Chemical Short-range Order Research Articles

Effect of Ta content on stacking fault energy and microstructure characteristics of (VCoNi)100-XTaX (X = 0, 0.05, 0.5 and 1) medium entropy alloy

Ultra-strong medium entropy alloy (VCoNi) is designed via severe lattice distortion. Simulation calculation suggests the stacking fault energy of VCoNi can be reduced as increasing Ta contents, but keep the deformation twinnability almost unchanged. The experiment result suggests Ta addition can’t remarkably increase the deformation twinning propensity due to the chemical short-range order in VCoNi.

Probing chemical short-range order and other nanoscale motifs in the solid state

Probing chemical short-range order and other nanoscale motifs in the solid state

Atomistic simulations of dislocation mobility in refractory high-entropy alloys and the effect of chemical short-range order

Refractory high-entropy alloys (RHEAs) are designed for high elevated-temperature strength, with both edge and screw dislocations playing an important role for plastic deformation. However, they can also display a significant energetic driving force for chemical short-range ordering (SRO). Here, we investigate mechanisms underlying the mobilities of screw and edge dislocations in the body-centered cubic MoNbTaW RHEA over a wide temperature range using extensive molecular dynamics simulations based on a highly-accurate machine-learning interatomic potential. Further, we specifically evaluate how these mechanisms are affected by the presence of SRO. The mobility of edge dislocations is found to be enhanced by the presence of SRO, whereas the rate of double-kink nucleation in the motion of screw dislocations is reduced, although this influence of SRO appears to be attenuated at increasing temperature. Independent of the presence of SRO, a cross-slip locking mechanism is observed for the motion of screws, which provides for extra strengthening for refractory high-entropy alloy system.

Temperature-dependent mixing behavior of Pb–Sb alloys in liquid state

We have extended a computational model used to research the thermodynamic properties of binary liquid alloys to study chemical short-range order parameters and diffusion coefficients. Numerous simulations have also been employed for surface properties such as surface concentration and surface tension. The propensities for phase separation of the Pb–Sb alloy were also observed with a temperature rise. The structural and thermodynamic properties of the alloys were measured using the same interaction parameters that are suitable for the free energy of mixing and mixing enthalpy. The interaction parameters are discovered to be temperature-dependent but concentration-independent. Our results indicate that the liquid alloy Pb–Sb has a weakly interacting composition. According to the surface property review, the element Sb having a higher surface concentration than its bulk concentration segregates over the surface of the alloy.

Understanding chemical short-range ordering/demixing coupled with lattice distortion in solid solution high entropy alloys

Chemical short-range ordering (CSRO) or demixing in solid solution high entropy alloys (HEAs) is a fundamental issue yet to be fully understood. In this work, we first developed a generalized quasi-chemical solid solution model that enables quantitative computation of the local chemical ordering or demixing in solid solution HEAs. After that, we performed synchrotron diffraction experiments, extensive Reverse Monte Carlo (RMC) simulations, and first principles calculations on the CoCrFeNi model alloy to study the development of local chemical environments after long time thermal annealing. The outcome of the combined research demonstrates that the development of local chemical ordering or demixing in CoCrFeNi is not only affected by the heat of mixing between dislike atoms but also coupled with local lattice distortion.

Kinetic Monte Carlo simulation framework for chemical short-range order formation kinetics in a multi-principal-element alloy

Multi-principal-element alloys—so-called high-entropy alloys (HEAs)—contain multiple equiatomic or nearly equiatomic elements and are attracting increasing attention in basic and applied research because of their superior mechanical properties. Recently, the existence of chemical short-range order (CSRO)/local chemical ordering in HEAs has been experimentally confirmed and its effects on the mechanical properties of HEAs have been studied. However, the formation process and kinetics of CSRO have not yet been fully clarified. In the present study, we propose a simulation framework to study CSRO formation kinetics based on Monte Carlo and kinetic Monte Carlo simulation methods. Applying the simulation framework to quinary face-centered-cubic multi-principal-element alloys described by Lennard–Jones interatomic model potentials, we obtained the temperature-dependent CSRO formation kinetics via vacancy diffusion and constructed a time–temperature–CSRO degree diagram, which enables the CSRO of HEAs to be tailored via thermal processing.

