DFT functionals, the hybrid HSE06, and the
state-of-the-art random phase approximation (RPA). The nonlocal functionals
such as HSE06 and RPA yield accurate formation energies of binary alloys with
completely-filled d-band metals, where semilocal functionals underperform. The
accuracy at the nonlocal functionals is greatly reduced when a partially-filled
d-band metal is present in an alloy, while PBE-GGA outperforms in these cases.
We show that the accurate prediction of formation energies by any DFT method
depends on its ability to predict the accurate electronic properties, e.g.,
valence d-band contribution to the density of states (DOS). The SCAN meta-GGA
often corrects the PBE-DOS, however, it does not provide accurate formation
energies compared to PBE. This is assumed to be due to the lack of proper error
cancellation that should be expected due to the similar bulk nature of both
alloys and their constituents, which may improve with the modification of
meta-GGA ingredients. RPA yields too negative formation energies of alloys with
partially-filled d-band metals. RPA results can be corrected by restoring the
exchange-correlation kernel, thereby improving the short-range
electron-electron correlation in metallic densities. | 2007.11150v2 |
2020-09-16 | Rolling contact fatigue deformation mechanisms of nickel-rich nickel-titanium-hafnium alloys | The tribological performance and underlying deformation behavior of Ni55Ti45,
Ni54Ti45Hf1 and Ni56Ti36Hf8 alloys were studied using rolling contact fatigue
(RCF) testing and transmission electron microscopy (TEM). TEM results of the
as-machined RCF rods, prepared using focus ion beam, revealed some damage very
close to the surface. TEM results after initial RCF cycling showed that
additional damage was mainly confined to deformation bands that propagated
several microns into the sample. These bands formed via localized dislocation
slip, possibly on multiple slip systems, within the B2 matrix and/or within
transformed B19 prime martensite phase under repeated applied contact stress.
Further cycling of Ni55Ti45 and Ni54Ti45Hf1 led to shearing and dissolution of
the strengthening precipitates within the deformation bands, followed by
formation of nanocrystalline grains and finally amorphization of the remaining
matrix material within the bands. The Ni56Ti36Hf8 alloy exhibited a notable
increase in RCF performance and a smaller damage zone (1.5 microns) compared to
the Ni55Ti45 and Ni54Ti45Hf1 alloys (over 6 microns). This was attributed to
the low fraction of B2 matrix phase (less than or equal to 13 %) in the
Ni56Ti36Hf8 alloy, leading to formation of narrow deformation bands (less than
11 nm) that were incapable of dissolving the much larger precipitates. Instead,
the deformation bands were restricted to narrow channels between the dense
cubic NiTiHf and H-phase precipitates. In contrast, broad deformation bands
accompanied by shearing and eventual dissolution of the Ni4Ti3 precipitates
were observed in the Ni55Ti45 and Ni54Ti45Hf1 alloys due to the high area
fractions of B2 matrix phase (~49 %). | 2009.07969v1 |
2020-09-22 | A cracking oxygen story: a new view of stress corrosion cracking in titanium alloys | Titanium alloys can suffer from halide-associated stress corrosion cracking
at elevated temperatures e.g., in jet engines, where chlorides and Ti-oxide
promote the cracking of water vapour in the gas stream, depositing embrittling
species at the crack tip. Here we report, using isotopically-labelled
experiments, that crack tips in an industrial Ti-6Al-2Sn-4Zr-6Mo alloy are
strongly enriched (>5 at.%) in oxygen from the water vapour, far greater than
the amounts (0.25 at.%) required to embrittle the material. Surprisingly,
relatively little hydrogen (deuterium) is measured, despite careful preparation
and analysis. Therefore, we suggest that a combined effect of O and H leads to
cracking, with O playing a vital role, since it is well-known to cause
embrittlement of the alloy. In contrast it appears that in alpha+beta Ti
alloys, it may be that H may drain away into the bulk owing to its high
solubility in beta-Ti, rather than being retained in the stress field of the
crack tip. Therefore, whilst hydrides may form on the fracture surface,
hydrogen ingress might not be the only plausible mechanism of embrittlement of
the underlying matrix. This possibility challenges decades of understanding of
stress-corrosion cracking as being related solely to the hydrogen enhanced
localised plasticity (HELP) mechanism, which explains why H-doped Ti alloys are
embrittled. This would change the perspective on stress corrosion embrittlement
away from a focus purely on hydrogen to also consider the ingress of O
originating from the water vapour, insights critical for designing corrosion
resistant materials. | 2009.10567v2 |
2020-10-01 | Low-temperature thermal conductivity of Co$_{1-x}$M$_x$Si (M=Fe, Ni) alloys | We study the low-temperature electrical and thermal conductivity of CoSi and
Co$_{1-x}$M$_x$Si alloys (M = Fe, Ni; $x \leq$ 0.06). Measurements show that
the low-temperature electrical conductivity of Co$_{1-x}$Fe$_{x}$Si alloys
decreases at $x > $ 0.01 by an order of magnitude compared with that of pure
CoSi. It was expected that both the lattice and electronic contributions to
thermal conductivity would decrease in the alloys. However, our experimental
results revealed that at temperatures below 20K the thermal conductivity of Fe-
and Ni-containing alloys is several times larger than that of pure CoSi. We
discuss possible mechanisms of the thermal conductivity enhancement. The most
probable one is related to the dominant scattering of phonons by charge
carriers. We propose a simple theoretical model that takes into account the
complex semimetallic electronic structure of CoSi with nonequivalent valleys,
and show that it explains well the increase of the lattice thermal conductivity
with increasing disorder and the linear temperature dependence of the thermal
conductivity in the Co$_{1-x}$Fe$_x$Si alloys below 20K. | 2010.00552v3 |
2020-12-15 | Comparison of long-term natural aging to artificial aging in Duralumin | The understanding of long-term aging of aeronautical materials, in particular
aluminium alloys used in the fuselage and structure of aircraft is of extreme
importance for airline fleets. In this work, a plate from an old aircraft
(Breguet) was retrieved and studied in terms of microstructure and mechanical
properties. A comparison was made between this naturally-aged alloy and a
modern alloy on which different artificial aging conditions were applied. The
old alloy exhibits a precipitation of $\theta$-Al2Cu at grain boundaries and of
$\Omega$-Al2Cu on dispersoids. This non-expected nanostructure for an alloy in
T4 state was attributed to the heat that the plate experienced during the
aircraft cycles. However, it is shown that this aging is reversible (after a
solution treatment). Moreover, the very long time of outdoors exposure seems to
have caused intergranular corrosion causing the early failure during tensile
tests on some of the specimens. The artificial aging (low temperature,
100{\textdegree}C for up to 10,000h) applied on the modern 2017A alloy did not
allow to reproduce the nanostructure of the old plate, meaning that isothermal
conditions for artificial aging might not be appropriate in this case. | 2012.08315v1 |
2021-02-25 | Phonon heat conduction in Al1-xScxN thin films | Aluminum scandium nitride alloy (Al1-xScxN) is regarded as a promising
material for high-performance acoustic devices used in wireless communication
systems. Phonon scattering and heat conduction processes govern the energy
dissipation in acoustic resonators, ultimately determining their performance
quality. This work reports, for the first time, on phonon scattering processes
and thermal conductivity in Al1-xScxN alloys with the Sc content (x) up to
0.26. The thermal conductivity measured presents a descending trend with
increasing x. Temperature-dependent measurements show an increase in thermal
conductivity as the temperature increases at temperatures below 200K, followed
by a plateau at higher temperatures (T> 200K). Application of a virtual crystal
phonon conduction model allows us to elucidate the effects of boundary and
alloy scattering on the observed thermal conductivity behaviors. We further
demonstrate that the alloy scattering is caused mainly by strain-field
difference, and less by the atomic mass difference between ScN and AlN, which
is in contrast to the well-studied Al1-xGaxN and SixGe1-x alloy systems where
atomic mass difference dominates the alloy scattering. This work studies and
provides the quantitative knowledge for phonon scattering and the thermal
conductivity in Al1-xScxN, paving the way for future investigation of materials
and design of acoustic devices. | 2102.12673v1 |
2021-04-15 | Correlation versus randomization of jerky flow in an AlMgScZr alloy using acoustic emission | Jerky flow in solids results from collective dynamics of dislocations which
gives rise to serrated deformation curves and a complex evolution of the strain
heterogeneity. A rich example of this phenomenon is the Portevin-Le Chatelier
effect in alloys. The corresponding spatiotemporal patterns showed some
universal features which provided a basis for a well-known phenomenological
classification. Recent studies revealed peculiar features in both the stress
serration sequences and the kinematics of deformation bands in Al-based alloys
containing fine microstructure elements, such as nanosize precipitates and (or)
submicron grains. In the present work, jerky flow of an AlMgScZr alloy is
studied using statistical analysis of stress serrations and the accompanying
acoustic emission. As in the case of coarse-grained binary AlMg alloys, the
amplitude distributions of acoustic events obey a power-law scaling which is
usually considered as evidence of avalanchelike dynamics. However, the scaling
exponents display specific dependences on the strain and strain rate for the
investigated materials. The observed effects bear evidence to a competition
between the phenomena of synchronization and randomization of dislocation
avalanches, which may shed light on the mechanisms leading to a high variety of
jerky flow patterns observed in applied alloys. | 2104.07305v1 |
2021-05-22 | Unexpected variations in the kinetics of solid solution alloys due to local interactions | Diffusion of atoms in solids is one of the most fundamental kinetic processes
that ultimately governs many materials properties. Here, we report on a
combined first-principles and kinetic Monte Carlo study of macroscopic
diffusion properties of disordered Ti-Ta alloys over the entire composition
range. Using simple cluster expansion model Hamiltonians parametrized on
density functional theory data, we compute transport properties explicitly
including local interactions between the two atomic species and compare them
with the non-interacting diffusion model for disordered, random alloys.
