Short-range Order Effects Research Articles

FCC-BCC phase transformation induced simultaneous enhancement of tensile strength and ductility at high strain rate in high-entropy alloy

FCC-BCC phase transformation-induced plasticity (TRIP) has been extensively studied in high-entropy alloys (HEAs) to customize their mechanical properties through compression/tension loading or thermal fabrication processes. In this study, we employed a combination of molecular dynamics (MD) and Monte-Carlo (MC) simulations to investigate the effects of TRIP on the uniaxial strain tensile deformation of Co25Ni25Fe25Al7.5Cu17.5 HEA. Our results demonstrate that a complete FCC-BCC phase transformation occurs, in accordance with the N-W relationship, resulting in a simultaneous enhancement of strength and ductility. This is attributed to the HEA's significantly low stacking fault energy and pronounced lattice distortion (LD). However, short-range order (SRO) acts as an obstacle on atomic sliding, which further reduces the degree of phase transformation, leading to an increase in Young's modulus but a decrease in ductility. Furthermore, an increase in strain rate can promote the occurrence of the phase transformation to a certain extent but also leads to an increase in the degree of disorder defects. We also found that the HEA maintains excellent thermal stability up to 900 K, but the amount of phase transformation decreases with increasing initial temperature. Our systematic study of FCC-BCC transformation, considering the effects of SRO, LD, strain rate, and temperature, provides insights into tailoring the mechanical properties of HEAs for practical design purposes.

Effects of grain boundary and chemical short-range order on mechanical properties of NiCoCr multi-principal element alloys: A molecular dynamics simulations

Due to their outstanding mechanical properties, multi-principal element alloys (MPEAs) are attracting exceptional interest. In this work, we used molecular dynamics (MD) simulations to investigate the effects of grain boundaries (GBs) and chemical short-range order (CSRO) on the mechanical properties of NiCoCr MPEAs. We demonstrated that the presence of various grain boundaries and CSRO affects these characteristics by analyzing lattice distortion, elastic constants, elastic moduli, the Kleinman parameter, the machinability index, and the Vickers hardness of NiCoCr MPEAs. Elastic moduli show a rising trend with grain boundaries and CSRO, indicating that NiCoCr becomes more rigid. Our studied MPEAs are ductile according to Cauchy pressure, Pugh’s ratio, and Poisson’s ratio. According to the results of the anisotropy factor analysis, NiCoCr MPEAs are inherently anisotropic, and the addition of CSRO exacerbates this property. The presence of CSRO and GBs has an important effect on the mechanical features of MPEAs including as stability, fracture toughness, and ductility. The findings suggest that a better understanding of these effects can contribute to the design and development of MPEAs with enhanced mechanical characteristics for a variety of applications.

Effect of short-range order on the high-temperature plastic deformation behavior of Ni–Cr–W, Ni–W, and Ni–Re single crystals

The development of a Ni-based superalloy that consists of only the γ-solid-solution phase is highly desired for some applications such as high-temperature gas-cooled reactors in power plants. In this study, the temperature dependence of the plastic deformation behavior of Ni–22Cr–8W (at%) alloy was examined for the first time by using a single crystal, focusing on the influence of the short-range order (SRO), especially upon high-temperature deformation behavior. The critical resolved shear stress (CRSS) for (111)[101¯] slip exhibited a rapid decrease as the temperature increased at low temperatures but an almost constant value of ∼45 MPa between 500 and 900 °C. In addition, a slight yield stress anomaly (YSA) was observed, reaching a maximum value at 1000 °C.To clarify the influence of the SRO on the YSA behavior, the mechanical properties of Ni–8W and Ni–5Re single crystals were also examined. Similar temperature dependence for the CRSS, including the occurrence of slight YSA, was observed for Ni–8W, in which SRO developed. In the Ni–5Re single crystal in which SRO did not significantly develop, however, the CRSS monotonically decreased as the temperature increased. The results suggest that the presence of SRO in a Ni–Cr–W alloy contributes to the strengthening of the alloy not only at low temperatures but also at high temperatures up to ∼1000 °C. The results strongly suggested that the pseudo-PLC effect is related to the origin of YSA appearing in Ni-based γ-solid-solution alloys at high temperatures.

