Effect of Sigma and L21 phases co-precipitation on the mechanical properties of CoCr0.6NiV0.6Al0.35Tix high entropy alloy

In this study, non-equiatomic CoCr0.6NiV0.6Al0.35Tix high entropy alloys (HEAs) were synthesized by vacuum arc melting. The addition of titanium modified the morphology and types of precipitated phases in the HEAs. Results indicated that the incorporation of titanium facilitated the formation of the tetragonal Sigma phase and stabilized the L21 phase, which exhibited a coherent interface with the BCC matrix. With addition of 4 wt% Ti, the L21 and Sigma phases co-precipitated as submicron short rods in the alloy. The synergistic effect of the two phases could effectively enhance the mechanical properties of the alloy. Therefore, the alloy has the highest compressive strength and work-hardening rate of 2731.7 MPa and 26.4 GPa, respectively. Compared to the CoCr0.6NiV0.6Al0.35 alloy, the compressive strength increased by 33.3 % while maintaining plasticity above 10 %. Excellent comprehensive mechanical properties made it suitable for engineering applications. The primary strengthening mechanism of the alloy was the precipitation strengthening, which was mainly due to the formation of Sigma and L21/B2 precipitation phases in the alloy. However, excessive addition of titanium in the alloy would decrease the compressive strength and plasticity of the alloy, which was related to the reticulate brittle interface and lamella structure phases formed in the alloy.