On the nature of chemical short-range order evolution

Abstract

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.