Dynamically compressive behaviors and plastic mechanisms of a CrCoNi medium entropy alloy at various temperatures

As an attractive class of metallic materials, single-phase CrCoNi medium-entropy alloy (MEA) has drawn much attention recently regarding their deformation behaviors, but the dynamically mechanical responses of this alloy at high strain rates remain less studied, especially coupled with extremely low temperatures. In this study, the dynamic deformation behaviors of this CrCoNi MEA were systematically investigated at room temperature (RT) of 298 K and liquid nitrogen temperature (LNT) of 77 K using the split Hopkinson pressure bar (SHPB). This alloy exhibited a combination of higher yield strength and stronger hardening rate upon dynamic compressive deformation when the loading conditions become much harsher (higher strain rate or lower temperature). Detailed microstructure analyses indicated that the strong strain hardening ability during dynamic deformation was mainly attributed to the continuous formation of nanoscale deformation twins. Furthermore, as loaded at LNT, multi-directional deformation twins were activated. Meanwhile, due to the interaction between Shockley partial dislocations and twin boundaries, large-sized deformation-induced FCC-HCP phase transformations at a micrometer scale were also observed within the grains, which not only accommodated the plasticity but also played an important role in improving the hardening capability owing to the appearance of newly generated interfaces.