Interplay between phase stability and deformation mechanisms through compositional tuning: Insights into alloy design strategy from the Co-Cr-Ni medium-entropy system

This study investigates the interplay between composition-dependent phase stability and deformation mechanisms in Co-Cr-Ni medium-entropy alloys (MEAs) of equiatomic and non-equiatomic compositions. Co-rich (Co 2 CrNi), Cr-rich (CoCr 2 Ni), and Ni-rich (CoCrNi 2 ) MEAs were designed with each composition determined by selecting a constituent element present in the highest proportion compared to the equiatomic CoCrNi system, which served as the master composition. The composition tuning resulted in distinct microstructure evolution, influencing the mechanical properties and flow mechanisms, closely correlated with variations in stacking fault energy (SFE). Notably, the results highlighted the superior formability, attributed to the distinct deformation mechanisms present in single-phase MEAs (i.e., Co 2 CrNi, CoCrNi 2 , and CoCrNi). On the other hand, the introduction of ∼50 at% Cr thermodynamically triggered the formation of σ-phase upon homogenization treatment, negatively affecting the stability of the face-centered cubic (FCC) matrix and the alloy's mechanical behavior. Macro-to-nanoscale hierarchical microstructure analyses confirmed the progression of deformation via deformation-induced twinning (CoCrNi) and ɛ-martensite formation (Co 2 CrNi) in low-SFE alloys. In contrast, the higher-SFE CoCrNi 2 alloy exhibited highly dense dislocation walls (HDDWs) with fewer planar dislocation arrays despite the presence of numerous annealing twins. This research provides a systematic alloy design strategy, integrating theoretical analyses with experimental observations to achieve an in-depth comprehension of composition tuning in the Co-Cr-Ni system.