With density functional theory, studied are the local magnetic moments in Fe-Al alloys depending on concentration and Fe nearest environment. At zero temperature, the system can be in different states: ferromagnetic, antiferromagnetic and spin-spiral waves (SSW) which has a minimum energy. Both SSW and negative moment of Fe atoms with many Al atoms around them agree with experiments. Magnetization curves taken from literature are analysed. Assumption on percolation character of size distribution of magnetic clusters describes well the experimental superparamagnetic behaviour above 150 K.
The unique optical and electronic properties of two-dimensional transition metal dichalcogenides (2D TMDs) make them promising materials for applications in (opto-)electronics, catalysis and more. Specifically, alloys of 2D TMDs have broad potential applications owing to their composition-controlled properties. Several important challenges remain regarding controllable and scalable fabrication of these alloys, such as achieving control over their atomic ordering (i.e. clustering or random mixing of the transition metal atoms within the 2D layers). In this work, atomic layer deposition (ALD) is…
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）
The transitional state of ordering in the long-period stacking ordered (LPSO) structure in Mg-Al-Gd alloy has been investigated by scanning transmission electron microscopy. It is found that the domains are the origin of observed transitional state. Three new patterns of clusters at this state can be fully explained by the superimposition of L12 type clusters located in different domains. The domains are further verified by different tilted views. Growth of preferred domains during ageing process will result in final ordered LPSO structure. Based on the idea of domains, less ordered LPSO struc…
The magnetic state of low temperature martensite phase in Co-substituted Ni-Mn-Sn-based ferromagnetic shape memory alloys (FSMAs) has been investigated, in view of numerous conflicting reports of occurrences of spin glass (SG), superparamagnetism (SPM) or long range anti-ferromagnetic (AF) ordering. Combination of dc magnetization, ac susceptibility and small angle neutron scattering (SANS) studies provide a clear evidence for AF order in martensitic phase of Ni45Co5Mn38Sn12 alloy and rule out SPM and SG orders. Identical studies on another alloy of close composition of Ni44Co6Mn40Sn10 point t…
We study the influence of the liquid-vapor surface on the crystallization kinetics of supercooled metal alloys. While a good glass former, $\rm{Cu}_{50}\rm{Zr}_{50}$, shows no evidence of surface enhancement of crystallization, $\rm{Ni}_{50}\rm{Al}_{50}$ exhibits an increased rate of crystallization due to heterogeneous nucleation at the free liquid surface. The difference in the compositional fluctuations at the interface is proposed as the explanation of the distinction between the two alloys. Specifically, we observe compositional ordering at the surface of $\rm{Ni}_{50}\rm{Al}_{50}$ while …
A central problem in multicomponent lattice systems is to systematically quantify multi-point ordering. Ordering in such systems is often described in terms of pairs, even though this is not sufficient when three-point and higher-order interactions are included in the Hamiltonian. Current models and parameters for multi-point ordering are often only applicable for very specific cases and/or require approximating a subset of correlated occupational variables on a lattice as being uncorrelated. In this work, a cluster order parameter (ClstOP) is introduced to systematically quantify arbitrary mu…
Tailoring the degree of structural disorder in Ge-Sb-Te alloys is important for the development of non-volatile phase-change memory and neuro-inspired computing. Upon crystallization from the amorphous phase, these alloys form a cubic rocksalt-like structure with a high content of intrinsic vacancies. Further thermal annealing results in a gradual structural transition towards a layered structure and an insulator-to-metal transition. In this work, we elucidate the atomic-level details of the structural transition in crystalline GeSb2Te4 by in situ high-resolution transmission electron microsco…
Why is EXAFS analysis for multicomponent metals so hard? Challenges and opportunities for measuring ordering in complex concentrated alloys using x-ray absorption spectroscopy
The development of long range order in nickel-chromium alloys is of great technological interest but the kinetics and mechanisms of the transformation are poorly understood. The present research utilizes a combined computational and experimental approach to elucidate the mechanism by which phosphorus accelerates the ordering rate of stoichiometric Ni_2Cr in Ni-Cr alloys. A series of Ni-33%Cr-x%P samples (in atomic percent) were fabricated with phosphorus concentrations, x = <0.005-0.1 at.% and aged between 373 and 470°C for times up to 3000 h. The first-principles modeling considers fcc Ni wit…
The physical properties of Fe2CoAl (FCA) Heusler alloy are systematically investigated using the first-principles calculations within generalized gradient approximation (GGA) and GGA+U. The influence of atomic ordering with respect to the Wyckoff sites on the phase stability, magnetism and half metallicity in both the conventional L21 and XA phases of FCA is focused in this study. Various possible hypothetical structures viz., L21, XA-I, and XA-II are prepared by altering atomic occupancies at their Wyckoff sites. At first, we have determined the stable phase of FCA considering various non-mag…
