Chemical short-range order (CSRO), the non-random local arrangement of atoms in solid solutions, strongly affects the phase stability and performance of medium- and high-entropy alloys (M/HEAs). Despite its importance, the fundamental nature of CSRO formation remains contested: is it a formal thermodynamic transition? Here, investigating CoCrNi as a model system, we indicate that the main CSRO transformations observed in alloys might not be classical thermodynamic transitions, but instead a kinetic arrest phenomenon analogous to the glass transition. Combining atomistic simulations and in situ synchrotron dilatometry experiments enabled the study of CSRO evolution and its structural impact across multiple length scales. For example, CSRO-driven changes in bond lengths and bond distribution significantly impact the observed lattice parameter and volume, allowing Warren-Cowley parameter values to be determined over a full experimental temperature range. We demonstrate that the degree of CSRO and the apparent transition temperatures, defined here as the komplex reaction temperatures (Tkr), are not intrinsic material constants. Rather, they are path-dependent quantities governed by thermal history and diffusional constraints, directly reflecting the frozen CSRO state. Our findings clarify the thermodynamic and kinetic mechanisms underlying CSRO evolution and establish a framework to distinguish thermodynamic transitions under kinetic constraints from genuine kinetically arrested phenomena. Understanding this distinction is crucial for controlling CSRO during alloy design and processing and provides a foundation for future investigations exploring the implications of CSRO in advanced materials. In situ synchrotron dilatometry and atomistic simulations indicate that changes between chemical short-range order states (evolving local chemical distributions) are governed by a kinetic arrest phenomenon rather than a thermodynamic transition.
Chemical short-range order (CSRO) is an intrinsic feature of complex concentrated alloys (CCAs), yet its influence on deformation mechanisms is controversial because of the inconclusive state of concurrent CSRO quantification during deformation. Here, we provide experimental evidence that CSRO acts as an intrinsic thermodynamic state variable governing stacking-fault energetics and deformation pathways in a Co30Cr40Ni30 alloy. By comparing quenched (CSRO-lean) and aged (CSRO-enriched) conditions with equivalent grain structure and phase constitution, we isolate the influence of atomic-scale chemical ordering on mechanical behavior. Calorimetry confirms reversible CSRO formation, while synchrotron X-ray diffraction and electron microscopy reveal that CSRO suppresses deformation-induced fcc-hcp martensitic transformation at both room and cryogenic temperatures. Despite differences in transformation dynamics, the macroscopic tensile response is still broadly similar. Atomistic simulations show that CSRO increases both stable and unstable stacking-fault energies, raising the energetic barrier for partial-dislocation activity and stabilizing the fcc lattice against transformation. Together, the experimental and computational results establish CSRO as an added degree of freedom for tuning stacking-fault energetics and controlling deformation pathways in complex concentrated alloys.
A. Andreoli, G. B. Ribeiro, G. C. Stumpf et al.· 0 citations
Metallic glasses challenge the classical structure–property doctrine because the lack of long-range periodicity makes their structures complex and inherently multiscale. Although short-range order (SRO) within the first-neighbor cage has provided a useful vocabulary (e.g., icosahedral versus liquid-like motifs), growing evidence shows that SRO alone is not a reliable state variable for properties, such as stability and mechanical performance. In particular, the same nominal SRO motif can correspond to distinct energetic states depending on its embedding at more extended length scale, and local propensity indicators do not uniquely predict whether local plastic activity remains isolated or evolves into avalanches and extensive plastic deformation. We argue that rethinking glass structure requires shifting from per-atom SRO coordinates toward collective variables, defined over the range of relevant cooperativity. Within this framework, we clarify two kinds of medium-range order (MRO) that are frequently conflated in the literature at the intermediate scales (around second nearest- neighbor shell, 0.5–1 nm). One is the topological MRO, manifested as SRO motif connectivity/networking in atomistic models or as crystal-like nanodomains, probed by fluctuation electron microscopy and 4D-STEM. Another is defined by the persistent oscillations of pair distribution functions (i.e., the density wave fluctuations). Their coherence length is also about the same scale, and provides an alternative, more general view of the MRO. This motivates an energy landscape-based viewpoint in which density-fluctuation fields do not map to single basins, but condition the statistical accessibility of basins and activation barriers, thereby shaping the rigidity network that steers cascade propagation. We conclude by outlining quantitative opportunities to connect these scale-dependent collective descriptors, response-based diffraction, and energy landscape sampling into predictive, experimentally anchored structure–property relations. Data-driven atomistic analysis of metallic-glass structures using smooth overlap of atomic positions (SOAP) descriptors and transparent machine learning to identify radius of informative structural environments (RISE) Data-driven atomistic analysis of metallic-glass structures using smooth overlap of atomic positions (SOAP) descriptors and transparent machine learning to identify radius of informative structural environments (RISE)
The Stokes-Einstein relation (SER) links diffusion and viscosity, but its applicability is challenged in systems with complex structure and chemical properties. Medium-entropy alloys (MEAs), with their inherent chemical complexity and propensity for diverse local ordering, offer a compelling platform to study such coupling phenomena in the liquid state. However, how element-specific structural preferences influence SER breakdown remains unexplored. Here, we investigate the microscopic mechanisms underlying SER breakdown in liquid equiatomic NiCoCr MEA using high-energy x-ray scattering and molecular dynamics simulations. A dynamic transition is observed near 1700 K (melting point 1682.9 K). While the self-diffusion coefficients of all elements retain Arrhenius behavior down to the deeply supercooled regime, the viscosity and structural relaxation times exhibit a clear cross-over between two Arrhenius regimes below this temperature, leading to the breakdown of the inverse scaling between diffusion and viscosity. This decoupling is primarily governed by the anomalous temperature dependence of the viscosity. Fractional SER analysis reveals element-specific decoupling, with Cr showing a fundamentally distinct departure from classical scaling compared to Ni and Co. Concurrently, non-Gaussian parameters reveal growing dynamic heterogeneity upon cooling. Structurally, short-range order strengthens with significant increases in icosahedral-like and mixed clusters, especially those centered on Cr, accompanied by an enhancement of local five-fold symmetry. The formation of these rigid, Cr-centered ordered domains amplifies local geometric constraints, which severely hinder cooperative atomic rearrangements, while leaving single-atom diffusion less affected. These results connect element-specific ordering to the viscosity-driven breakdown of SER in NiCoCr, providing a structural perspective on diffusion-viscosity decoupling in MEA liquids.