Structure and thermal stability of amorphous Co23Fe60B17 film on Si substrate

Structural evolution with thermal annealing of the thin film of amorphous Co23Fe60B17 depositors on Si/SiO2 substrate has been studied using grazing incidence x-ray diffraction, soft x-ray absorption spectroscopy (SXAS), and secondary ion mass spectrometry (SIMS). Amorphous to crystalline transformation starts occurring in the film at a temperature between 200 and 300 °C, which is significantly below the crystallization temperature of amorphous ribbon of similar composition. In the process, the body centered cubic CoFe is the primary phase to precipitate out. Combined SIMS and SXAS measurements show that at the surface iron gets preferentially oxidized as compared to cobalt. Surface oxidation is found to decreases with thermal annealing in vacuum. Boron is found to gradually migrate to the surface and get sublimated. Higher surface and interface energies and depletion of B with thermal annealing are expected to be the cause of lower thermal stability of the film. Magneto-optical Kerr effect measurements show that the as-deposited film possesses a well-defined uniaxial magnetic anisotropy. The anisotropy does not disappear even after the partial crystallization of the film. This suggests that the origin of the observed anisotropy is a combined effect of long-range internal stresses and possible chemical short range order in the film.

Magnetism in curved geometries

Curvature impacts physical properties across multiple length scales, ranging from the macroscopic scale, where the shape and size vary drastically with the curvature, to the nanoscale at interfaces and inhomogeneities in materials with structural, chemical, electronic, and magnetic short-range order. In quantum materials, where correlations, entanglement, and topology dominate, the curvature opens the path to novel characteristics and phenomena that have recently emerged and could have a dramatic impact on future fundamental and applied studies of materials. Particularly, magnetic systems hosting non-collinear and topological states and 3D magnetic nanostructures strongly benefit from treating curvature as a new design parameter to explore prospective applications in the magnetic field and stress sensing, microrobotics, and information processing and storage. This Perspective gives an overview of recent progress in synthesis, theory, and characterization studies and discusses future directions, challenges, and application potential of the harnessing curvature for 3D nanomagnetism.

Molecule-like chemical units in metallic alloys

Each conventional alloy has its own specific compositions but the compositional origin is largely unknown due to our insufficient understanding about chemical short-range ordering in the alloy, in particular, in the solid-solution state. In the present paper, the compositions of metallic alloys are discussed and formulated, by unveiling the basic moleculelike structural units in solid solutions. Friedel oscillation theory, which describes the partial charge screening behavior in solid solutions, and henceforth the origin of short-range ordering, is applied to pin down the ideal chemical compositions of conventional metallic alloys. We propose that, at a specific composition, atoms self-assemble into an ideally ordered structure consisting of atoms residing in the nearest-neighbor shell (denoted as cluster) plus those in the next outer shell (denoted as glue atoms), which can be formulated as [cluster](glue atoms). This simplified version of short-range-order structure represents the smallest charge-neutral and mean-density zone (termed as “chemical units”) and can be regarded as the ‘molecules’ of solid solutions. Accordingly, the chemical units and the corresponding molecule-like formulas for face-centered-cubic (FCC), hexagonal close-packed (HCP), and body-centered cubic (BCC) structures are analyzed and equations are obtained to identify the chemical formulas for FCC solid solutions. For instance, well-known α-brass Cu-30Zn alloy is formulated as [Zn-Cu12]Zn4. Examples of aluminum alloys, superalloys and stainless steels are also illustrated, demonstrating the versatility of the present model to interpret chemically complex alloys.