Surprisingly, we find that although these alloys thermodynamically behave as
nearly random solid solutions, their kinetic properties deviate significantly
from the behavior predicted by diffusion models for non-interacting systems. We
attribute these differences in transport properties to the local interactions
that create a rather corrugated potential energy landscape and consequently
give rise to energetically non-degenerate end-states of diffusion processes
which cannot be realized in a non-interacting disordered or other simpler
diffusion models. The findings emphasize the limitations of the widely known
non-interacting disordered diffusion model for such systems. Furthermore, we
explain that changes in mobility in these alloys is predominantly due to
changes in the correlation factor caused by the local interactions. Our work
thus highlights the importance of explicitly including local interactions when
assessing the transport properties of thermodynamically nearly disordered
alloys. | 2105.10629v1 |
2021-06-10 | Spin-Orbit Torque Engineering in β-W/CoFeB Heterostructures via Ta and V Alloying at Interfaces | Spin-orbit torque manifested as an accumulated spin-polarized moment at
nonmagnetic normal metal, and ferromagnet interfaces is a promising
magnetization switching mechanism for spintronic devices. To fully exploit this
in practice, materials with a high spin Hall angle, i.e., a charge-to-spin
conversion efficiency, are indispensable. To date, very few approaches have
been made to devise new nonmagnetic metal alloys. Moreover, new materials need
to be compatible with semiconductor processing. Here we introduce W-Ta and W-V
alloys and deploy them at the interface between $\beta$-W/CoFeB layers. First,
spin Hall conductivities of W-Ta and W-V structures with various compositions
are carried out by first-principles band calculations, which predict the spin
Hall conductivity of the W-V alloy is improved from $-0.82 \times 10^3$ S/cm
that of W to $-1.98 \times 10^3$ S/cm. Subsequently, heterostructure
fabrication and spin-orbit torque properties are characterized experimentally.
By alloying $\beta$-W with V at a concentration of 20 at%, we observe a large
enhancement of the absolute value of spin Hall conductivity of up to $-(2.77
\pm 0.31) \times 10^3$ S/cm. By employing X-ray diffraction and scanning
transmission electron microscopy, we further explain the enhancement of
spin-orbit torque efficiency is stemmed from W-V alloy between W and CoFeB. | 2106.05460v1 |
2021-08-11 | Ising superconductivity in monolayer niobium dichalcogenide alloys | NbSe$_{2}$ and NbS$_{2}$ are isostructural two-dimensional materials that
exhibit contrasting superconducting properties when reduced to the single
monolayer limit. Monolayer NbSe$_{2}$ is an Ising superconductor, while there
have been no reports of superconductivity in monolayer NbS$_{2}$.
NbS$_{x}$Se$_{2-x}$ alloys exhibit an intriguing non-monotonic dependence of
the superconducting transition temperature with sulfur content, which has been
interpreted as a manifestation of fractal superconductivity. However, several
key questions about this result are not known: (1) Does the electronic
structure of the alloy differ from the parent compounds, (2) Are spin
fluctuations which have been shown to be prominent in monolayer NbSe$_{2}$ also
present in the alloys? Using first-principles calculations, we show that the
density of states at the Fermi level and the proximity to magnetism in
NbS$_{x}$Se$_{2-x}$ alloys are both reduced compared to the parent compound;
the former would decrease the transition temperature while the latter would
increase it. We also show that Se vacancies, which are likely magnetic
pair-breaking defects, may form in large concentrations in NbSe$_{2}$. Based on
our results, we suggest an alternative explanation of the non-monotonic
behavior the superconducting transition temperature in NbS$_{x}$Se$_{2-x}$
alloys, which does not require the conjecture of multifractality. | 2108.05426v2 |
2021-08-26 | Effects of minor alloying on the mechanical properties of Al based metallic glasses | Minor alloying is widely used to control mechanical properties of metallic
glasses (MGs). The present understanding of how a small amount of alloying
element changes strength is that the additions lead to more efficient packing
of atoms and increased local topological order, which then increases the
barrier for shear transformations and the resistance to plastic deformation.
Here, we discover that minor alloying can improve the strength of MGs by
increasing the chemical bond strength alone and show that this strengthening is
distinct from changes in topological order. The results were obtained using
Al-Sm based MGs minor alloyed with transition metals (TMs). The addition of TMs
led to an increase in the hardness of the MGs which, however, could not be
explained based on changes in the topological ordering in the structure.
Instead we found that it was the strong bonding between TM and Al atoms which
led to a higher resistance to shear transformation that resulted in higher
strength and hardness, while the topology around the TM atoms had no influence
on their mechanical response. This finding demonstrates that the effects of
topology and chemistry on mechanical properties of MGs are independent of each
other and that they should be understood as separate, sometimes competing
mechanisms of strengthening. This understanding lays a foundation for design of
MGs with improved mechanical properties. | 2108.12028v1 |
2022-01-22 | High-throughput calculations combining machine learning to investigate the corrosion properties of binary Mg alloys | Magnesium (Mg) alloys have shown great prospects as both structural and
biomedical materials, while poor corrosion resistance limits their further
application. In this work, to avoid the time-consuming and laborious experiment
trial, a high-throughput computational strategy based on first-principles
calculations is designed for screening corrosion-resistant binary Mg alloy with
intermetallics, from both the thermodynamic and kinetic perspectives. The
stable binary Mg intermetallics with low equilibrium potential difference with
respect to the Mg matrix are firstly identified. Then, the hydrogen adsorption
energies on the surfaces of these Mg intermetallics are calculated, and the
corrosion exchange current density is further calculated by a hydrogen
evolution reaction (HER) kinetic model. Several intermetallics, e.g. Y3Mg, Y2Mg
and La5Mg, are identified to be promising intermetallics which might
effectively hinder the cathodic HER. Furthermore, machine learning (ML) models
are developed to predict Mg intermetallics with proper hydrogen adsorption
energy employing work function (W_f) and weighted first ionization energy
(WFIE). The generalization of the ML models is tested on five new binary Mg
intermetallics with the average root mean square error (RMSE) of 0.11 eV. This
study not only predicts some promising binary Mg intermetallics which may
suppress the galvanic corrosion, but also provides a high-throughput screening
strategy and ML models for the design of corrosion-resistant alloy, which can
be extended to ternary Mg alloys or other alloy systems. | 2201.09059v1 |
2022-05-19 | Intermetallic particle heterogeneity controls shear localization in high-strength nanostructured Al alloys | The mechanical behavior of two nanocrystalline Al alloys, Al-Mg-Y and
Al-Fe-Y, is investigated with in-situ micropillar compression testing. Both
alloys were strengthened by a hierarchical microstructure including grain
boundary segregation, nanometer-thick amorphous complexions, carbide nanorod
precipitates with sizes of a few nanometers, and submicron-scale intermetallic
particles. The maximum yield strength of the Al-Mg-Y system is measured to be
950 MPa, exceeding that of the Al-Fe-Y system (680 MPa), primarily due to a
combination of more carbide nanorods and more amorphous complexions. Both
alloys exhibited yield strengths much higher than those of commercial Al
alloys, and therefore have great potential for structural applications.
However, some micropillar specimens were observed to plastically soften through
shear banding. Post-mortem investigation revealed that intermetallic-free
deformation pathways of a few micrometers in length were responsible for this
failure. Further characterization showed significant grain growth within the
shear band. The coarsened grains maintained the same orientation with each
other, pointing to grain boundary mechanisms for plastic flow, specifically
grain rotation and/or grain boundary migration. The presence of intermetallic
particles makes it difficult for both matrix and intermetallic grains to rotate
into the same orientation due to the different lattice parameters and slip
systems. Therefore, we are able to conclude that a uniform distribution of
intermetallic particles with an average spacing less than the percolation
length of shear localization can effectively prevent the maturation of shear
bands, offering a design strategy for high-strength nanocrystalline Al alloys
with both high strength and stable plastic flow. | 2205.09820v2 |
2022-08-02 | Materials Swelling Revealed Through Automated Semantic Segmentation of Cavities in Electron Microscopy Images | Accurately quantifying swelling of alloys that have undergone irradiation is
essential for understanding alloy performance in a nuclear reactor and critical
for the safe and reliable operation of reactor facilities. However, typical
practice is for radiation-induced defects in electron microscopy images of
alloys to be manually quantified by domain-expert researchers. Here, we employ
an end-to-end deep learning approach using the Mask Regional Convolutional
Neural Network (Mask R-CNN) model to detect and quantify nanoscale cavities in
irradiated alloys. We have assembled the largest database of labeled cavity
images to date, which includes 400 images, >34k discrete cavities, and numerous
alloy compositions and irradiation conditions. We have evaluated both
statistical (precision, recall, and F1 scores) and materials property-centric
(cavity size, density, and swelling) metrics of model performance, and
performed in-depth analysis of materials swelling assessments. We find our
model gives assessments of material swelling with an average (standard
deviation) swelling mean absolute error based on random leave-out
cross-validation of 0.30 (0.03) percent swelling. This result demonstrates our
approach can accurately provide swelling metrics on a per-image and
per-condition basis, which can provide helpful insight into material design
(e.g., alloy refinement) and impact of service conditions (e.g., temperature,
irradiation dose) on swelling. Finally, we find there are cases of test images
with poor statistical metrics, but small errors in swelling, pointing to the
need for moving beyond traditional classification-based metrics to evaluate
object detection models in the context of materials domain applications. | 2208.01460v1 |
2022-08-10 | Impact of random alloy fluctuations on the electronic and optical properties of (Al,Ga)N quantum wells: Insights from tight-binding calculations | Light emitters based on the semiconductor alloy aluminium gallium nitride
((Al,Ga)N) have gained significant attention in recent years due to their
potential for a wide range of applications in the ultraviolet (UV) spectral
window. However, current state-of-the-art (Al,Ga)N light emitters exhibit very
low internal quantum efficiencies (IQEs). Therefore, understanding the
fundamental electronic and optical properties of (Al,Ga)N-based quantum wells
is key to improving the IQE. Here, we target the electronic and optical
properties of c-plane Al$_x$Ga$_{1-x}$N/AlN quantum wells by means of an
empirical atomistic tight-binding model. Special attention is paid to the
impact of random alloy fluctuations on the results as well as the aluminium
content x in the well. We find that across the studied Al content range (from
10% to 75% Al) strong hole wave function localization effects are observed.
