Precipitation and grain-boundary strengthening of Al-added CoCrNi medium-entropy alloys

Herein, we report the hardening effect of Al in CoCrNi medium-entropy alloys. Al contents of up to 25 at% were added to CoCrNi and hardness measurements and phase investigation of the as-cast alloys were subsequently performed. The hardness of the as-cast alloys increased with increasing Al content and the BCC/B2 and sigma phase fraction increased as well. An additional hardness measurement was performed on cold-rolled and annealed alloys, and revealed a sharp increase in the hardness of Al7.7CoCrNi. Microstructural analysis of Al7.7CoCrNi was also performed and significant heterogeneous precipitation of the B2 phase was observed. Our findings demonstrated that heterogeneous precipitation owing to Al addition resulted in the abrupt hardening of Al7.7CoCrNi. However, the retarded grain growth due to heterogeneous precipitation may have more significantly contributed to the hardening effect. (C) 2019 Elsevier B.V. All rights reserved.