Implications for the Nb aggregation inherited from melt to γ phase of U-Nb alloy

By means of ab initio molecular dynamics (AIMD) simulations, Nudged Elastic Band (NEB) method and cluster expansion (CE) combined with the Monte Carlo (MC) techniques, we have investigated that the elemental interactions and local structural dynamics in the melt and γ phase of U-Nb alloy and hydrogen diffusion behavior in γ-U-Nb alloy. It has been found that the aggregation of Nb atoms inherited from melt to γ phase plays a significant role in improving mechanical property and protecting γ-U-Nb alloy from hydrogen damage. The analyses of chemical short-range order parameters attest that there exists the similarity of element interactions between the melt and γ phase of U-Nb alloy. The U-Nb pairs in both melt and γ phase exhibit the repulsive interactions, whereas the attractive U-U and Nb-Nb interactions exits. These results demonstrate that the U and Nb atoms prefer to stay in their own shells to form the localized clusters in both melt and γ phase of U-Nb alloy. Furthermore, the analyses of hydrogen diffusion behavior in γ-U-Nb alloy, our NEB calculations have further proved that the hydrogen atoms prefer to be trapped by Nb atoms and, this trend is further reinforced by the aggregation of Nb atoms. It implies that, once Nb is added into U alloys, the Nb addition plays an important role in slowing down the diffusion of hydrogen atoms in γ-U-Nb alloy. The uniform distribution of Nb clusters helps to avoid the occurrence of excessive local hydrogen concentration.

Chemical short range order strengthening in BCC complex concentrated alloys

Atomistic methods are used to anneal two body-centered cubic (BCC) chemically complex alloys (CCAs) in order to assess the effect of chemical short-range order on alloy strength. The two alloys, a model quaternary Co16.67Fe36.67Ni16.67Ti30 and ternary Nb33.33Ti33.33Zr33.33, are represented using simple Zhou interatomic potentials. Chemically random cells are annealed at temperatures 65% of the average melting temperature of the individual elements using a Monte Carlo approach, and the critical stress required to move a/2[111] screw dislocations in the initial and annealed cells are estimated using molecular dynamics simulations. It is shown that annealing leads to a softening of the BCC quaternary alloy relative to the random state, especially for deformation at low temperatures. On the other hand, short-range order has minimal effect on solid solution strengthening in the ternary alloy. These results are modeled using an extension of the Suzuki model of substitutional solid solution strengthening developed for BCC chemically complex alloys. Here the model is modified to account for the effects of chemical short-range order. Good agreement is shown between the model results and direct atomistic simulation data. The model presented should be useful in predicting BCC CCAs with adequate high temperature strength, which will accelerate development of high temperature, high strength BCC alloys for aerospace applications.

Unveiling anneal hardening in dilute Al-doped AlxCoCrFeMnNi (x = 0, 0.1) high-entropy alloys

Anneal hardening has been one of the approaches to improve mechanical properties of solid solution alloys with the face-centered cubic (FCC) structure, whereby a considerable strengthening can be attained by annealing of cold-worked alloys below the recrystallization temperature (Trx). Microscopically, this hardening effect has been ascribed to several mechanisms, i.e. solute segregation to defects (dislocation and stacking fault) and short-range chemical ordering, etc. However, none of these mechanisms can well explain the anneal hardening recently observed in phase-pure and coarse-grained FCC-structured high-entropy alloys (HEAs). Here we report the observations, using high-resolution electron channeling contrast imaging and transmission electron microscopy, of profuse and stable dislocation substructures in a cold-rolled CoCrFeMnNi HEA subject to an annealing below Trx. The dislocation substructures are observed to be thermally stable up to Trx, which could arise from the chemical complexity of the high-entropy system where certain elemental diffusion retardation occurs. The microstructure feature is markedly different from that of conventional dilute solid solution alloys, in which dislocation substructures gradually vanish by recovery during annealing, leading to a strength drop. Furthermore, dilute addition of 2 at.% Al leads to a reduction in both microhardness and yield strength of the cold-rolled and subsequently annealed (≤500 °C) HEA. This Al induced softening effect, could be associated with the anisotropic formation of dislocation substructure, resulting from enhanced dislocation planar slip due to glide plane softening effect. These findings suggest that the strength of HEAs can be tailored through the anneal hardening effect from dislocation substructure strengthening.

Direct observation of chemical short-range order in a medium-entropy alloy.