Additionally, with increasing Al content, electron wave functions start also to
exhibit carrier localization features. Overall, our investigations on the
electronic structure of c-plane Al$_x$Ga$_{1-x}$N/AlN quantum wells reveal that
already random alloy fluctuations are sufficient to lead to (strong) carrier
localization effects. Furthermore, our results indicate that random alloy
fluctuations impact the degree of optical polarization in c-plane
Al$_x$Ga$_{1-x}$N quantum wells. We find that the switching from transverse
electric to transverse magnetic light polarization occurs at higher Al contents
in the atomistic calculation, which accounts for random alloy fluctuations,
when compared to the outcome of widely used virtual crystal approximations.
This observation is important for light extraction efficiencies in
(Al,Ga)N-based light emitting diodes operating in the deep UV. | 2208.05337v1 |
2022-08-23 | Theory of Nb-Zr Alloy Superconductivity and First Experimental Demonstration for Superconducting Radio-Frequency Cavity Applications | Niobium-zirconium (Nb-Zr) alloy is an old superconductor that is a promising
new candidate for superconducting radio-frequency (SRF) cavity applications.
Using density-functional and Eliashberg theories, we show that addition of Zr
to a Nb surface in small concentrations increases the critical temperature
$T_c$ and improves other superconducting properties. Furthermore, we calculate
$T_c$ for Nb-Zr alloys across a broad range of Zr concentrations, showing good
agreement with the literature for disordered alloys as well as the potential
for significantly higher $T_c$ in ordered alloys near 75%Nb/25%Zr composition.
We provide experimental verification on Nb-Zr alloy samples and SRF sample test
cavities prepared with either physical vapor or our novel electrochemical
deposition recipes. These samples have the highest measured $T_c$ of any Nb-Zr
superconductor to date and indicate a reduction in BCS resistance compared to
the conventional Nb reference sample; they represent the first steps along a
new pathway to greatly enhanced SRF performance. Finally, we use
Ginzburg-Landau theory to show that the addition of Zr to a Nb surface
increases the superheating field $B_{sh}$, a key figure of merit for SRF which
determines the maximum accelerating gradient at which cavities can operate. | 2208.10678v1 |
2023-01-18 | Multiscale statistical quantum transport in porous media and random alloys with vacancies | We have developed a multi-scale self-consistent method to study the charge
conductivity of a porous system or a metallic matrix alloyed by randomly
distributed nonmetallic grains and vacancies by incorporating Schr\"{o}dinger's
equation and Poisson's equation. To account for the random distribution of the
nonmetallic grains and clusters within the alloy system, we have used an
uncorrelated white-noise Monte-Carlo sampling to generate numerous random
alloys and statistically evaluate the charge conductance. We have performed a
parametric study and investigated various electrical aspects of random porous
and alloy systems as a function of the inherent parameters and density of the
random grains. Our results find that the charge conductance within the
low-voltage regime shows a highly nonlinear behavior against voltage variations
in stark contrast to the high-voltage regime where the charge conductance is
constant. The former finding is a direct consequence of the quantum scattering
processes. The results reveal the threshold to the experimentally observable
quantities, e.g., voltage difference, so that the charge current is activated
for values larger than the threshold. The numerical study determines the
threshold of one quantity as a function of the remaining quantities. Our method
and results can serve to guide future experiments in designing circuital
elements, involving this type of random alloy system. | 2301.07569v2 |
2023-01-28 | Critical resolved shear stresses for slip and twinning in Mg-Y-Ca alloys and their effect on the ductility | The deformation mechanisms of an extruded Mg-5Y-0.08Ca (wt. %) alloy were
analyzed by means of micropillar compression tests on single crystals along
different orientations -- selected to activate specific deformation modes -- as
well as slip trace analysis, transmission electron microscopy and transmission
Kikuchi diffraction. The polycrystalline alloy presented a remarkable ductility
in tension (~32%) and negligible differences in the yield strength between
tension and compression. It was found that the presence of Y and Ca in solid
solution led to a huge increase in the CRSS for <a> basal slip (29 $\pm$ 5
MPa), <c+a> pyramidal slip (203 $\pm$ 7 MPa) and tensile twin nucleation (above
148 MPa), while the CRSS for <a> prismatic slip only increases up to 105 $\pm$
4 MPa. The changes in the CRSS for slip and tensile twinning in Mg-Y-Ca alloys
expectedly modify the dominant deformation mechanisms in polycrystals. In
particular, tensile twinning is replaced by <a> prismatic slip during
compressive deformation along the a-axis. The reduction of twinning (which
generally induces strong anisotropy in the plastic deformation in textured
alloys), and the activation of <a> prismatic slip (which provides an additional
plastic deformation mechanism with limited hardening) were responsible for the
large tensile ductility of the alloy. | 2301.12154v1 |
2023-02-07 | What can one learn about Fe-Cr alloys using Mössbauer spectroscopy? | Applications of the M\"ossbauer spectroscopy (MS) in the investigation of
Fe-Cr alloys are reviewed. A high sensitivity of the hyperfine magnetic field
to the presence of Cr atoms in the vicinity of the probe Fe atoms permits
quantitative investigation of various aspects related both to the
crystallographic as well as to the magnetic phase diagram of this alloy system.
Concerning the former, presented is the relevance of MS for determining borders
of the miscibility gap and kinetics of the phase decomposition, distinguishing
between nucleation and growth and spinodal decomposition, identifying the
sigma-phase and studying kinetics of its precipitation. Regarding the magnetic
phase diagram, MS is useful for determining the Curie, the N\'eel and the
spin-freezing temperature, hence studying paramagnetic-ferromagnetic,
paramagnetic-antiferromagnetic and paramagnetic-spin-glass transitions. An
effect of different heat treatments, strain and irradiation with various
particles on a distribution of Cr atoms in the Fe matrix is demonstrated, too.
For bcc-FeCr alloys relevance of MS for determining changes in spin and charge
densities at Fe-sites induced by neighboring Cr atoms is illustrated, as well
as its usefulness in studying changes caused by a high-temperature sulphidation
and oxidation. Concerning properties of sigma-FeCr alloys the application of MS
for determining the Curie and Debye temperature is reviewed. Application of MS
to study an effect of magnetism on the lattice dynamics of Fe atoms in
sigma-FeCr is also exemplified. Last but not least, determining a magnetic
texture and mechanical alloying is addressed. | 2302.03436v1 |
2023-02-09 | Strengthening of Mg-Al-Ca alloys with C15 and C36 Laves phases | The Laves phase skeleton in cast Mg-Al-Ca alloys is known to provide
considerable strengthening. Laves phases such as CaMg$_2$ (C14), Ca(Al,Mg)$_2$
(C36), and CaAl2 (C15) have high melting points, high hardness at room and
elevated temperatures, but unfortunately are inherently brittle. Mg-Al-Ca
alloys thus have good creep properties but limited ductility. An understanding
of the co-deformation behaviour of $\alpha$-Mg and Laves phases is essential
for optimising the strength-ductility balance of these alloys. Here, we study
the mechanical behaviour of a Mg-4.65Al-2.82Ca alloy using micropillar
compression in the $\alpha$-Mg matrix, at $\alpha$-Mg/C36 and $\alpha$-Mg/C15
interfaces and in the C15 phase in combination with scanning electron
microscopy (SE imaging), electron backscatter diffraction (EBSD), transmission
Kikuchi diffraction (TKD), and low-kV scanning transmission electron microscopy
(STEM). We show that both, C15 and C36, Laves phases provide considerable
strengthening to the $\alpha$-Mg matrix by delaying the onset of basal slip and
extension twinning, while only the C36 phase appears to allow a certain extent
of slip transfer/ plastic co-deformation, in spite of its greater anisotropy
compared with the cubic C15 phase. We therefore conclude based on these results
that strengthening of the $\alpha$-Mg matrix by the C36 Laves phase is
preferable given that it combines easy skeleton formation with some
co-deformation and considerable stability at common application temperatures of
magnesium alloys. | 2302.04756v1 |
2023-07-12 | Machine learning accelerated discovery of corrosion-resistant high-entropy alloys | Corrosion has a wide impact on society, causing catastrophic damage to
structurally engineered components. An emerging class of corrosion-resistant
materials are high-entropy alloys. However, high-entropy alloys live in
high-dimensional composition and configuration space, making materials designs
via experimental trial-and-error or brute-force ab initio calculations almost
impossible. Here we develop a physics-informed machine-learning framework to
identify corrosion-resistant high-entropy alloys. Three metrics are used to
evaluate the corrosion resistance, including single-phase formability, surface
energy and Pilling-Bedworth ratios. We used random forest models to predict the
single-phase formability, trained on an experimental dataset. Machine learning
inter-atomic potentials were employed to calculate surface energies and
Pilling-Bedworth ratios, which are trained on first-principles data fast
sampled using embedded atom models. A combination of random forest models and
high-fidelity machine learning potentials represents the first of its kind to
relate chemical compositions to corrosion resistance of high-entropy alloys,
paving the way for automatic design of materials with superior corrosion
protection. This framework was demonstrated on AlCrFeCoNi high-entropy alloys
and we identified composition regions with high corrosion resistance. Machine
learning predicted lattice constants and surface energies are consistent with
values by first-principles calculations. The predicted single-phase formability
and corrosion-resistant compositions of AlCrFeCoNi agree well with experiments.
