On the nature of chemical short-range order evolution

Chemical short-range order (CSRO) refers to the local, non-random arrangement of atoms in solid solutions, where certain chemical pairs are more or less likely as first neighbors than expected from stoichiometry1,2. This subtle deviation from ideal disorder has been increasingly recognized as a foundational aspect of alloys, especially chemically complex ones such as medium- and high-entropy alloys (M/HEAs), without which their phase stability and properties cannot be fully explained3,4. Traditional studies have focused primarily on detecting CSRO and assessing its potential effects, but have rarely addressed whether, by its fundamental nature, it emerges as a true thermodynamic phase transition, or if its formation is instead dominated by kinetic constraints4,5. Here we show that the main CSRO transformations observed in alloys are not a classical thermodynamic transition, but rather a kinetic arrest phenomenon analogous to the glass transition. Combining atomistic simulations and synchrotron dilatometry experiments enabled the study of in situ evolution of CSRO and its structural impact across multiple length scales, supporting this new finding. For example, CSRO-driven changes in bond lengths and bond distribution significantly impact the observed lattice parameter and volume. Furthermore, we demonstrate that the degree of CSRO and the apparent transition temperatures reported in many previous works, defined here as the komplex reaction temperatures (Tkr), are not intrinsic to a material, but are instead governed by its thermal history and diffusional constraints. Our findings clarify the thermodynamic and kinetic mechanisms driving CSRO and establish a framework to distinguish genuine phase transitions from kinetically arrested states. 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.