Complex concentrated solutions of multiple principal elements are being widely investigated as high- or medium-entropy alloys (HEAs or MEAs)1-11, often assuming that these materials have the high configurational entropy of an ideal solution. However, enthalpic interactions among constituent elements are also expected at normal temperatures, resulting in various degrees of local chemical order12-22. Of the local chemical orders that can develop, chemical short-range order (CSRO) is arguably the most difficult to decipher and firm evidence of CSRO in these materials has been missing thus far16,22. Here we discover that, using an appropriate zone axis, micro/nanobeam diffraction, together with atomic-resolution imaging and chemical mapping via transmission electron microscopy, can explicitly reveal CSRO in a face-centred-cubic VCoNi concentrated solution. Our complementary suite of tools provides concrete information about the degree/extent of CSRO, atomic packing configuration and preferential occupancy of neighbouring lattice planes/sites by chemical species. Modelling of the CSRO order parameters and pair correlations over the nearest atomic shells indicates that the CSRO originates from the nearest-neighbour preference towardsunlike (V-Co and V-Ni) pairs and avoidance of V-V pairs. Our findings offer a way of identifying CSRO in concentrated solution alloys. We also use atomic strain mapping to demonstrate the dislocation interactions enhanced by the CSROs, clarifying the effects of these CSROs on plasticity mechanisms and mechanical properties upon deformation.

Atomistic insights into the deformation mechanism of a CoCrNi medium entropy alloy under nanoindentation

Understanding the fundamental of contact-induced behavior in metals is critical for the studies of associated mechanical properties. Although nanocontact plasticity in the conventional pure metals or solutions have been well established, such theoretical frameworks may break down in the recently emerging medium entropy alloys (MEAs), owing to their unique disordered feature and inevitable chemical fluctuations. Here combined with crystal defect theories, we investigate the nucleation and evolution of dislocations in CoCrNi MEA during nanoindentation. Molecular dynamics simulations of nanoindentation are carried out to analyze the effects of composition inhomogeneity and temperature on the defect behavior. In contrast to the usual homogeneous nucleation criterion in pure metals, heterogeneous dislocation nucleation is preferred to occur at Cr-rich clusters in CoCrNi MEA at a lower indentation depth. Although the local composition fluctuation can facilitate the nucleation of dislocations, these partials are pinned significantly by the local atomic sites with a high level of CoCr chemical short-range order. Moreover, nanotwin and phase-transformation are promoted at low temperature, due to the low stacking fault energy and enhanced glide resistance. In particular, the formation and evolution of the distorted prismatic dislocation loop are visualized at high temperature. The new phenomena reported herein could provide a basis for tuning local composition and atomic arrangement to obtain excellent mechanical properties in medium or even high entropy alloys.

A tale of two phase diagrams: Interplay of ordering and hydrogen uptake in Pd–Au–H

Due to their ability to reversibly absorb/desorb hydrogen without hysteresis, Pd–Au nanoalloys have been proposed as materials for hydrogen sensing. For sensing, it is important that absorption/desorption isotherms are reproducible and stable over time. A few studies have pointed to the influence of short and long range chemical order on these isotherms, but many aspects of the impact of chemical order have remained unexplored. Here, we use alloy cluster expansions to describe the thermodynamics of hydrogen in Pd–Au in a wide concentration range. We investigate how different chemical orderings, corresponding to annealing at different temperatures as well as different external pressures of hydrogen, impact the behavior of the material with focus on its hydrogen absorption/desorption isotherms. In particular, we find that a long-range ordered L12 phase is expected to form if the H2 pressure is sufficiently high. Furthermore, we construct the phase diagram at temperatures from 250 K to 500 K, showing that if full equilibrium is reached in the presence of hydrogen, phase separation can often be expected to occur, in stark contrast to the phase diagram in para-equilibrium. Our results explain the experimental observation that absorption/desorption isotherms in Pd–Au are often stable over time, but also reveal pitfalls for when this may not be the case.

Lightweight Zr1.2V0.8NbTi Al high-entropy alloys with high tensile strength and ductility

High-entropy alloys (HEAs) provide a new perspective to design novel lightweight alloys, whereas the complex precipitates in current lightweight HEAs (LHEAs) generally limit the mechanical properties. By controlling Ti and Al, the B2 and C14 Laves precipitates in Zr 1.2 V 0.8 NbTi x Al y (Ti x Al y ) LHEA system were successfully suppressed, and a series of LHEAs with outstanding specific yield strength-ductility matching were designed. Especially, with the same deformability (tensile ductility over 25%), the specific yield strength of Ti3.6Al0.6 LHEA is over 40% higher than current developed HEAs. The excellent mechanical properties are attributed to be benefited by a combination of solid solution strengthening and chemical short-range order strengthening. Our results bring new insights into the phase stability and precipitation suppression of HEAs, which is constructive for the novel lightweight alloy development. • Ti–Al control achieved both precipitate suppression and lightweight in HEA design. • A specific strength improvement of 40% was achieved in HEAs with ductility over 25%. • Strengthening of solid solution and chemical short-range order benefit the property.

