Point defects in body-centred cubic Fe, Cr and concentrated random magnetic Fe-Cr are investigated using density functional theory and theory of elasticity. The volume of a substitutional Cr atom in ferromagnetic bcc Fe is approximately 18\% larger than the volume of a host Fe atom, whereas the volume of a substitutional Fe atom in antiferromagnetic bcc Cr is 5\% smaller than the volume of a host Cr atom. Elastic dipole $\boldsymbol{P}$ and relaxation volume $\boldsymbolΩ$ tensors of vacancies and self-interstitial atom (SIA) defects exhibit large fluctuations, with vacancies having negative and SIA large positive relaxation volumes. Dipole tensors of vacancies are nearly isotropic across the entire alloy composition range, with diagonal elements $P_{ii}$ decreasing as a function of Cr content. Fe-Fe and Fe-Cr SIA dumbbells are more anisotropic than Cr-Cr dumbbells. Fluctuations of elastic dipole tensors of SIA defects are primarily associated with the variable crystallographic orientations of the dumbbells. Statistical properties of tensors $\boldsymbol{P}$ and $\boldsymbolΩ$ are analysed using their principal invariants, suggesting that point defects differ significantly in alloys containing below and above 10\% at. Cr. The relaxation volume of a vacancy depends sensitively on whether it occupies a Fe or a Cr lattice site. A correlation between elastic relaxation volumes and magnetic moments of defects found in this study suggests that magnetism is a significant factor influencing elastic fields of defects in Fe-Cr alloys.
The vacancy concentration at finite temperatures is studied for a series of (CoCrFeMn)$_{1-x_\mathrm{Ni}}$Ni$_{x_\mathrm{Ni}}$ alloys by grand-canonical Monte-Carlo (MC) simulations. The vacancy formation energies are calculated from a classical interatomic potential and exhibit a distribution due to the different chemical environments of the vacated sites. In dilute alloys, this distribution features multiple discrete peaks, while concentrated alloys exhibit an unimodal distribution as there are many different chemical environments of similar vacancy formation energy. MC simulations using a n…
Why is EXAFS analysis for multicomponent metals so hard? Challenges and opportunities for measuring ordering in complex concentrated alloys using x-ray absorption spectroscopy
Grain boundaries amplify local chemical ordering in complex concentrated alloys
University of California, Irvine（加州大学尔湾分校）；Hopkins Extreme Materials Institute, Johns Hopkins University（约翰斯·霍普金斯大学霍普金斯极端材料研究所）；Johns Hopkins University（约翰斯·霍普金斯大学）
Whilst it has long been known that disorder profoundly affects transport properties, recent measurements on a series of solid solution 3d-transition metal alloys reveal two orders of magnitude variations in the residual resistivity. Using ab-initio methods, we demonstrate that, while the carrier density of all alloys is as high as in normal metals, the electron mean-free-path can vary from ~10 Å (strong scattering limit) to ~10$^3$ Å (weak scattering limit). Here, we delineate the underlying electron scattering mechanisms responsible for this disparate behavior. While spin dependent site-diago…
Enhanced Elevated-Temperature Strength in Refractory Complex Concentrated Alloys via Temperature-Induced Transition from Screw-to-Edge Dislocation Control
University of California, Davis（加州大学戴维斯分校）；Oak Ridge National Laboratory（橡树岭国家实验室）；University of Tennessee, Knoxville（田纳西大学诺克斯维尔分校）
Using ab initio calculations and special quasirandom structures, we have characterized the distribution of defect formation energy and migration barrier in Ni-based solid-solution alloys: Ni_{0.5}Co_{0.5}, Ni_{0.5}Fe_{0.5}, Ni_{0.8}Fe_{0.2} and Ni_{0.8}Cr_{0.2}. As defect formation energies depend sensitively on elemental chemical potential, we have developed a computationally efficient method for determining it which takes into account the global composition and local short-range order. We find that Fe has the biggest alloy effects for Ni among these four elements. Our results show that the d…
The ability to predict the composition- and temperature-dependent stability of refractory complex concentrated alloys (RCCAs) is vital to the design of high-temperature structural alloys. Here, we present a model based on first-principles calculations to predict the thermodynamic stability of multicomponent equimolar solid solutions in a high-throughput manner and apply it to screen over 20,000 compositions. We develop a database that contains pairwise mixing enthalpy of 17 refractory metals using density-functional theory (DFT)-based total energy calculations. To these, we fit thermodynamic s…
