Superior tensile properties at cryogenic temperature induced by dual-graded structures in medium entropy alloys

Abstract

The dual-graded structure with both gradients of grain size and coherent L12 precipitation was applied to a (CoCrNi)96Al3Ti3 medium-entropy alloy for achieving superior tensile properties. Simultaneous improvement on yield strength and uniform elongation is observed in the dual-graded structure at cryogenic temperature, compared to those at room temperature. Stronger hetero-deformation-induced hardening and higher density of geometrically necessary dislocations are observed at cryogenic temperature in both single-grain-size-graded and dual-graded structures. The hardening mechanisms at room temperature are characterized by low density of deformation twins and stacking faults (SFs) on one slip system, while are dominated by high density SFs at two slip systems and formation of Lomer-Cottrell (L-C) locks at cryogenic temperature for the single-graded structure. The hardening mechanisms at both room and cryogenic temperatures are revealed by high density of SFs and L-C locks in the dual-grade structure. These SFs and L-C locks can provide strong hardening themselves as dynamic Hall-Petch effect on one hand, and can interact with coherent L12 precipitates for intense precipitation hardening on the other hand. The better tensile properties for the dual-graded structure at cryogenic temperature should be attributed to a special SF-induced plasticity, i.e., the formation of SF networks and their interactions with nanoprecipitates.