This framework is general in its application and applicable to other materials,
enabling high-throughput screening of material candidates and potentially
reducing the turnaround time for integrated computational materials
engineering. | 2307.06384v3 |
2023-10-07 | ScaleLat: A chemical structure matching algorithm for mapping atomic structure of multi-phase system and high entropy alloys | ScaleLat (Scale Lattice) is a computer program written in C for performing
the atomic structure analysis of multi-phase system or high entropy alloys
(HEAs). The program implements an atomic cluster extraction algorithm to obtain
all independent and symmetry-reduced characteristic chemical structures for the
complex atomic configurations which are usually obtained from molecular
dynamics or kinetic Monte-Carlo simulations for supercell containing more than
104 atoms. ScaleLat employes an efficient and unique chemical structure
matching algorithm to map all extracted atomic clusters from a large supercell
(>10^4 atoms) to a representative small one (~ 10^3 or less), providing the
possibility to directly use the highly accurate quantum mechanical methods to
study the electronic, magnetic, and mechanical properties of multi-component
alloys with complex microstructures. We demonstrate the capability of ScaleLat
code by conducting both the atomic structure analysis and chemical structure
matching procedure for Fe-12.8 at.% Cr binary alloy and equiatomic CrFeCoNiCu
high entropy alloy, and by successfully obtaining the representatively
supercells containing 10^2~10^3 atoms of the two alloys. Overall, ScaleLat
program provides a universal platform to efficiently project all essential
chemical structures of large complex atomic structures to a relatively
easy-handling small supercell for quantum mechanical calculations of various
user interested properties. | 2310.04754v1 |
2023-10-09 | A Reactive Force Field Approach to Modeling Corrosion of NiCr Alloys in Molten FLiNaK Salts | The interface between NiCr alloys and FLiNaK molten salt exhibits complex
corrosion behavior, mainly driven by intricate chemical interactions involving
Cr and F$\mathrm{^-}$ ions. Understanding these dynamic reactions is crucial
for developing effective corrosion mitigation strategies to ensure the
long-term durability of Ni-based alloy components in molten salt technologies.
However, obtaining molecular-level understanding through experiments is
challenging. To address this, we utilize reactive molecular dynamics
simulations enabled by a reactive force field, ReaxFF, to investigate detailed
reaction dynamics at the atomic level. Since there is currently no available
force field involving fluoride salt and Ni-based alloys, we first present the
development of the ReaxFF parameter set for Ni/Cr/F/Li/Na/K based on extensive
first-principles calculations. With this force field, we achieve a strong
agreement for the structure of FLiNaK molten salt by comparing the pair
distribution functions with experimental and simulation results. Furthermore,
it successfully reproduces the experimental phenomenon of Cr dissolution in
fluoride salt, with the corrosion rate depending on the alloy and salt
compositions. Particularly, it reveals that increasing the concentration of Li
can enhance the formation of a compact double layer, mitigating Cr dissolution.
This work enables a fundamental understanding of the interfacial behavior
between fluoride salt and NiCr alloys. | 2310.05856v1 |
2024-01-10 | Laser induced ultrafast Gd 4f spin dynamics at the surface of amorphous CoxGd100-x ferrimagnetic alloys | We have investigated the laser induced ultrafast dynamics of Gd 4f spins at
the surface of CoxGd100-x alloys by means of surface-sensitive and
time-resolved dichroic resonant Auger spectroscopy. We have observed that the
laser induced quenching of Gd 4f magnetic order at the surface of the
CoxGd100-x alloys occur on a much longer time scale than that previously
reported in bulk sensitive time-resolved experiments. In parallel, we have
characterized the static structural and magnetic properties at the surface and
in the bulk of these alloys by combining Physical Property Measurement System
(PPMS) magnetometry with X-ray Magnetic Circular Dichroism in absorption
spectroscopy (XMCD) and X-Ray Photoelectron spectroscopy (XPS). The PPMS and
XMCD measurements give information regarding the composition in the bulk of the
alloys. The XPS measurements show non-homogeneous composition at the surface of
the alloys with a strongly increased Gd content within the first layers
compared to the nominal bulk values. Such larger Gd concentration results in a
reduced indirect Gd 4f spin-lattice coupling. It explains the slower Gd 4f
demagnetization we have observed in our surface-sensitive and time-resolved
measurements compared to that previously reported by bulk-sensitive
measurements. | 2401.05130v3 |
2024-02-29 | Tuning chemical short-range order for stainless behavior at reduced chromium concentrations in multi-principal element alloys | Single-phase multi-principal element alloys (MPEAs) hold promise for improved
mechanical properties as a result of multiple operative deformation modes.
However, the use of many of these alloys in structural applications is limited
as a consequence of their poor aqueous corrosion resistance. Here we introduce
a new approach for significantly improving the passivation behavior of alloys
by tuning the chemical short-range order (CSRO) parameter. We show that the
addition of only 0.03 to 0.06 mole fraction of Al to a (FeCoNi)0.9Cr0.1 alloy
changed both the magnitude and sign of the Cr-Cr CSRO parameter resulting in
passivation behavior similar to 304L stainless steel containing twice the
amount of Cr. Our analysis is based on comparing electrochemical measures of
the kinetics of passive film formation with CSRO characterizations using
time-of-flight neutron scattering, cluster expansion methods, density
functional theory and Monte Carlo techniques. Our findings are interpreted
within the framework of a recently proposed percolation theory of passivation
that examines how selective dissolution of the non-passivating alloy components
and CSRO results in excellent passive films at reduced levels of the
passivating component. | 2403.00086v1 |
2020-02-18 | Anisotropic RKKY interactions mediated by $j=3/2$ quasiparticles in half-Heusler topological semimetal | We theoretically explore the RKKY interaction mediated by spin-3/2
quasiparticles in half-Heusler topological semimetals in quasi-two-dimensional
geometries. We find that while the Kohn-Luttinger terms gives rise to
generalized Heisenberg coupling of the form ${\cal H}_{\rm RKKY} \propto
{\sigma}_{1,i} {\cal I}_{ij} {\sigma}_{2,j}$ with a symmetric matrix ${\cal
I}_{ij}$, addition of small antisymmetric linear spin-orbit coupling term leads
to Dzyaloshinskii-Moriya (DM) coupling with an antisymmetric matrix ${\cal
I}'_{ij}$. We demonstrate that besides the oscillatory dependence on the
distance, all coupling strengths strongly depend on the relative orientation of
the two impurities with respect to the lattice. This yields a strongly
anisotropic behavior for ${\cal I}_{ij}$ such that by only rotating one
impurity around another at a constant distance, we can see further oscillations
of the RKKY couplings. This unprecedented effect is unique to our system which
combines spin-orbit coupling with strongly anisotropic Fermi surfaces. We
further find that all of the RKKY terms have two common features: a tetragonal
warping in their map of spatial variations, and a complex beating pattern.