The harmonic Kanzaki-Krivoglaz-Khachaturyan model of strain-induced interaction is generalized to concentrated size-mismatched alloys and adapted to first-principles calculations. The configuration dependence of both Kanzaki forces and force constants is represented by real-space cluster expansions that can be constructed based on the calculated forces. The model is implemented for the fcc lattice and applied to Cu$_{1-x}$Au$_x$ and Fe$_{1-x}$Pt$_x$ alloys for concentrations $x=0.25$, 0.5, and 0.75. The asymmetry between the $3d$ and $5d$ elements leads to large quadratic terms in the occupati…
Defects and microstructural features spanning the atomic level to the microscale play deterministic roles in the expressed properties of materials. Yet studies of material evolution in response to environmental stimuli most often correlate resulting performance with one dominant microstructural feature only. Here, the dynamic evolution of swelling in a series of Ni-based concentrated solid solution alloys under high-temperature irradiation exposure is observed using continuous, in situ measurements of thermoelastic properties in bulk specimens. Unlike traditional evaluation techniques which ac…
Chemical short-range order controls deformation pathways in a complex concentrated alloy
Federal University of São Carlos（圣卡洛斯联邦大学）；Uppsala University（乌普萨拉大学）；Wallenberg Initiative Materials Science for Sustainability, Uppsala University（乌普萨拉大学瓦伦堡可持续发展材料科学倡议）；Massachusetts Institute of Technology（麻省理工学院）；The University of Manchester（曼彻斯特大学）；Baylor University（贝勒大学）；AF Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson AFB（赖特-帕特森空军基地材料与制造局空军研究实验室）
We report a detailed study of dc magnetization and ac susceptibility performed on the zero field cooled (ZFC) and field cooled (FC) state of polycrystalline AuFe(11%) alloy. The temperature variation of ZFC and FC dc magnetization at low fields show a distinct peak around Tf = 33 K, which indicates the cooperative freezing of the finite size spin clusters. A weak thermomagnetic irreversibility between ZFC and FC magnetization appears at a temperature Tir, which is slightly below Tf. The ZFC ac susceptibility shows a sharp cusp at Tf, which shifts towards higher temperatures with an increase in…
Effect of cold rolling strain on the microstructural evolution in equimolar MoNbTaTiZr refractory complex concentrated alloy: Comprehensive characterization
We present a proof-of-principle demonstration of a universal law for the element Mn, which greatly enhances vacancy diffusion through an anomalous Friedel Oscillation effect in a series of Ni-based concentrated solid-solution alloys, regardless of the type of atom involved. The antiferromagnetic element Mn possesses a unique half-filled 3d electron structure, creating split virtual bound states near the Fermi energy level and producing a large local magnetic moment after vacancy formation. The resultant electron spin oscillations reduce the number of electrons involved in charge density oscill…
Microstructure evolution during rapid solidification of hypoeutectic Al-Ag alloys near absolute stability
Colorado School of Mines（科罗拉多矿业学院）；Northeastern University（东北大学）；Lawrence Livermore National Laboratory（劳伦斯利弗莫尔国家实验室）；Los Alamos National Laboratory（洛斯阿拉莫斯国家实验室）
Quantum critical behavior has been associated with some of the most exotic emergent states of matter including high-temperature superconductivity. Much of the research into quantum critical point (QCP) physics has been hampered by the lack of model systems simple enough to be analyzed by theory. Here, we show that the concentrated solid solution fcc alloys, including the so-called high-entropy alloys, are ideal model systems to study the effects of chemical disorder on emergent properties near a quantum critical region. The face centered cubic (fcc) alloy NiCoCrx with x near 1 is found to be c…
Interfacial segregation and chemical short-range ordering influence the behavior of grain boundaries in complex concentrated alloys. In this study, we use atomistic modeling of a NbMoTaW refractory complex concentrated alloy to provide insight into the interplay between these two phenomena. Hybrid Monte Carlo and molecular dynamics simulations are performed on columnar grain models to identify equilibrium grain boundary structures. Our results reveal extended near-boundary segregation zones that are much larger than traditional segregation regions, which also exhibit chemical patterning that b…
Dislocations in single-phase concentrated random alloys, including high- entropy alloys (HEAs), repeatedly encounter pinning during glide, resulting in jerky dislocation motion. While solute-dislocation interaction is well understood in conventional alloys, the origin of individual pinning points in concentrated random alloys is a matter of debate. In this work, we investigate the origin of dislocation pinning in the CoCrFeMnNi HEA. In- situ transmission electron microscopy studies reveal wavy dislocation lines and a jagged glide motion under external loading, even though no segregation or clu…
