Effect of μ-precipitates on the microstructure and mechanical properties of non-equiatomic CoCrFeNiMo medium-entropy alloys

Non-equiatomic Co17.5Cr12.5Fe55Ni10Mo5 (Mo5) and Ca18Cr12.5Fe55Ni7Mo7.5 (Mo7.5) medium-entropy alloys were synthesized by vacuum induction melting, cold rolling, and subsequent annealing treatment at various temperatures (900-1200 degrees C) and they were investigated to exploit the precipitation strengthening in addition to solid solution strengthening of alloys. The effect of annealing temperature and Mo content on the microstructure and mechanical properties are systematically analyzed. From the microstructural analysis, a Mo-rich mu phase is observed in the face-centered cubic (fcc) matrix. Increasing the Mo content and low annealing temperature enhance the formation of mu phase, which is consistent with the thermodynamic calculation results. The formation of mu phase effectively enhances the strength of the Mo7.5 alloy by precipitation strengthening, and suppression of grain growth and recrystallization by Zener pinning effect. These lead to superior combination of tensile strength as high as 1100 MPa and large ductility. Our results provide insights not only into mu-phase strengthening of fcc-structured alloys, but also into the future development of high-performance MEAs. (C) 2018 Elsevier B.V. All rights reserved.