Intriguingly, all these features survive in all dopings and we see them in both
electron- and hole-doped cases. In addition, due to the lower dimensionality
combined with the effects of different spin-orbit couplings, we see that only
one symmetric off-diagonal term, ${\cal I}_{xy}$ and two DM components ${\cal
I}'_{xz}$ and ${\cal I}'_{yz}$ are nonvanishing, while the remaining three
off-diagonal components are identically zero. This manifests another drastic
difference of RKKY interaction in half-Heusler topological semimetals compared
to the electronic systems with spin-1/2 effective description. | 2002.07736v1 |
2020-03-11 | High Thermoelectric Performance and Defect Energetics of Multi-pocketed Full-Heusler Compounds | We report first-principles density-functional study of electron-phonon
interactions and thermoelectric transport properties of full-Heusler compounds
Sr$_{2}$BiAu and Sr$_{2}$SbAu. Our results show that ultrahigh intrinsic bulk
thermoelectric performance across a wide range of temperatures is physically
possible and point to the presence of multiply degenerate and highly dispersive
carrier pockets as the key factor for achieving it. Sr$_{2}$BiAu, which
features ten energy-aligned low effective mass pockets (six along $\Gamma-X$
and four at $L$), is predicted to deliver $n$-type $zT=0.4-4.9$ at
$T=100-700$~K. Comparison with the previously investigated Ba$_{2}$BiAu
compound shows that the additional $L$-pockets in Sr$_{2}$BiAu significantly
increase its low-temperature power factor to a maximum value of
$12$~mW~m$^{-1}$~K$^{-2}$ near $T=300$~K. However, at high temperatures the
power factor of Sr$_{2}$BiAu drops below that of Ba$_{2}$BiAu because the $L$
states are heavier and subject to strong scattering by phonon deformation as
opposed to the lighter $\Gamma-X$ states that are limited by polar-optical
scattering. Sr$_{2}$SbAu is predicted to deliver lower $n$-type of $zT=3.4$ at
$T=750$~K due to appreciable misalignment between the $L$ and $\Gamma-X$
carrier pockets, generally heavier scattering, and slightly higher lattice
thermal conductivity. Soft acoustic modes, responsible for low lattice thermal
conductivity, also increase vibrational entropies and high-temperature
stability of the Heusler compounds, suggesting that their experimental
synthesis may be feasible. The dominant intrinsic defects are found to be Au
vacancies, which drive the Fermi level towards the conduction band and work in
favor of $n$-doping. | 2003.05506v3 |
1999-03-23 | Magnetism and magnetic asphericity in NiFe alloys | We here study magnetic properties of Ni$_{x}$Fe$_{1-x}$ using Augmented space
recursion technique coupled with tight-binding linearized muffin tin orbital
method. Also the spectral properties of this alloy has been studied here. | 9903348v1 |
2001-03-14 | Phase-Field Formulation for Quantitative Modeling of Alloy Solidification | A phase-field formulation is introduced to simulate quantitatively
microstructural pattern formation in alloys. The thin-interface limit of this
formulation yields a much less stringent restriction on the choice of interface
thickness than previous formulations and permits to eliminate non-equilibrium
effects at the interface. Dendrite growth simulations with vanishing solid
diffusivity show that both the interface evolution and the solute profile in
the solid are well resolved. | 0103289v1 |
2001-07-09 | Formation of magnetic characteristics and hyperfine fields in metal-metalloid alloys | This work deals with the analysis of peculiarities of formation of the
hyperfine fields (HFF) at the Fe nuclei in disordered alloys metal- metalloid
using the "first-principles" calculations. Some phenomenological models and
justification of their usage for the interpretation of the experimental HFF
distributions are discussed. | 0107181v1 |
2002-01-11 | More on``Atomic motions in the crystalline Al$_{50}$Cu$_{35}$Ni$_{15}$ alloy'' | We refute recent claims that ultrafast atomic jumps as observed in
quasicrystals (QC) could be called phasons in many crystalline alloys by
pointing out that there is a genuine conceptual difference between the hopping
dynamics in an imperfect crystal containing a substantial number of vacancies,
and the hopping dynamics due to phason motion in QC. | 0201172v1 |
2004-02-05 | The thermal waves induced by ultra-short laser pulses in n-dimensional space-time | In this paper the heat waves, induced by ultra-short laser pulses are
considered. The hyperbolic heat transport in n-dimensional space-time is
formulated and solved. It is shown that only for n-odd for heat waves the
Huygens principle is fulfilled. The heat transport experiment for Cu_3Au alloy
is considered. Key words: Hyperbolic heat transport; Thermal waves; Huygens
principle; Cu_3Au alloy. | 0402159v1 |
2006-06-23 | Effect of Short-ranged Order on the electronic structure and optical properties of the CuZn alloy : an augmented space approach | We report here a study of the effect of short-ranged ordering on the
electronic structure and optical properties of CuZn alloys. We shall use the
augmented space recursion technique developed by us in conjunction with the
tight-binding linear muffin tin orbitals basis. | 0606623v1 |
2006-08-31 | Observation of Phasons in Magnetic Shape Memory Alloy Ni2MnGa | An inelastic neutron scattering study of the lattice dynamics of the
martensite phase of the ferromagnetic shape memory alloy, Ni2MnGa, reveals the
presence of well-defined phasons associated with the charge density wave (CDW)
resulting from Fermi surface (FS) nesting. The velocity and the temperature
dependence of the phason are measured as well as the anomalous [110]-TA2
phonon. | 0608729v1 |
2006-11-27 | Formation of ferromagnetic bulk amorphous Fe40Ni40P14B6 alloys | Ferromagnetic bulk amorphous Fe40Ni40P14B6 alloy rods with a diameter of 1.2
mm can be prepared by means of a rapid quenching technique. If a fluxing
technique is also used, amorphous rods with a diameter as large as 2.5 mm can
be synthesized. The critical cooling rate Rc for the glass formation
Fe40Ni40P14B6 is estimated to be on the order of 100 K.s-1 | 0611661v1 |
1998-06-02 | Cross-projective representations of pairs of anticommutative algebras, alloys and finite-dimensional irreducible representations of some infinite-dimensional Lie algebras | The article is devoted to some ``strange'' phenomena of representation theory
and their interrelations. Cross-projective representations of pairs of
anticommutative algebras, alloys, their universal envelopping Lie algebras and
their representations, quaternary algebras and their alloyability are
discussed. Considered examples allow to conclude that new representations have
some intriguing features (continuous moduli of finite-dimensional irreducible
representations, sophisticated Clebsch-Gordan coefficient calculus, etc.). | 9806005v1 |
1998-11-21 | Approximate Models of Dynamic Thermoviscoelasticity Describing Shape-Memory-Alloy Phase Transitions | We consider problems of dynamic viscoelasticity taking into account the
coupling of elastic and thermal fields. Efficient approximate models are
developed and computational results on thermomechanical behaviour of
shape-memory-alloy structures are presented. | 9811125v1 |
1996-07-02 | Point-charge electrostatics in disordered alloys | A simple analytic model of point-ion electrostatics has been previously
proposed in which the magnitude of the net charge q_i on each atom in an
ordered or random alloy depends linearly on the number N_i^(1) of unlike
neighbors in its first coordination shell. Point charges extracted from recent
large supercell (256-432 atom) local density approximation (LDA) calculations
of Cu-Zn random alloys now enable an assessment of the physical validity and
accuracy of the simple model. We find that this model accurately describes (i)
the trends in q_i vs. N_i^(1), particularly for fcc alloys, (ii) the magnitudes
of total electrostatic energies in random alloys, (iii) the relationships
between constant-occupation-averaged charges <q_i> and Coulomb shifts <V_i>
(i.e., the average over all sites occupied by either $A$ or $B$ atoms) in the
random alloy, and (iv) the linear relation between the site charge q_i and the
constant- charge-averaged Coulomb shift (i.e., the average over all sites with
the same charge) for fcc alloys. However, for bcc alloys the fluctuations
predicted by the model in the q_i vs. V_i relation exceed those found in the
LDA supercell calculations. We find that (a) the fluctuations present in the
model have a vanishing contribution to the electrostatic energy. (b)
Generalizing the model to include a dependence of the charge on the atoms in
the first three (two) shells in bcc (fcc) - rather than the first shell only -
removes the fluctuations, in complete agreement with the LDA data. We also
demonstrate an efficient way to extract charge transfer parameters of the
generalized model from LDA calculations on small unit cells. | 9607001v1 |
2003-08-19 | Study of Friction at the Mesoscale using Nitinol Shape Memory Alloy | Elastic and dissipative properties of a NiTi shape memory alloy have been
studied in both the martensite and austenite phases, using the free-decay of a
vibrating Nitinol wire. The influence of air was estimated from a martensite
measurement in vacuum. | 0308077v1 |
2007-04-24 | Slowly rotating pulsars | In the present work we investigate one possible variation on the usual static
pulsars: the inclusion of rotation. We use a formalism proposed by Hartle and
Thorne to calculate the properties of rotating pulsars with all possible
compositions. All calculations were performed for zero temperature and also for
fixed entropy equations of state. | 0704.3233v1 |
2007-04-24 | Electrically charged pulsars | n the present work we investigate one possible variation on the usual
electrically neutral pulsars: the inclusion of electric charge. We study the
effect of electric charge in pulsars assuming that the charge distribution is
proportional to the energy density. All calculations were performed for zero
temperature and fixed entropy equations of state. | 0704.3245v1 |
2007-05-01 | Direct Measurement of 2D and 3D Interprecipitate Distance Distributions from Atom-Probe Tomographic Reconstructions | Edge-to-edge interprecipitate distance distributions are critical for
predicting precipitation strengthening of alloys and other physical phenomena.
A method to calculate this 3D distance and the 2D interplanar distance from
atom-probe tomographic data is presented. It is applied to nanometer-sized
Cu-rich precipitates in an Fe-1.7 at.% Cu alloy. Experimental interprecipitate
distance distributions are discussed. | 0705.0052v1 |
2008-05-04 | Observation of Relaxation Phenomena in Thermophysical Properties of Alloys and Metals | In this paper the proposal for the study of the second sound in medium is
presented. The master equation is derived and its solution is obtained, The
properties of alloys with very long relaxation times, as the examples for the
proposed study are discussed | 0805.0436v1 |
2010-04-08 | Anderson localization for a multi-particle model with alloy-type external potential | We establish exponential localization for a multi-particle Anderson model in
a Euclidean space of an arbitrary dimension, in presence of a non-trivial
short-range interaction and an alloy-type random external potential.
