Dual heterogeneous structured medium-entropy alloys showing a superior strength-ductility synergy at cryogenic temperature

The tensile properties and the corresponding deformation mechanisms at both room and cryogenic temperatures for a CoCrNi-based medium entropy alloy with both a homogeneous structure and a dual heterogeneous structure have been studied and compared. The dual heterogeneous structure is found to be composed of both heterogeneous grain structure and coherent L12 nanoprecipitates. Both the homogeneous structure and the dual heterogeneous structure show a simultaneous improvement on yield strength and uniform elongation at cryogenic temperature as compared to those at room temperature, which can be attributed to higher hetero-deformation-induced hardening rate and higher density of geometrically necessary dislocations induced at cryogenic temperature. For the homogeneous structure, nano-spaced stacking faults along two slip systems with much smaller interspacing and higher density of Lomer-Cottrell locks should play the essential role in sustaining the strain hardening for better tensile ductility at cryogenic temperature. L12 nanoprecipitates are observed to be very effective on blocking dislocation slip and accumulation of dislocations around them. For the dual heterogeneous structure, more intense interactions between defects and L12 nanoprecipitates are observed after tensile deformation at cryogenic temperature, resulting in fragmentation and refinement of L12 nanoprecipitates for better tensile properties than at room temperature.