Short-range order and its impacts on the BCC MoNbTaW multi-principal element alloy by the machine-learning potential

We utilize a machine-learning force field, trained by a neural network (NN) with bispectrum coefficients as descriptors, to investigate the chemical short-range order (SRO) influences on the BCC MoNbTaW alloy strengthening mechanism. The NN interatomic potential offers a transferable force field that exhibits accuracy comparable to density functional theory. This innovative NN potential is employed to examine the SRO effects on various aspects such as elasticity, vibrational modes, plasticity, and strength in the MoNbTaW multi-principal element alloy (MPEA). The findings reveal a significant attraction between Mo-Ta pairs, resulting in the formation of locally ordered B2 clusters. These clusters can be adjusted via temperature and enhanced by Nb content. The presence of SRO leads to an increase in high-frequency phonon modes and introduces additional lattice friction to dislocation motion. This approach facilitates efficient compositional screening, paving the way for computational-guided materials design of novel MPEAs with enhanced performance. Furthermore, it opens up avenues for tuning the mechanical properties through optimization of the processing parameters.

Accurate prediction of chemical short-range order and its effect on thermodynamic, structural, and electronic properties of disordered alloys: Exemplified in Cu3Au

Electronic-structure methods based on density-functional theory (DFT) were used to directly quantify the effect of chemical short-range order (SRO) on thermodynamic, structural, and electronic properties of archetypal face-centered-cubic (fcc) Cu3Au alloy. We show that SRO can be tuned to alter bonding and lattice dynamics (i.e., phonons) and detail how these properties are changed with SRO. Thermodynamically favorable SRO improves phase stability of Cu3Au from − 0.0343 eV-atom−1 to −0.0682 eV-atom−1. We use DFT-based linear-response theory to predict SRO and its electronic origin, and accurately estimate the transition temperature, ordering instability (L12), and Warren-Cowley SRO parameters, observed in experiments. The accurate prediction of real-space SRO gives an edge over computationally and resource intensive approches such as Monte-Carlo methods or experiments, which will enable large-scale molecular dynamic simulations by providing supercells with optimized SRO. We also analyze phonon dispersion and estimate the vibrational entropy changes in Cu3Au (from 9kB at 300–6kB at 100 K). We establish from SRO analysis that exclusion of chemical interactions may lead to a skewed view of true properties in chemically complex alloys. The first-principles methods described here are applicable to any arbitrary complex solid-solution alloys, including multi-principal-element alloys, so hold promise for designing technologically useful materials.

Effects of lattice distortion and chemical short-range ordering on the incipient behavior of Ti-based multi-principal element alloys: MD simulations and DFT calculations

One of key factors in designing high-performance alloys is to understand their incipient behavior. To get detailed real-time atomic scale evolutions of multi-principal element alloys (MPEAs) during nanoindentation incipient yielding, molecular dynamics (MD) simulations were carried out. Emphases are given to the effects of lattice distortion (LD) and chemical short-range ordering (CSRO) on the initiation of plasticity in body-centered cubic (BCC) MPEAs. The initial plastic mechanism is embryo nucleation assisted and the incipient behavior is strongly affected by LD and CSRO. Specifically, the embryo nucleation load Pe is reduced by LD and CSRO and the incipient drop P0 is lowered by LDI that only comes from atomic size difference and further declined by CSRO. This is because LD and CSRO could boost embryo nucleation, which can act as precursors for dislocation nucleation. This is consistent with the free-end nudged elastic band observations. Although Pe is weakened by LDII generated by introducing extra elements, P0 is intensified because severe LD could dominate the following process. The dislocation starvation mechanism is found in an average-atom (AA) model. However, it is replaced by dislocation interactions due to LD and CSRO. Density functional theory (DFT) calculations suggest that charge density distribution and the amount of directional Al-Al bonding would play a critical role in the onset of plastic deformation. When a larger indenter radius was used in MD simulations, the dislocation starvation mechanism is absent in AA model due to the reduced maximum shear stress. Instead, the dislocation propagation prevails. With respect to the orientation effect, three investigated orientations demonstrate a distinct yielding behavior.

Partial Order-Disorder Transition Driving Closure of Band Gap: Example of Thermoelectric Clathrates.

In the quest for efficient thermoelectrics, semiconducting behavior is a targeted property. Yet, this is often difficult to achieve due to the complex interplay between electronic structure, temperature, and disorder. We find this to be the case for the thermoelectric clathrate Ba_{8}Al_{16}Si_{30}: Although this material exhibits a band gap in its ground state, a temperature-driven partial order-disorder transition leads to its effective closing. This finding is enabled by a novel approach to calculate the temperature-dependent effective band structure of alloys. Our method fully accounts for the effects of short-range order and can be applied to complex alloys with many atoms in the primitive cell, without relying on effective medium approximations.