Sluggish and Chemically-Biased Interstitial Diffusion in Concentrated Solid Solution Alloys: Mechanisms and Methods
City University of Hong Kong（香港城市大学）；City University of Hong Kong Shenzhen Research Institute（香港城市大学深圳研究院）；Hong Kong Institute for Advanced Study, City University of Hong Kong（香港城市大学香港高等研究院）
We present an ab initio theory of transport quantities of metallic ferromagnets developed in the framework of the fully relativistic tight-binding linear muffin-tin orbital method. The approach is based on the Kubo-Streda formula for the conductivity tensor, on the coherent potential approximation for random alloys, and on the concept of interatomic electron transport. The developed formalism is applied to pure 3d transition metals (Fe, Co, Ni) and to random Ni-based ferromagnetic alloys (Ni-Fe, Ni-Co, Ni-Mn). High values of the anisotropic magnetoresistance (AMR), found for Ni-rich alloys, ar…
In the present work, the aspects of magnetic and structural properties of Ni-Mn-Ga alloys are described in the framework of fist-principles approach and mapped into ternary composition diagrams. The stable atomic arrangement and magnetic alignment for compositions with cubic austenite and tetragonal martensite structures across phase diagrams are predicted. It is shown that Ni- and Ga-rich compositions possess the regular Heusler structure in contrast to Mn-rich compositions with inverse Heusler structure as favorable one. Compositions with unstable austenite structure are concentrated in the …
Edge Dislocation Mediated Anomalous Charge Transfer in Face Centered Cubic High Entropy Alloys
School of Materials Science and Technology, IIT-BHU（印度理工联合学院瓦拉纳西分校材料科学与技术学院）；National Centre for Computational Sciences, Oak Ridge National Laboratory（橡树岭国家实验室国家计算科学中心）；Department of Metallurgy and Materials Engineering, IIEST Shibpur（IIEST Shibpur冶金与材料工程系）
Effect of He ion irradiation on three model Fe(100-x)Cr(x) alloys (x =5.8, 10.75 and 15.15) was investigated with the conversion electron Mössbauer spectroscopy. The study of the alloys irradiated with 25 keV ions revealed that the strongest effect occured in the Fe(84.85)Cr(15.15) sample where an inversion of a short-range-order (SRO) parameter was found. Consequently, the investigation of the influence of the irradiation dose, D, was carried out on the chromium-most concentrated sample showing that the average hyperfine field, <B>, the average angle between the normal to the sample's surface…
The exceptional mechanical strengths of medium and high-entropy alloys have been attributed to hardening in random solid solutions. Here, we evidence non-random chemical mixings in CrCoNi alloys, resulting from short range ordering. A novel data-mining approach of electron nanodiffraction patterns enabled the study, which is assisted by neutron scattering, atom probe tomography, and diffraction simulation using first principles theory models. Results reveal two critical types of short range orders in nanoclusters that minimize the Cr and Cr nearest neighbors (L11) or segregate Cr on alternatin…
Intrinsic Electric Field Driven High Sensitive Photodetection in Alloy TMDC MoSSe
Institute for Basic Science (IBS)（韩国基础科学研究所）；Bose Institute（玻色研究所）；S.N. Bose National Centre for Basic Sciences（S.N. 玻色基础科学国家中心）；Indian Institute of Science Education and Research Kolkata（印度科学教育研究学院加尔各答分校）；Indian Institute of Technology Bhubaneswar（印度技术学院布巴内斯瓦尔分校）；Central University of Kashmir（克什米尔中央大学）
Magnetic high entropy alloys (HEAs) are a new category of high-performance magnetic materials, with multi-component concentrated compositions and complex multi-phase structures. Although there have been numerous reports of their interesting magnetic properties, there is very limited understanding about the interplay between their hierarchical multi-phase structures and their local magnetic structures. By employing high spatial resolution correlative magnetic, structural and chemical studies, we reveal the influence of a hierarchically decomposed B2 + A2 structure in an AlCo0.5Cr0.5FeNi HEA on …
This work extends our Object Kinetic Monte Carlo model for neutron irradiation-induced nanostructure evolution in Fe-C alloys to consider higher irradiation temperatures. The previous study concentrated on irradiation temperatures < 370 K. Here we study the evolution of vacancy and self-interstitial atom (SIA) cluster populations at the operational temperature of light water reactors, by simulating specific reference irradiation experiments. Following our previous study, the effect of carbon on radiation defect evolution can be described in terms of formation of immobile complexes with vacanci…
