Local Chemical Order in High-Entropy Alloys

Medium- and high-entropy alloys (M/HEAs) mix several principal elements with near-equiatomic composition and represent a model-shift strategy for designing previously unknown materials in metallurgy1-8, catalysis9-14 and other fields15-18. One of the core hypotheses of M/HEAs is lattice distortion5,19,20, which has been investigated by different numerical and experimental techniques21-26. However, determining the three-dimensional (3D) lattice distortion in M/HEAs remains a challenge. Moreover, the presumed random elemental mixing in M/HEAs has been questioned by X-ray and neutron studies27, atomistic simulations28-30, energy dispersive spectroscopy31,32 and electron diffraction33,34, which suggest the existence of local chemical order in M/HEAs. However, direct experimental observation of the 3D local chemical order has been difficult because energy dispersive spectroscopy integrates the composition of atomic columns along the zone axes7,32,34 and diffuse electron reflections may originate from planar defects instead of local chemical order35. Here we determine the 3D atomic positions of M/HEA nanoparticles using atomic electron tomography36 and quantitatively characterize the local lattice distortion, strain tensor, twin boundaries, dislocation cores and chemical short-range order (CSRO). We find that the high-entropy alloys have larger local lattice distortion and more heterogeneous strain than the medium-entropy alloys and that strain is correlated to CSRO. We also observe CSRO-mediated twinning in the medium-entropy alloys, that is, twinning occurs in energetically unfavoured CSRO regions but not in energetically favoured CSRO ones, which represents,to our knowledge, the first experimental observation of correlating local chemical order with structural defects in any material. We expect that this work will not only expand our fundamental understanding of this important class of materials but also provide the foundation for tailoring M/HEA properties through engineering lattice distortion and local chemical order.

Effect of local chemical order on monovacancy diffusion in CoNiCrFe high-entropy alloy

Atomistic simulation of local chemical order in NbTiZrMoV high entropy alloy based on a newly-developed interatomic potential

Local chemical inhomogeneities in TiZrNb-based refractory high-entropy alloys

High-entropy alloys (HEAs) balance mixing entropy and intermetallic phase formation enthalpy, creating a vast compositional space for structural and functional materials1-6. They exhibit exceptional strength-ductility trade-offs in metallurgy4-10 and near-continuum adsorbate binding energies in catalysis11-16. A deep understanding of crystal nucleation and growth in HEAs is essential for controlling their formation and optimizing their structural and functional properties. However, atomic-scale nucleation in HEAs challenges traditional theories based on one or two principal elements17-23. The intricate interplay of structural and chemical orders among multiple principal elements further obscures our understanding of nucleation pathways5,24-27. Due to the lack of direct three-dimensional (3D) atomic-scale observations, previous studies have relied on simulations and indirect measurements28-32, leaving HEA nucleation and growth fundamentally elusive. Here, we advance atomic electron tomography33,34 to resolve the 3D atomic structure and chemical composition of 7,662 HEA and 498 medium-entropy alloy nuclei at different nucleation stages. We observe local structural order that decreases from core to boundary, correlating with local chemical order. As nuclei grow, structural order improves. At later stages, most nuclei coalesce without misorientation, while some form coherent twin boundaries. To explain these experimental observations, we propose the gradient nucleation pathways model, in which the nucleation energy barrier progressively increases through multiple evolving intermediate states. We expect these findings to not only provide fundamental insights into crystal nucleation and growth in HEAs, but also offer a general framework for understanding nucleation mechanisms in other materials.

Nanoscale high-entropy alloy for electrocatalysis

Enhancing the radiation tolerance of high-entropy alloys via solute-promoted chemical heterogeneities

High-entropy alloys (HEAs) are an intriguing new class of metallic materials due to their unique mechanical behavior. Achieving a detailed understanding of structure-property relationships in these materials has been challenged by the compositional disorder that underlies their unique mechanical behavior. Accordingly, in this work, we employ first-principles calculations to investigate the nature of local chemical order and establish its relationship to the intrinsic and extrinsic stacking fault energy (SFE) in CrCoNi medium-entropy solid-solution alloys, whose combination of strength, ductility, and toughness properties approaches the best on record. We find that the average intrinsic and extrinsic SFE are both highly tunable, with values ranging from -43 to 30 mJ⋅m-2 and from -28 to 66 mJ⋅m-2, respectively, as the degree of local chemical order increases. The state of local ordering also strongly correlates with the energy difference between the face-centered cubic (fcc) and hexagonal close-packed (hcp) phases, which affects the occurrence of transformation-induced plasticity. This theoretical study demonstrates that chemical short-range order is thermodynamically favored in HEAs and can be tuned to affect the mechanical behavior of these alloys. It thus addresses the pressing need to establish robust processing-structure-property relationships to guide the science-based design of new HEAs with targeted mechanical behavior.