Short-range ordering plays a determining role in the low-cycle fatigue life improvement of fcc metals: A conclusive study on low solid-solution hardening Ni-Cr alloys

The effect of short-range ordering (SRO) on the low-cycle fatigue (LCF) behavior of low solid-solution hardening Ni-Cr alloys with high stacking fault energies (SFEs) was systematically studied under cycling at constant total strain amplitude (Δεt/2) in the range of 0.1%–0.7%. The results show that an inducement of SRO structures can notably improve the fatigue life of the alloy regardless of Δεt/2, and several unique fatigue characteristics have been detected, including the transition of fatigue cracking mode from intergranular cracking to slip band cracking, the non-negligible evolution from non-Masing behavior in pure Ni to Masing behavior in the Ni-40Cr alloy, and the secondary cyclic hardening behavior in the Ni-10Cr and Ni-20Cr alloys. All these experimental phenomena are tightly associated with the transformation in cyclic deformation mechanisms that is induced by SRO based on the “glide plane softening” effect. Furthermore, a comprehensive fatigue life prediction model based on total hysteresis energy has been reasonably proposed, focusing on the analyses of the macroscopic model parameters (namely the fatigue cracking resistance exponent β and the crack propagation resistance parameter W0) and microscopic damage mechanisms. In brief, on the premise that the effects of SFE and friction stress can be nearly ignored, as in the case of the present low solid-solution hardening Ni-Cr alloys with high SFEs, an enhancement of SRO in face-centered cubic metals has been convincingly confirmed to be an effective strategy to improve their LCF performance.

Dislocation plasticity in equiatomic NiCoCr alloys: Effect of short-range order

Equiatomic NiCoCr solid solutions have been shown by recent experiments and atomistic simulations to display exceptional mechanical properties that have been suggested to be linked to nanostructural short-range order (SRO) features that may arise from thermal treatments, such as annealing or/and aging. Here we use hybrid Monte Carlo--molecular dynamics simulations to gain further insights of thermal effects on the SRO formation as well as the edge dislocation plasticity mechanisms of equiatomic NiCoCr face-centered cubic solid solution. For that purpose, we utilize two well-known NiCoCr interatomic potentials, one of which displays well-documented SRO, believed to be linked to experimental evidence and labeled as the Li-Sheng-Ma potential, while the other (Farkas-Caro) does not. We use these two potentials to discern short-range ordering (from inherent randomness in random solid solutions) and understand how SROs influence dislocation depinning dynamics in various thermal annealing scenarios. In this context, we used robust, scale-dependent metrics to infer a characteristic SRO size in the Li-Sheng-Ma case by probing local concentration fluctuations which otherwise indicate uncorrelated patterns in the Farkas-Caro case in a close agreement with random alloys. Our Voronoi-based analysis shows meaningful variations of local misfit properties owing to the presence of SROs. Using relevant order parameters, we also report on the drastic increase of chemical ordering within the stacking fault region. More importantly, we find that the Li-Sheng-Ma potential leads to excellent edge dislocation depinning strength with low stacking fault width. Our findings indicate an enhanced roughening mechanism due to the SROs-misfit synergy that leads to significant improvements in dislocation glide resistance. We argue that the improvements in alloy strength have their atomistic origins in the interplay between nanoscopic SROs and atomic-level misfit properties.

Spatial inhomogeneity of point defect properties in refractory multi-principal element alloy with short-range order: A first-principles study

Short-range order can be developed in multi-principal element alloys and influences the point defect behavior due to the large variation of the local chemical environment. The effect of short-range order on vacancy and interstitial formation energy and migration behavior was studied in body-centered cubic multi-principal element alloy NbZrTi by first-principles calculations. Two short-range order structures created by density functional theory and Monte Carlo method at 500 and 800 K were compared with the structure of random solid solution. Both vacancy and interstitial formation energies increase with the degree of short-range order. Point defect formation energies tend to be higher in regions enriched in Nb and lower in regions enriched in Zr and Ti. Both vacancies and interstitials prefer to migrate toward Zr,Ti-rich regions and away from Nb-rich regions, suggesting that Zr,Ti-rich regions can potentially act as recombination centers for point defect annihilation. Compared to an ideal random solid solution, the short-range order increases the spatial inhomogeneity of point defect energy landscape. Tuning the degree of short-range order by different processing techniques can be a viable strategy to optimize the point defect behavior to achieve enhanced radiation resistance in multi-principal element alloys.