Hanmei Chen, Pengfei Yu, Jiang Ren et al.· Journal of Chemical Physics· 0 citations
Chemical short‐range order (CSRO) in Multi‐Principal Element Alloys (MPEAs) arises from non‐random atomic arrangements within local coordination shells and plays a critical role in governing mechanical properties, diffusion kinetics, and phase stability. Capturing CSRO requires synergistic computational and experimental strategies. Advanced first‐principles methods evaluate mixing enthalpies and local distortions, while atomistic simulations enable large‐scale configurational sampling. Analytical frameworks, such as the Cluster Variation Method, predict temperature‐dependent ordering trends. Concurrent experimental approaches, including pair distribution function analysis, in situ synchrotron diffraction, atom probe tomography, and resistivity measurements, provide indirect yet complementary insights into local ordering. However, the field lacks a unified framework linking the thermodynamic origin, kinetic evolution, and experimental observability of CSRO, limiting the ability to systematically interpret and compare different approaches. This perspective examines computational and experimental methods in the context of CSRO formation, evolution, and measurement, emphasizes the importance of modeling–experiment cross‐validation pathways for integrating these complementary approaches, and provides a critical assessment of their capabilities, limitations, and roles in advancing the understanding and design of MPEAs.
M. Asle Zaeem, Peter K. Liaw· Advancement of science· 0 citations
The formation energy of point defects is a key parameter governing their concentration, stability, and evolution under both equilibrium and nonequilibrium conditions. In medium‐ and high‐entropy alloys (M/HEAs), intrinsic chemical complexity produces a broad distribution of defect‐formation energies, in contrast to the comparatively well‐defined values typical of pure metals and dilute alloys. However, how chemical short‐range order (CSRO) modifies these energetics remains incompletely understood. Here, we employ first‐principles calculations to systematically investigate the formation energies of vacancies and interstitials in random solid‐solution (RSS) and CSRO‐containing CrCoNi alloys. The results show that local compositional fluctuations and chemical ordering generate pronounced site‐to‐site variations in the formation energies of both defect types. More importantly, CSRO shifts the energy distributions toward higher values, making the formation of vacancies and interstitials thermodynamically less favorable. These findings provide quantitative atomic‐scale insight into the influence of CSRO on point‐defect energetics and establish a thermodynamic basis for understanding defect stability and evolution in irradiated CrCoNi alloys.
Bozhao Zhang, Jun Ding· Advanced Engineering Materia...· 0 citations
Relations between structural properties and their variations due to phase transitions under variable thermodynamic conditions are of central interest in material science. More than a decade ago, the discovery of elastic bending in single crystals of caffeine cocrystal solvates opened up a new area of research in the mechanical properties of molecular crystals. Here, we report on a structural transition at T > 320 K that is accompanied by negative area expansion of the plane normal to the bending axis, and a nonlinear increase of intermolecular distances within π···π stacks and the unit cell volume. Upon applying heating–cooling cycles, the transition at Tc is found to be reversible; albeit, from 360 K ≤ T ≤ 380 K to T = 100 K, structurally irreversible states are observed that are characterized by hysteresis in the area of the planes and intermolecular distances. The phase transformation, as well as irreversibilities, are associated with significant crystal degradation, as suggested by increasing mosaicity as a function of T, which qualitatively indicates the evolution of defects in the crystals. During these various thermal processes, the acid–base dimers rotate as well as move. The ratio (η) between variation in rotations and distances reveals a similarity in its order in the high-temperature phase, as well as those in the irreversible structural states, to that calculated from reported deformations during mechanical bending at T = 100 K.
Somnath Dey, Hans Gildenast, Shibabrata Nandi et al.· Crystal Growth & Design· 0 citations
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