Achieving the compositionally ordered state in a substitutional alloy of two or more species can often be even critical for improving its functional properties. To produce a highly ordered alloy, a longtime high-temperature (up to T=1000 K) treatment of the alloy is typically necessary because of insufficient vacancy concentration (c_v) and their mobility. However, such processing affects the morphology and complicates the technology of functional alloys. We show theoretically that the ordering in the practically important FePt system (Fe_xPt_1-x with x being close to 0.5) is already achievabl…
The behavior of magnetic ordering and the KOndo effect in the alloys, Ce2Rh(1-x)Co(x)Si3: Evidence from bulk studies for Fermi-surface change during magnetic ordering - QCP transformation and applicability of SDW pictur
Short-range ordering (SRO) tendency for disordered alloys is considered as competition between chemical ordering and geometric (mainly, difference in atomic radius for constituents) effects. Especially for multicomponent (including the so-called high entropy alloys (HEAs) near equiatomic composition), it has been considered as difficult to systematically characterize the SRO tendency only by geometric effects, due mainly to the fact that (i) chemical effects typically plays significant role, (ii) near equiatomic composition, we cannnot classify which elements belong to solute or solvent, and (…
Co2MnAl has been predicted to have Weyl points near Fermi level which is expected to give rise to exotic transverse transport properties such as large anomalous Hall(AHE) and Nernst effects(ANE) due to large Berry curvature. In this study, the effect of Fermi level position and atomic ordering on AHE and ANE in Co2MnAl1-xSix were studied systematically. The Co2MnAl film keeps B2-disordred structure regardless of annealing temperature, which results in much smaller anomalous Hall conductivity sigma_xy and transverse Peltier coefficient sigma_xy than those calculated for L21-ordered Co2MnAl. Our…
Statistical Thermodynamics and Ordering Kinetics of D019-Type Phase: Application of the Models for H.C.P.-Ti-Al Alloy
采用自洽场近似、静态浓度波近似和Onsager型动力学方程，建立了D 019相原子有序化的统计热力学和动力学描述，并应用于h. c. p. - Ti-Al合金有序-无序相变模型描述了h. c. p.固溶体向D 019相的相变。原子间相互作用参数估计的两个近似：一个假设温度无关的原子间相互作用参数，而另一个包括温度依赖的Ti-Al合金的交换能。部分Ti-Al相图（共存的有序相和无序相的平衡成分）的评估为这两种情况。分析了D 019型长程序（LRO）参量的时间依赖关系。的曲线（显示的LRO参数的演变）数值得到的温度无关的相互作用能和温度相关的。原子间相互作用能的温度依赖性加速了LRO弛豫，并减小了瞬时和平衡LRO参数随温度变化的值的扩散。热力学-热力学和动力学结果表明，在高温下，非化学计量比（其中合金组分的原子分数大于0.25）的平衡LRO参数可以高于化学计量比。实验相图证实了h. c. p.的预测（有序或无序）状态。钛铝合金
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…
Increasing attention is being paid to $α$$_2$ Ti$_3$(Al,Sn) precipitation from the $α$ phase of titanium alloys owing to its effect on slip band formation, localisation and the implications for fatigue performance in jet engine titanium. However, the early stages of $α$$_2$ precipitation have historically been difficult to observe in TEM, neutron diffraction or atom probe analysis. Here, small angle X-ray scattering is used to reexamine the phase boundary in binary Ti-Al and Ti-Sn alloys with around 500 ppmw O. It is found that the phase boundaries in the literature are approximately correct, …
Microscopic structures for fcc-based quaternary high-entropy alloys (HEA) in thermodynamically equilibrium state is examined based on first-principles (FP) calculation combined with our recently-developed theoretical approach. We find that (i) hierarchical ordering tendency for whole quaternary, and ternary and binary subsystems for the present five HEAs cannot be reasonably characterized by conventional Goldschmidt atomic radius or by those obtained from unary system for FP calculation, but can be systematically characterized by atomic radius from specially fluctuated configuration. (ii) orde…
We report a study on the electronic and magnetic properties of a novel Rb$_{x}$Sr$_{1-x}$C alloy. We find that the variation of the lattice parameter as function of Rb composition exhibits a deviation from the Vegard's law of about 0.4 Å. The total energy results predict a ferromagnetic ordering for Sr-rich alloy, and antiferromagnetic ordering for RbC-rich alloy with a total magnetic moment ranging from 2 to 3$μ_\text{B}$ per cation. As for the parent compounds, RbC and SrC, the origin of magnetism arises from the polarization of the carbon $p$-orbitals. From the spin-polarized calculations, …
Crystallization represents a fundamental process engendering solidification of a material and determines its microstructure. Driven by complex phenomena at the atomic scale, its understanding for alloys still remains elusive. The present work proposes a large scale molecular dynamics simulation study of the homogeneous crystal nucleation pathways of prototypical undercooled Al-Ni binary alloys. An unsupervised topological learning analysis shows that the nucleation sets in first from a chemical ordering, followed by a bond-orientational ordering of the underlying crystal phase. Our results ind…
Structure-Dependent Chemical Order Modification in Strained Alloy Nanoparticles
Guangdong Technion – Israel Institute of Technology（广东以色列理工学院）；Universidad de Castilla-La Mancha（卡斯蒂利亚-拉曼恰大学）；Aristotle University of Thessaloniki（塞萨洛尼基亚里士多德大学）