Unraveling the bond structure, porosity, and mechanical properties amorphous ZIF-4 and its topological equivalents: Large scale ab initio calculations

Four large-scale amorphous zeolite imidazolate frameworks (a-ZIFs) have been constructed based on the continuous random network model. Structural properties correlation (SPC) between the mechanical properties and structural order (short-range, mid-range, or long-range orders) of ZIFs samples have been investigated by well-defined density functional theory calculations. The results demonstrated the effect of short-range and mid-range local order on the mechanical properties of the a-ZIFs, respectively. By combining different metal nodes and organic ligands, the porosity of a-ZIFs can be dramatically changed, leading to changes in Young’s modulus and shear modulus. The most important, SPC between structural order and mechanical properties of a-ZIFs is established, which will pave the way to designing high-strength and toughness ZIF-based glass.

Effects of lattice distortion and chemical short-range order on creep behavior of medium-entropy alloy CoCrNi

Creep behavior of multi-principal element alloys (MPEAs) is an intriguing topic to explore for their potential high-temperature applications. The challenge on this topic is to elucidate what effect the local chemical fluctuation has on the creep behavior of MPEAs. By using large-scale molecular dynamics (MD) simulations, we investigate the creep performances of CoCrNi medium-entropy alloys (MEAs) with and without chemical short-range order (CSRO) as well as the Average-atom (A-atom) counterpart (no lattice distortion (LD)) under different uniaxial tensile stresses at various elevated temperatures. A power-law model is adopted to analyze the implicit stress exponent and activation energy, which are associated with the creep mechanism and creep resistance, respectively. The results reveal CSRO rather than LD plays an important role in creep performance. Specifically, with the introduction of CSRO, the activation energy for creep in CoCrNi MEA is significantly increased and finally close to the activation energy for the diffusion of the Cr element that is the highest among the three elements (Co, Cr, and Ni). The CoCr clusters in the MEA with CSRO make it difficult to increase the shear strain in the creep process, resulting in a much lower creep rate than that of the CoCrNi MEA without CSRO. For the CoCrNi MEA without CSRO, grain boundary diffusion is the main creep mechanism. For the MEAs with CSRO, the predominant creep mechanism depends on the applied stress. At low stress, grain boundary diffusion still prevails and there is no obvious dislocation glide or grain boundary sliding. When the applied stress is high enough, grain boundary diffusion and dislocation slip dominate the creep behavior, accompanied by the occurrence of grain boundary sliding.

Effect of Short-Range Ordering on the Grain Boundary Character Distribution Optimization of FCC Metals with High Stacking Fault Energy: A Case Study on Ni-Cr Alloys

The critical roles of short-range ordering (SRO) in the grain boundary character distribution (GBCD) optimization of Ni-Cr alloys with high stacking fault energies were experimentally studied by thermomechanical treatments. It is found that, with the enhancement of the SRO degree (or the increase in Cr content), the dislocation slip mode changes from wavy slip to planar slip, and even deformation twins (DTs) appear in the cold-rolled Ni-40at.%Cr alloy. Within the lower level of Cr content (≤20 at.%), the optimized result of GBCD is conspicuous with the increase in Cr content. As the Cr content is higher than 20 at.%, the GBCD optimization of Ni-Cr alloys cannot be further enhanced, since the cold rolling induced DTs would hinder the growth of twin related domains during subsequent annealing.

Effects of short-range order on phase equilibria and opto-electronic properties of ternary alloy ZnxCd1-xTe