As a promising candidate for next-generation aviation structural materials, lightweight refractory high entropy alloys (HEAs) exhibit high strength, low density, and excellent high-temperature performance. In this study, we investigated the influence of local chemical ordering on the properties of Ti-V-Zr-Nb-Al HEAs using Monte Carlo (MC) simulations based on density functional theory (DFT) calculations. We established that the chemical short-range ordering (SRO) in Ti-V-Zr-Nb-Al HEAs increases with the Al content, resulting in a gradual increase in stacking fault energy (SFE). This theoretical investigation suggests that SRO can be utilized to tailor the performance of HEAs, thereby providing guidance for the scientific design of macroscopic mechanical properties.

Owing to superior strength-ductility combination and great potential for applications in extreme conditions, high-entropy alloys (HEAs) with the face-centered cubic (FCC) structure have drawn enormous attention. However, the FCC structure limits yield strength and makes the alloys unable to meet ever-increasing demands for exploring the universe. Here, we report a strategy to obtain FCC materials with outstanding mechanical properties in both ambient and cryogenic environments, via exploiting dynamic development of the interstitial-driven local chemical order (LCO). Dense laths composed of the multiscaled LCO domains evolve from planar-slip bands that form in the prior thermomechanical processing, contributing to ultrahigh yield strengths over a wide temperature range. During cryogenic tensile deformation, LCO further develops and promotes remarkable dislocation cross-slip. Together with the deformation-driven transformation and twinning, these factors lead to satisfactory work hardening. The cryogenic loading-promoted LCO, also revealed by ab initio calculations, opens an avenue for designing advanced cryogenic materials.

High-entropy alloys (HEAs) have attracted considerable attention in recent years due to their exceptional mechanical and physical properties. However, the existence of local chemical order (LCO) in these alloys—particularly in equiatomic CrFeCoNiPd face-centered cubic HEAs—remains a subject of ongoing debate. In this study, we employ a robust and high-fidelity cluster expansion (CE) method, combined with Monte Carlo simulations, to investigate the potential presence of LCO in CrFeCoNiPd HEAs. Our CE model demonstrates high predictive accuracy, achieving a correlation coefficient of 0.987 for configurational energy. The analysis reveals distinct chemical short-range order, with a pronounced preference for Pd–Pd pairs. Moreover, significant nanoscale compositional fluctuations are observed, indicating a non-uniform distribution of elements within the alloy. These findings offer valuable guidance for future experimental and theoretical studies aimed at understanding chemical ordering phenomena in complex alloys.

Temper embrittlement, characterized by a dramatic loss of ductility within a narrow temperature window, is ubiquitous in conventional alloys but is not reported in compositionally complex or high-entropy alloy systems. Here, an unexpected ductile-brittle-ductile transition is discovered in a multiphase high-entropy alloy (HEA) aged in an intermediate temperature regime. Unlike the classical thermal embrittlement driven by grain boundary effects, this transition originates from the dynamic evolution of local chemical order (LCO) and phase boundary (PB) configurations in HEAs. Aging within the embrittlement-prone regime enhances chemical ordering, increases the density of ordered domains, and induces jagged PBs, collectively triggering plastic instability and the ductile-to-brittle transition. In contrast, aging outside this regime suppresses excessive ordering and promotes the formation of ductile interphase transition zones, facilitating a brittle-to-ductile recovery. The findings offer new insights into the thermal behavior of HEAs and challenge the established paradigm of thermal embrittlement. These insights provide valuable guidance for the design and processing of high-performance HEAs, thereby unlocking their potential as advanced high-temperature structural materials.

Compositional fluctuation and local chemical ordering in multi-principal element alloys

A Ti1.5Nb1Ta0.5Zr1Mo0.5 (TNTZM) refractory high entropy alloy (HEA) with a cellular structure was successfully fabricated by laser powder bed fusion (L-PBF). Compression testing and cyclic deformation testing results revealed that, in the cellular structure, the cell walls could store dislocations. Furthermore, the local chemical order (LCO) plays a crucial role in controlling dislocations within the cell wall region. The LCO not only facilitates dislocation slip but also generates additional lattice distortion upon stress-induced LCO destruction to enable dislocation pinning. This work offers novel insights into the microstructure of additively manufactured refractory HEAs and uncovers a distinct dislocation regulation mechanism.

Trade-off between local chemical order and lattice distortion in affecting dislocation motion in NbTiZr multi-principal element alloys