Effect of age hardening on microstructure and typical mechanical properties of CoCrFeNiTa_0.43 eutectic high entropy alloy

In this study, effects of aging at 750 °C for 250 h on microstructure, micromechanical and macromechanical properties of CoCrFeNiTa_0.43 eutectic high entropy alloy were investigated in detail. After aging at 750 °C for 250 h, Ni, Ta-rich γ''-Ni₃Ta phases with body-center tetragonal (BCT) structure precipitated from γ matrix. After aging, microhardness of CoCrFeNiTa_0.43 alloy increases ∼2.5%. Meanwhile, room temperature compression tests indicated that aged CoCrFeNiTa_0.43 alloy exhibited higher 0.2% yield strength than as-cast CoCrFeNiTa_0.43 alloy (increased from 1068 MPa to 1340 MPa, i.e. increasing 25.5%), and aged CoCrFeNiTa_0.43 alloy also maintained relatively high plasticity (with fracture strain>13%). The strengthening effect of γ''-Ni₃Ta phases was mainly attributed to Orowan bypass mechanism, and the contribution of γ''-Ni₃Ta phases to 0.2% yield strength was calculated to be 412.2 MPa by using the Orowan bypass model. Moreover, as compared with other eutectic high entropy alloys, aged CoCrFeNiTa_0.43 alloy exhibited a relatively high yield strength and relatively high fracture strain.