Specifically, we prove that all eigenfunctions with eigenvalues near the lower
edge of the spectrum decay exponentially. | 1004.1300v1 |
2010-07-22 | Multi-particle dynamical localization in a continuous Anderson model with an alloy-type potential | This paper is a complement to our earlier work \cite{BCSS10b}. With the help
of the multi-scale analysis, we derive, from estimates obtained in
\cite{BCSS10b}, dynamical localization for a multi-particle Anderson model in a
Euclidean space $\D{R}^{d}$, $d\geq 1$, with a short-range interaction, subject
to a random alloy-type potential. | 1007.3815v1 |
2012-04-06 | Tin Pest: A Forgotten Issue in the Field of Applied Superconductivity? | Shear ruptures of Cu samples soldered with Sn96Ag4 and Sn60Pb40 alloys have
been measured at 300 K and 77 K. An average degradation of about 37 % of the
shear rupture strength has been observed at cold for samples soldered with the
lead-free alloy. This effect can be attributed to the tin pest. | 1204.1443v1 |
2015-12-17 | Interaction of 3D mesostructures composed of Pd-Ni alloy nanowires with low-temperature oxygen plasma | In this article we report about active interaction of volumetric mesoscopic
structures composed of PdNi alloy nanowires with low temperature nonequilibrium
oxygen plasma. Object of our study is fine 3D meso-structures, which were
fabricated via a self-organization of nanowires growing during the
electrodeposition of metals on a template. | 1512.05590v1 |
2016-04-13 | Radiation-induced segregation in dilute Re-W solid solutions | The occurrence of segregation in highly dilute alloys under irradiation is an
unusual phenomenon that has so far eluded theoretical explanation. Using ab
initio calculations, we are able to explain the origin of radiation-induced
rhenium segregation in dilute tungsten-rhenium alloys. | 1604.03746v1 |
2018-06-07 | On Spectral and Energetic Characteristics of Erosional Plasma on the Basis of a Tin Alloy and of "Jumping Fireballs" | In this paper we present the results of the spectral studies of erosive
discharge with tin alloy electrodes and of the generated JFs, and
experimentally determine internal energy of JFs using the calorimetric
technique. | 1806.02910v1 |
2020-01-03 | Effect of N, C and B interstitials on the structural and magnetic properties of alloys with Cu$_3$Au-structure | High-throughput density functional calculations are used to investigate the
effect of interstitial B, C and N atoms on 21 alloys reported to crystallize in
the cubic Cu$_3$Au structure. It is shown that the interstitials can have a
significant impact on the magneto-crystalline anisotropy energy (MAE), the
thermodynamic stability and the magnetic ground state structure, making these
alloys interesting for hard magnetic, magnetocaloric and other applications.
For 29 alloy/interstitial combinations the formation of stable alloys with
interstitial concentrations above 5\% is expected. In Ni$_3$Mn interstitial N
induces a tetragonal distortion with substantial uniaxial MAE for realistic N
concentrations. Mn$_3X$N$_x$ ($X$=Rh, Ir, Pt and Sb) are identified as alloys
with strong magneto-crystalline anisotropy. For Mn$_3$Ir we find a strong
enhancement of the MAE upon N alloying in the most stable collinear
ferrimagnetic state as well as in the non-collinear magnetic ground state.
Mn$_3$Ir and Mn$_3$IrN show also interesting topological transport properties.
The effect of N concentration and strain on the magnetic properties are
discussed. Further, the huge impact of N on the MAE of Mn$_3$Ir and a possible
impact of interstitial N on amorphous Mn$_3$Ir, a material that is
indispensable in today's data storage devices, are discussed at hand of the
electronic structure. For Mn$_3$Sb, non-collinear, ferrimagnetic and
ferromagnetic states are very close in energy, making this material potentially
interesting for magnetocaloric applications. For the investigated Mn alloys and
competing phases, the determination of the magnetic ground state is essential
for a reliable prediction of the phase stability. | 2001.00959v1 |
2020-01-16 | A dual-phase cobalt alloy with a triple yielding phenomenon under compression test | The compressive mechanical behavior of a dual phase cobalt alloy
(Al$_{14}$Co$_{41}$Cr$_{16}$Fe$_{11}$Ni$_{18}$) is reported in this
communication. An uncommon triple yielding phenomenon is observed in the
as-cast condition. Microstructural studies suggest that the observed behavior
may be due to a stress/strain-induced martensitic phase transformation. | 2001.05695v2 |
2017-04-11 | Atomistic simulations of dislocation/precipitation interactions in Mg-Al alloys and implications for precipitation hardening | Atomistic simulations were carried out to analyze the interaction between $<
a>$ basal dislocations and precipitates in Mg-Al alloys and the associated
strengthening mechanisms. | 1704.03487v2 |
2018-07-05 | Variational principle for shape memory alloys | The quasistatic problem of shape memory alloys is reviewed within the
phenomenological mechanics of solids without microphysics analysis. The
assumption is that the temperature variation rate is small. Reissner's type of
generalized variational principle is presented, and its mathematical
justification is given for three-dimensional bodies made of shape memory
materials. | 1807.03153v1 |
2018-07-25 | Shape memory alloys as gradient-polyconvex materials | We show existence of an energetic solution to a model of shape memory alloys
in which the elastic energy is described by means of a gradient-polyconvex
functional. This allows us to show existence of a solution based on weak
continuity of nonlinear minors of deformation gradients in Sobolev spaces.
Resulting deformations are orientation-preserving and injective everywhere in a
domain representing the specimen. | 1807.09855v1 |
2019-07-17 | Seeking high temperature superconductors in ambient from exemplary beryllium-based alloys | With the help of the McMillan formula and virtual crystal model, we predict
$T_c$ may exceed 34 K in a beryllium-based alloy with a specific composition,
reminiscent of $T_c$ = 35 K in the first cuprate superconductor. This may
similarly inspire research efforts to seek high temperature superconductors in
ambient. | 1907.07597v2 |
2019-06-13 | What Stabilizes the Intermediate Structure of an Amorphous Alloy? | We present the results of simulation studies of a model binary
metal-metalloid alloy in which we characterize and explain the local
coordination structure, the intermediate structure associated with the packing
of these coordination polyhedra and the thermal stability of the various
structural elements of this model amorphous solid. | 1906.06175v1 |
2022-03-19 | The effect of isovalent doping on the electronic band structure of group IV semiconductors | The band gap engineering of group IV semiconductors has not been well
explored theoretically and experimentally, except for SiGe. Recently, GeSn has
attracted much attention due to the possibility of obtaining a direct band gap
in this alloy, thereby making it suitable for light emitters. Other group IV
alloys may also potentially exhibit material properties useful for device
applications, expanding the space for band gap engineering in group IV. In this
work the electronic band structure of all group IV semiconductor alloys is
investigated. Twelve possible A:B alloys, where A is a semiconducting host (A =
C, Si, and Ge) and B is an isovalent dopant (B = C, Si, Ge, Sn, and Pb), were
studied in the dilute regime (0.8%) of the isovalent dopant in the entire
Brillouin zone (BZ), and the chemical trends in the evolution of their
electronic band structure were carefully analyzed. Density functional theory
with state-of-the-art methods such as meta-GGA functionals and a spectral
weight approach to band unfolding from large supercells was used to obtain
dopant-related changes in the band structure, in particular the direct band gap
at the {\Gamma} point and indirect band gaps at the L(X) points of the BZ.
Analysis of contributions from geometry distortion and electronic interaction
was also performed. Moreover, the obtained results are discussed in the context
of obtaining a direct fundamental gap in Ge:B (B = C, Sn, and Pb) alloys, and
intermediate band formation in C:B (B = Sn and Pb) and Ge:C. An increase in
localization effects is also observed: a strong hole localization for alloys
diluted with a dopant of a larger covalent radius and a strong electron
localization for alloys with a dopant of smaller radius. Finally, it is shown
that alloying Si and Ge with other elements from group IV is a promising way to
enhance the functionality of group IV semiconductors. | 2203.10361v1 |
2019-05-10 | First-principles study of the effect of compressive strain on oxygen adsorption in PdNiCu-alloy-core@PdIr-alloy-shell catalysts | A palladium-based (Pd-based) core@shell catalyst can be modified to achieve
the desired oxygen adsorption properties by selecting an appropriate core
composition, surface alloying, and compressive strain. Herein, we present the
effects of compressive strain, core composition, and surface alloying in
Pd3Ni@PdIr(111), Pd3CuNi@PdIr(111), and Pd3Cu@PdIr(111) alloy-core@alloy-shell
catalysts on dioxygen adsorption. Using experimental lattice parameters for the
unstrained catalysts, -1% to -5%, the strain was systematically introduced. The
calculated dioxygen-adsorption energies for the surface Pd and surface Ir atoms
reveal that the Pd3CuNi@PdIr catalyst has the lowest dioxygen-adsorption energy
at a given compressive strain. Bader charge calculations show that the
Pd3CuNi@PdIr catalyst surface is the most charge depleted. The d-band model
displays an intermediate d-band center downshift for the surface Pd atoms, and
the highest downshift for the surface Ir atoms. Due to synergism between charge
depletion, the d-band center shift, and the surface alloy effect, the
Pd3CuNi@PdIr catalyst has the lowest dioxygen-adsorption energy. The
relationship between the experimentally obtained catalyst-surface mass activity
and the theoretically calculated d-band center of the surface Pd and the
surface Ir is volcano shaped, with the Pd3CuNi@PdIr catalyst at the apex of the
volcano. The catalytic activities of these catalysts were observed to follow
the order: Pd3CuNi@PdIr > Pd3Cu@PdIr > Pd3Ni@PdIr. This work sheds light on the
importance of ligand and strain effects, as well as surface alloying for the
fine-tuning of alloy-core@alloy-shell-catalysts during the rational design of
catalysts from first principles. | 1905.03958v1 |
2020-07-03 | Enhanced creep performance in a polycrystalline superalloy driven by atomic-scale phase transformation along planar faults | Predicting the mechanical failure of parts in service requires understanding
their deformation behavior, and associated dynamic microstructural evolution up
to the near-atomic scale. Solutes are known to interact with defects generated
by plastic deformation, thereby affecting their displacement throughout the
microstructure and hence the material mechanical response to solicitation. This
effect is studied here in a polycrystalline Ni-based superalloy with two
different Nb contents that lead to a significant change in their creep
lifetime. Creep testing at 750C and 600 MPa shows that the high-Nb alloy
performs better in terms of creep strain rate. Considering the similar initial
microstructures, the difference in mechanical behavior is attributed to a phase
transformation that occurs along planar faults, controlled by the different
types of stacking faults and alloy composition. Electron channeling contrast
imaging reveals the presence of stacking faults in both alloys. Microtwinning
is observed only in the low-Nb alloy, rationalizing in part the higher creep
strain rate. In the high-Nb alloy, atom probe tomography evidences two
different types of stacking faults based on their partitioning behavior.