We employ first principles methods based on density functional theory and beyond to study ZnxCd1-xTe, 0≤x≤1, alloys in the zinc blende (B3) crystal structure. Cluster Expansion and Monte Carlo formalisms were deployed to provide a phase diagram determining consolute temperature of 387 K at 40% Zn concentration. Opto-electronic properties are computed with the hybrid HSE06 functional for disordered ZnxCd1-xTe alloys, which were simulated using special quasi-random structures. Bowing effects in the bandgap and effective carrier masses were observed in alloying which can be attributed to local geometrical distortions as portrayed by bond length distributions. Downward bowing in the electronic bandgap will be beneficial in photovoltaic applications through increased net photocurrent. Absorption coefficients show robust absorption in ZnxCd1-xTe as indicated by substantial optical absorption throughout all Zn concentrations, aided by low reflectivity that ensures a high absorption. Presence of short-range order in Zn0.25Cd0.75Te was observed through clustering and anti-clustering among different cationic species of Zn and Cd. It has a value of 0.11 at 1000 K compared to −0.79 at 400 K. The bandgap of Zn0.25Cd0.75Te at 1000 K was found to be slightly higher, 0.05 eV, than at 400 K, consistent with the value of short-range order. Thus, short-range order and possibly composition can be used in the design and synthesis of the appropriate solar cell, thereby improving the performance in this system.

Atomistic simulations of dislocation plasticity in concentrated VCoNi medium entropy alloys: Effects of lattice distortion and short range order

Face-centered cubic (fcc) high and medium entropy alloys (H/MEAs) have been shown to display superior mechanical properties at low temperatures, but significant improvement of their strength at high temperatures is required for industrial applications at extreme conditions. Recently, it has been shown that the breakthrough of the MEAs from equiatomic/near-equiatomic to non-equiatomic ratios leads to strong MEAs with good ductility. To design new H/MEAs, we consider two important factors that may influence strength: the chemical composition and chemical short range order (CSRO). In this study, we investigate the depinning stress (σc) as a criterion of strength of several compositions of VCoNi concentrated solid solution alloys (CSSAs) including V0.33Co0.33Ni0.33, V0.35Co0.2Ni0.45, V0.33Co0.17Ni0.5, and V0.17Co0.33Ni0.5 at 5 K and 300 K, using atomistic simulations. The chosen interatomic potential is shown to be reliable by comparing experimental/ab initio values and calculated parameters such as lattice constant, shear modulus, depinning stress, and temperature variation of stacking fault width for equimolar VCoNi. We find a good agreement between experimental friction stress and the depinning stress extracted from our results for equimolar VCoNi. Also, we find that Vclusters are the main pinning points of dislocations, and With a random distribution of atoms, we find that the alloy composition V0.33Co0.17Ni0.5 displays the largest depinning stress at both 5 and 300 K. Furthermore, to investigate how CSRO affects the strength of these alloys, we design CSRO into the microstructure using two different methods: In the first method, hybrid Molecular-dynamics/Monte-Carlo simulations were employed to simulate annealing at various temperatures. We observe that such simulations create CSRO so that it increases with decreasing annealing temperature. Recently, the CSRO motif and its concentration in an equimolar VCoNi have been determined by experiment. By modeling this experiment, we also implemented the CSRO into microstructure as the second method. By using both methods, the effect of CSRO on the magnitude of the depinning stress is discussed. It was shown that in both methods, CSRO significantly influences the strength of non-equimolar VCoNi alloys.

Crystal Plasticity Model Analysis of the Effect of Short-Range Order on Strength-Plasticity of Medium Entropy Alloys

Numerous studies have demonstrated the widespread presence of chemical short-range order (SRO) in medium and high entropy alloys (M/HEAs). However, the mechanism of their influence on macroscopic mechanical behavior remains to be understood. In this paper, we propose a novel dislocation-based model of crystal plasticity, by considering both the dislocation blocking and coplanar slip induced by SRO. The effect of SRO on the plastic deformation of CoCrNi MEAs was investigated. We found that the yield strength increases monotonically with increasing SRO-induced slip resistance, but the elongation first appeared to increase and then decreased. Further analysis suggested that the plastic elongation is a result of the competition between grain rotation-induced deformation coordination and stress concentration, which depends on the slip resistance of the SRO.

The effect of short-range order on mechanical properties of high entropy alloy Al0.3CoCrFeNi

Recent studies show that the atoms in high entropy alloys (HEAs) are not randomly distributed but have a certain short-range order (SRO). However, the study of the effect of SRO on Al0.3CoCrFeNi properties is insufficient. In this paper, the effect of SRO on the mechanical properties of Al0.3CoCrFeNi was investigated by molecular dynamics (MD). Melting/quenching and hybrid Monte Carlo (MC)/MD optimization were used to order the random distribution model. The effects of SRO on tensile and shear properties, stacking fault energy (SFE) and dislocation slip were comparatively analyzed and systematically discussed. The results show that the SRO inhibited the propagation of SFs and enhanced dislocation storage capacity. The ordering of the single crystals promoted the formation of deformation twins. The ordering of the polycrystals leads to martensite, BCC phase transformation, and activation of the sub-slip planes. SRO resulted in more severe SFE fluctuation, a more tortuous dislocation path, and a stronger pinning effect. The strength and toughness enhancement resulted from plane slip, multi-system slip, deformation twins, and nanoscale precipitates.