Local Ordering Tendency in Body-Centered Cubic (BCC) Multi-Principal Element Alloys

The recent development of high-entropy alloys (HEAs) has opened a new avenue for alloy design by incorporating multiple principal elements into a simple crystal lattice. In HEAs, short-range chemical ordering may arise due to the different mixing properties of various constituent elements. In this work, we explore the trend of elemental arrangement in five typical body-centered cubic (BCC) multi-principal element alloys (MPEAs), including CrFeV, HfNbZr, TaVW, NbTaV, and TaTiV. Combining the Monte Carlo method and density-functional theory calculations, we calculated the evolution of short-range order (SRO) parameters in these alloys at 500 K. Our results demonstrated that all these MPEAs might develop a certain degree of SRO, depending on the mixing properties of different element pairs. Using a modified quasi-chemical model, we show that the SRO values in these MPEAs can be accurately predicted as long as the mixing enthalpy values between different atomic pairs are known. The presence of SRO has a profound impact on the local structure of MPEAs, which further alters the electronic and elastic properties of MPEAs. Our results provide a theoretical understanding of the basic alloy structure of novel MPEAs, which is an essential step toward tailoring their properties.

Chemical short-range ordering and its strengthening effect in refractory high-entropy alloys

In this work, we perform first-principles calculations, coupled with synchrotron small-angle x-ray scattering (SAXS) to investigate electronic origins of the chemical short-range ordering (CSRO) and to reveal the relationship between CSROs and mechanical properties in a typical bcc (body-centered-cubic) TiZrHfNb high-entropy alloys (HEAs). It was found that existence of an appropriate degree of CSROs not only stabilizes the HEA lattice structure, but also enhances the hardness, elastic modulus, and ideal strength. Comprehensive electronic structure analyses uncover that the reduction of $d$ electrons at the Fermi level due to the development of (Ti, Zr)-based CSROs stabilizes the bcc HEA, and the CSRO-induced strengthening is attributed to the local lattice distortion and the $d$-electron transfer from high-energy to low-energy states under applied strains. This finding not only gives insight into understanding the nature of CSRO strengthening in bcc HEAs, but also provides a paradigm for achieving desired mechanical properties via tailoring CSROs in HEAs.

Magnetically driven short-range order can explain anomalous measurements in CrCoNi.

The presence, nature, and impact of chemical short-range order in the multi-principal element alloy CrCoNi are all topics of current interest and debate. First-principles calculations reveal that its origins are fundamentally magnetic, involving repulsion between like-spin Co-Cr and Cr-Cr pairs that is complemented by the formation of a magnetically aligned sublattice of second-nearest-neighbor Cr atoms. Ordering models following these principles are found to predict otherwise anomalous experimental measurements concerning both magnetization and atomic volumes across a range of compositions. In addition to demonstrating the impact of magnetic interactions and resulting chemical rearrangement, the possible explanation of experiments would imply that short-range order of this type is far more prevalent than previously realized.

Ab initio molecular dynamics study on local structure and dynamic properties of liquid Ni62Nb38 alloy

In the temperature range between 1873 K and 1233 K, structural evolution and dynamic properties of liquid Ni62Nb38 alloy have been studied by ab initio molecular dynamics. The evolution of local structure was characterized by pair distribution functions, structure factors, bond angle distributions, coordination numbers and chemical short-range order parameters, and electronic structure is mainly reflected by the density of states. Self-diffusion coefficients were calculated out to represent dynamic properties. It is found that with the decrease of temperature, short-range order tends to increase and medium-range order can be discovered in the under-cooling system. Meanwhile, a series of complex local structure like tetrahedron and icosahedron gradually forms in the liquid. Moreover, the whole system is hetero-coordinating. During the whole process, both Ni and Nb diffusion coefficients show a decreasing trend, effectively prohibiting nucleation and crystal growth, and it is beneficial to the amorphous transformation of liquid Ni62Nb38 alloy. The formation of chemical bonds between Ni and Nb atoms is mainly because of the hybridization interaction between Ni and Nb d-states.