Superlattice intrinsic stacking faults (SISF) were found enriched in Nb, Co, Cr
and Mo while only Nb and Co was segregated at superlattice extrinsic stacking
faults. Based on their composition, a local phase transformation occurring
along the faults is suggested, resulting in slower creep strain rate in the
high-Nb alloy. In comparison, mainly SISF enriched in Co, Cr, Nb and Mo were
found in the low-Nb alloy. Following the results presented here, and those
available in the literature, an atomic-scale driven alloy design approach that
controls and promotes local phase transformation along planar faults at 750C is
proposed, aiming to design superalloys with enhanced creep resistance. | 2007.01676v2 |
2021-11-24 | Effects of structure and temperature on the nature of excitons in the Mo0.6W0.4S2 alloys | We have studied the nature of excitons in the transition metal dichalcogenide
alloy Mo0.6W0.4 S2, compared to pure MoS2 and WS2 grown by atomic layer
deposition (ALD). For this, optical absorption/transmission spectroscopy and
time-dependent density functional theory (TDDFT) were used. Effects of
temperature on the A and B exciton peak energies and linewidths in the optical
transmission spectra were compared between the alloy and pure MoS2 and WS2. On
increasing the temperature from 25 K to 293 K the energy of the A and B exciton
peaks decreases, while their linewidth increases due to exciton-phonon
interactions. The exciton-phonon interactions in the alloy are closer to those
for MoS2 than WS2. This suggests that the exciton wave functions in the alloy
have a larger amplitude on Mo atoms than on W atoms. The experimental
absorption spectra could be reproduced by TDDFT calculations. Interestingly,
for the alloy the Mo and W atoms had to be distributed over all layers.
Conversely, we could not reproduce the experimental alloy spectrum by
calculations on a structure with alternating layers, in which every other layer
contains only Mo atoms and the layers in between also W atoms. For the latter
atomic arrangement, the TDDFT calculations yielded an additional optical
absorption peak that could be due to excitons with some charge transfer
character. From these results we conclude that ALD yields an alloy in which Mo
and W atoms are distributed uniformly among all layers. | 2111.12376v1 |
2022-04-28 | Anti-microbial properties of a multi-component alloy | High traffic touch surfaces such as doorknobs, countertops, and handrails can
be transmission points for the spread of pathogens, emphasizing the need to
develop materials that actively self-sanitize. Metals are frequently used for
these surfaces due to their durability, but many metals also possess
antimicrobial properties which function through a variety of mechanisms. This
work investigates metallic alloys comprised of several bioactive metals with
the target of achieving broad-spectrum, rapid bioactivity through synergistic
activity. An entropy-motivated stabilization paradigm is proposed to prepare
scalable alloys of copper, silver, nickel and cobalt. Using combinatorial
sputtering, thin-film alloys were prepared on 100 mm wafers with 50%
compositional grading of each element across the wafer. The films were then
annealed and investigated for alloy stability. Bioactivity testing was
performed on both the as-grown alloys and the annealed films using four
microorganisms -- Phi6, MS2, Bacillus subtilis and Escherichia coli -- as
surrogates for human viral and bacterial pathogens. Testing showed that after
30 s of contact with some of the test alloys, Phi6, an enveloped,
single-stranded RNA bacteriophage that serves as a SARS-CoV 2 surrogate, was
reduced up to 6.9 orders of magnitude (>99.9999%). Additionally, the
non-enveloped, double-stranded DNA bacteriophage MS2, and the Gram-negative E.
coli and Gram-positive B. subtilis bacterial strains showed a 5.0, 6.4, and 5.7
log reduction in activity after 30, 20 and 10 minutes, respectively.
Bioactivity in the alloy samples showed a strong dependence on the composition,
with the log reduction scaling directly with the Cu content. Concentration of
Cu by phase separation after annealing improved activity in some of the
samples. The results motivate a variety of themes which can be leveraged to
design ideal bioactive surfaces. | 2205.00886v1 |
2023-03-29 | Comprehensive ab initio study of effects of alloying elements on generalized stacking fault energies of Ni and Ni$_3$Al | Excellent high-temperature mechanical properties of Ni-based single crystal
superalloys (NSCSs) are attributed to the yield strength anomaly of Ni$_{3}$Al
that is intimately related to generalized stacking fault energies (GSFEs).
Therefore, clarifying the effects of alloying elements on the GSFEs is of great
significance for alloys design. Here, by means of ab initio density functional
theory calculations, we systematically calculated the GSFEs of different slip
systems of Ni and Ni$_{3}$Al without and with alloying elements using the alias
shear method. We obtained that for Ni, except for magnetic elements Mn, Fe, and
Co, most of alloying elements decrease the unstable stacking fault energy
($\gamma_{usf}$) of the $[01\bar{1}](111)$ and $[11\bar{2}](111)$ slip systems
and also decrease the stable stacking fault energy ($\gamma_{sf}$) of the
$[11\bar{2}](111)$ slip system. For Ni$_{3}$Al, most of alloying elements in
groups IIIB-VIIB show a strong Al site preference. Except for Mn and Fe, the
elements in groups VB-VIIB and the first column of group VIII increase the
values of $\gamma_{usf}$ of different slip systems of Ni$_{3}$Al. On the other
hand, the elements in groups IIIB-VIIB also increase the value of
$\gamma_{sf}$. We found that Re is an excellent strengthening alloying element
that significantly increases the slip barrier of the tailing slip process for
Ni, and also enhances the slip barrier of the leading slip process of three
slip systems for Ni$_{3}$Al. W and Mo exhibit similar effects as Re. We
predicted that Os, Ru, and Ir are good strengthening alloying elements as well,
since they show the strengthening effects on both the leading and tailing slip
process for Ni and Ni$_{3}$Al. | 2303.16379v1 |
2023-11-16 | Tailoring hot-carrier distributions of plasmonic nanostructures through surface alloying | Alloyed metal nanoparticles are a promising platform for plasmonically
enabled hot-carrier generation, which can be used to drive photochemical
reactions. Although the non-plasmonic component in these systems has been
investigated for its potential to enhance catalytic activity, its capacity to
affect the photochemical process favorably has been underexplored by
comparison. Here, we study the impact of surface alloy species and
concentration on hot-carrier generation in Ag nanoparticles. By
first-principles simulations, we photoexcite the localized surface plasmon,
allow it to dephase, and calculate spatially and energetically resolved
hot-carrier distributions. We show that the presence of non-noble species in
the topmost surface layer drastically enhances hot-hole generation at the
surface at the expense of hot-hole generation in the bulk, due to the
additional d-type states that are introduced to the surface. The energy of the
generated holes can be tuned by choice of the alloyant, with systematic trends
across the d-band block. Already low surface alloy concentrations have a large
impact, with a saturation of the enhancement effect typically close to 75% of a
monolayer. Hot-electron generation at the surface is hindered slightly by
alloying but here an judicious choice of the alloy composition allows one to
strike a balance between hot electrons and holes. In this context, it is also
important to consider that increasing the alloy concentration broadens the
localized surface plasmon resonance, and thus decreases hot-carrier generation
overall. Our work underscores the promise of utilizing multicomponent
nanoparticles to achieve enhanced control over plasmonic catalysis, and
provides guidelines for how hot-carrier distributions can be tailored by
designing the electronic structure of the surface through alloying. | 2311.09996v1 |
2024-01-31 | Effect of annealing on the corrosion-fatigue strength and hot salt corrosion resistance of fine-grained titanium near-α alloy Ti-5Al-2V obtained by Rotary Swaging | The corrosion-fatigue strength in 3% aqueous NaCl solution and the resistance
against hot salt corrosion (HSC) of the fine-grained near-a alloy Ti-5Al-2V
(Russian analog of Grade 9 titanium alloy with increased aluminum content) has
been studied. The properties of the Ti-5Al-2V alloy in the coarse-grained
state, in the fine-grained after cold Rotary Swaging (RS), in partly
recrystallized state, and in fully recrystallized one have been investigated.