Effect of short-range ordering and grain boundary segregation on shear deformation of CoCrFeNi high-entropy alloys with Al addition

A method combining molecular dynamics (MD) and Monte Carlo (MC) simulation is used to analyze the short-range ordering and grain boundary (GB) segregation in the bi-crystals of equiatomic CoCrFeNi and Al (8 at%)-CoCrFeNi high-entropy alloys (HEAs). Based on the structures of the two HEAs obtained by the combined MC/MD method, their defect evolution and deformation mechanisms during shear deformation at 300 K are studied. In addition, the bi-crystals of the Al (8 at%)-CoCrFeNi HEA with the inclusion of B2 intermetallic AlNi particles at the GBs are considered. For the CoCrFeNi HEA, the Cr and Fe atoms are revealed to segregate to GBs. In contrast, it is observed in the Al (8 at%)-CoCrFeNi HEA that Al and Fe have a strong tendency to segregate to GBs, while local ordering results in the formation of Fe3Al clusters, which in turn increase the stacking fault energy of the alloy. The GB segregation and the deformation behaviour of the alloys are found to be highly sensitive to the crystallographic orientation of the bi-crystals. The GB segregation, especially by the Al atoms, stabilizes the GBs and resists the plastic deformation through GB sliding and the GB migration. Overall, the Al (8 at%)-CoCrFeNi HEA with Al-atom segregation at GBs demonstrates an increased shear yield strength compared with the material without the Al addition. On the other hand, the AlNi particles reduce the yield strength of the HEA owing to the formation of amorphous structure at the face-centered cubic/B2 interface and thus facilite the GB sliding. The obtained results provide insights into designing HEAs with improved mechanical properties through GB engineering.

Retainable short-range order effects on the strength and toughness of NbMoTaW refractory high-entropy alloys

Short range order (SRO) can affect the dislocation glide and twin nucleation in high-entropy alloys (HEAs), leading to varying mechanical performance. However, the mechanical implications of SRO in refractory HEAs (RHEAs) remain largely unknown due to the lack of understanding of the nanoscale deformation. Here, the role of SRO on the tensile mechanical properties of single crystal equiatomic NbMoTaW RHEAs is investigated using the hybrid Monte Carlo method and molecular dynamics simulation. We find that the presence of SRO can simultaneously enhance the strength and toughness over a wide range of temperatures under very fast strain rate. The enhancement is larger at lower temperature due to higher degree of SRO. The effects of SRO strengthening and toughening are retainable after unloading from the plastic stage, due to the unexpected twinning-assisted pseudo-elastic deformation. The fundamental principles of SRO are provided to facilitate the rational design and applications of RHEAs.

Vacancy Energetics and Diffusivities in the Equiatomic Multielement Nb-Mo-Ta-W Alloy.

In this work, we study vacancy energetics in the equiatomic Nb-Mo-Ta-W alloy, especially vacancy formation and migration energies, using molecular statics calculations based on a spectral neighbor analysis potential specifically developed for Nb-Mo-Ta-W. We consider vacancy properties in bulk environments as well as near edge dislocation cores, including the effect of short-range order (SRO) by preparing supercells through Metropolis Monte-Carlo relaxations and temperature on the calculation. The nudged elastic band (NEB) method is applied to study vacancy migration energies. Our results show that both vacancy formation energies and vacancy migration energies are statistically distributed with a wide spread, on the order of 1.0 eV in some cases, and display a noticeable dependence on SRO. We find that, in some cases, vacancies can form with very low energies at edge dislocation cores, from which we hypothesize the formation of stable ‘superjogs’ on edge dislocation lines. Moreover, the large spread in vacancy formation energies results in an asymmetric thermal sampling of the formation energy distribution towards lower values. This gives rise to effective vacancy formation energies that are noticeably lower than the distribution averages. We study the effect that this phenomenon has on the vacancy diffusivity in the alloy and discuss the implications of our findings on the structural features of Nb-Mo-Ta-W.