Phase decomposition in the equiatomic CoCrNi alloy

Complex, concentrated alloys (CCAs) are composed of multiple principal elements in significant proportions and have attracted substantial interest due to their distinctive properties. It was initially thought that CCAs formed primarily as single-phase structures; however, subsequent research has revealed that CCAs may undergo phase decomposition when subjected to intermediate temperatures over extended durations. This study investigates the phase stability of equiatomic CoCrNi alloy, commonly recognized as a single-phase face-centered cubic (FCC) material. The alloy was subjected to severe plastic deformation, resulting in a high density of grain boundaries and deformation-induced structures. Guided by the calculation of phase diagrams (CALPHAD) predictions, prolonged annealing at a selected temperature was conducted to evaluate its phase stability. Microstructural characterization from the micro- to atomic-scale revealed that the FCC matrix undergoes structural decomposition into an HCP phase, accompanied by elemental partitioning within this phase. Transmission electron microscopy confirmed the presence of the HCP phase, while high-throughput CALPHAD and hybrid Monte Carlo/Molecular Dynamics simulations provided mechanistic insights into its formation. The emergence of this HCP phase, and the associated redistribution of elements, explains the observed differences in phase constitution compared to previously studied alloys. These findings highlight the critical role of processing-dependent phase evolution and elemental partitioning in dictating the performance of complex concentrated alloys (CCAs), thereby influencing their mechanical properties and long-term reliability in demanding applications.