The mechanical properties of the alloy were characterized using compression
tests and microhardness measurements. The effects of RS and of the annealing
temperature and time on the character of corrosion destruction of the surface
and on the composition of the products of the HSC were studied. RS was shown to
result in an increase in the depth of the intergranular corrosion defects while
the recrystallization annealing promotes the increasing of the corrosion
resistance of the Ti-5Al-2V titanium alloy. The parameters of the Basquin
equation for the corrosion-fatigue curves for the near-a Ti-5Al-2V alloy in the
coarse-grained state, in the severely strained one, and after recrystallization
annealing were determined for the first time. An effect of nonmonotonous
dependencies of the slopes of the corrosion-fatigue curves for the strained
near-a Ti-5Al-2V alloy on the recrystallization annealing temperature has been
observed. | 2401.17659v1 |
2024-02-23 | Weak Reproductive Solutions for a Convection-Diffusion Model Describing a Binary Alloy Solidification Processes | We study the existence of reproductive weak solutions for a system of
equations describing a solidification process of a binary alloy confined into a
bounded and regular domain in $\mathbb{R}^3$, having mixed boundary conditions. | 2402.15221v1 |
2005-09-23 | Semiclassical Theory of Chaotic Conductors | We calculate the Landauer conductance through chaotic ballistic devices in
the semiclassical limit, to all orders in the inverse number of scattering
channels without and with a magnetic field. Families of pairs of
entrance-to-exit trajectories contribute, similarly to the pairs of periodic
orbits making up the small-time expansion of the spectral form factor of
chaotic dynamics. As a clue to the exact result we find that close
self-encounters slightly hinder the escape of trajectories into leads. | 0509598v1 |
2005-11-11 | Semiclassical Prediction for Shot Noise in Chaotic Cavities | We show that in clean chaotic cavities the power of shot noise takes a
universal form. Our predictions go beyond previous results from random-matrix
theory, in covering the experimentally relevant case of few channels. Following
a semiclassical approach we evaluate the contributions of quadruplets of
classical trajectories to shot noise. Our approach can be extended to a variety
of transport phenomena as illustrated for the crossover between symmetry
classes in the presence of a weak magnetic field. | 0511292v1 |
2006-10-20 | Semiclassical Approach to Chaotic Quantum Transport | We describe a semiclassical method to calculate universal transport
properties of chaotic cavities. While the energy-averaged conductance turns out
governed by pairs of entrance-to-exit trajectories, the conductance variance,
shot noise and other related quantities require trajectory quadruplets; simple
diagrammatic rules allow to find the contributions of these pairs and
quadruplets. Both pure symmetry classes and the crossover due to an external
magnetic field are considered. | 0610560v1 |
1997-02-05 | Matters of Gravity, the newsletter of the APS TG on gravitation | Contents:
News:
April 1997 Joint APS/AAPT Meeting, by Beverly Berger
TGG News, by Jim Isenberg
Report from NSF, by David Berley
We hear that..., by Jorge Pullin
Research briefs:
GR in GPS, by Neil Ashby
What happens near the innermost stable circular orbit? by Doug Eardley
Conference reports:
Journees Relativistes 96, by D. Brill, M. Heusler, G. Lavrelashvili
TAMA Workshop, by Peter Saulson
Midwest gravity meeting, by Comer Duncan
OMNI-1 Workshop by N.S. Magalhaes, W. F. Velloso Jr and O.D. Aguiar
Chandra Symposium, by Robert Wald
Penn State Meeting, by Lee Smolin
Aspen Winter Conference, by Syd Meshkov | 9702010v1 |
1997-07-29 | Rotating solitons and non-rotating, non-static black holes | It is shown that the non-Abelian black hole solutions have stationary
generalizations which are parameterized by their angular momentum and electric
Yang-Mills charge. In particular, there exists a non-static class of stationary
black holes with vanishing angular momentum. It is also argued that the
particle-like Bartnik-McKinnon solutions admit slowly rotating, globally
regular excitations. In agreement with the non-Abelian version of the staticity
theorem, these non-static soliton excitations carry electric charge, although
their non-rotating limit is neutral. | 9707057v1 |
2000-10-19 | Self-adjoint wave equations for dynamical perturbations of self-gravitating fields | It is shown that the dynamical evolution of linear perturbations on a static
space-time is governed by a constrained wave equation for the extrinsic
curvature tensor. The spatial part of the wave operator is manifestly elliptic
and self-adjoint. In contrast to metric formulations, the curvature-based
approach to gravitational perturbation theory generalizes in a natural way to
self-gravitating matter fields. It is also demonstrated how to obtain symmetric
pulsation equations for self-gravitating non-Abelian gauge fields, Higgs fields
and perfect fluids. For vacuum fluctuations on a vacuum space-time, the
Regge-Wheeler and Zerilli equations are rederived. | 0010067v1 |
2006-07-30 | Semiclassical Theory for Parametric Correlation of Energy Levels | Parametric energy-level correlation describes the response of the
energy-level statistics to an external parameter such as the magnetic field.
Using semiclassical periodic-orbit theory for a chaotic system, we evaluate the
parametric energy-level correlation depending on the magnetic field difference.
The small-time expansion of the spectral form factor $K(\tau)$ is shown to be
in agreement with the prediction of parameter dependent random-matrix theory to
all orders in $\tau$. | 0607070v1 |
2006-10-20 | Periodic-Orbit Theory of Level Correlations | We present a semiclassical explanation of the so-called
Bohigas-Giannoni-Schmit conjecture which asserts universality of spectral
fluctuations in chaotic dynamics. We work with a generating function whose
semiclassical limit is determined by quadruplets of sets of periodic orbits.
The asymptotic expansions of both the non-oscillatory and the oscillatory part
of the universal spectral correlator are obtained. Borel summation of the
series reproduces the exact correlator of random-matrix theory. | 0610053v1 |
2007-07-11 | Interface effects at a half-metal/ferroelectric junction | Magnetoelectric effects are investigated ab-initio at the interface between
half-metallic and ferroelectric prototypes: Heusler Co$_2$MnSi and perovskite
BaTiO$_3$. For the Co-termination ferroelectricity develops in BaTiO$_3$ down
to nanometer thicknesses, whereas for the MnSi-termination a paraelectric and a
ferroelectric state energetically compete, calling for a full experimental
control over the junction atomic configuration whenever a ferroelectric barrier
is needed. Switch of the electric polarization largely affects magnetism in
Co$_2$MnSi, with magnetoelectric coupling due to electronic hybridization at
the MnSi termination and to structural effects at the Co-termination.
Half-metallicity is lost at the interface, but recovered already in the
subsurface layer. | 0707.1665v1 |
2007-11-15 | Non-quasiparticle states in Co$_2$MnSi evidenced through magnetic tunnel junction spectroscopy measurements | We investigate the effects of electronic correlations in the full-Heusler
Co$_2$MnSi, by combining a theoretical analysis of the spin-resolved density of
states with tunneling-conductance spectroscopy measurements using Co$_2$MnSi as
electrode. Both experimental and theoretical results confirm the existence of
so-called non-quasiparticle states and their crucial contribution to the
finite-temperature spin polarisation in this material. | 0711.2476v2 |
2009-08-21 | Nonzero macroscopic magnetization in half-metallic antiferromagnets at finite temperatures | Combining density-functional theory calculations with many-body
Green's-function technique, we reveal that the macroscopic magnetization in
half-metallic antiferromagnets does not vanish at finite temperature as for the
T=0 limit. This anomalous behavior stems from the inequivalent magnetic
sublattices which lead to different intrasublattice exchange interactions. As a
consequence, the spin fluctuations suppress the magnetic order of the
sublattices in a different way leading to a ferrimagnetic state at finite
temperatures. Computational results are presented for the half-metallic
antiferromagnetic CrMnZ (Z=P,As,Sb) semi-Heusler compounds. | 0908.3044v1 |
2010-04-28 | Triplet supercurrent due to spin-active zones in a Josephson junction | Motivated by a recent experiment evidencing triplet superconductivity in a
ferromagnetic Josephson junction with a Cu$_2$MnAl-Heusler barrier, we
construct a theoretical model accounting for this observation. The key
ingredients in our model which generate the triplet supercurrent are
\textit{spin-active zones}, characterised by an effective canted interface
magnetic moment. Using a numerical solution of the quasiclassical equations of
superconductivity with spin-active boundary conditions, we find qualitatively
very good agreement with the experimentally observed supercurrent. Further
experimental implications of the spin-active zones are discussed. | 1004.5124v2 |
2013-09-25 | Band structure calculations of Ti\raisebox{-.2ex}{\scriptsize 2}FeSn: a new half-metallic compound | Within the framework of density functional theory, the electronic structure
and magnetic properties have been studied for the
Ti\raisebox{-.2ex}{\scriptsize 2}FeSn full-Heusler compound. The ferromagnetic
state is found to be energetically more favorable than paramagnetic and
antiferromagnetic states. The spin-polarized results show that
Ti\raisebox{-.2ex}{\scriptsize 2}FeSn compound has half-metallic ferromagnetic
character with a total spin moment of $2 \mu_{B}$ and a band gap in the
minority spin channel of 0.489 eV, at the equilibrium lattice constant a=6.342
A. | 1309.6442v1 |
2013-09-25 | Magnetic and transport properties of Mn2CoAl oriented thin films | The structure, magnetic and transport properties of thin films of the Heusler
ferrimagnet Mn_{2}CoAl have been investigated for properties related to spin
gapless semiconductors. Oriented films were grown by molecular beam epitaxy on
GaAs substrates and the structure was found to transform from tetragonal to
cubic for increasing annealing temperature. The anomalous Hall resistivity is
found to be proportional to the square of the longitudinal resistivity and
magnetization expected for a topological Berry curvature origin. A delicate
balance of the spin-polarized carrier type when coupled with voltage
gate-tuning could significantly impact advanced electronic devices. | 1309.6660v1 |
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