Co-free non-equilibrium medium-entropy alloy with outstanding tensile properties

In this work, we report a novel Co-free non-equiatomic Ni_57.6Cr_19.2Fe_19.2Nb₄ medium-entropy alloy (MEA) with outstanding tensile properties. We systematically investigate its microstructure, strengthening and deformation mechanisms combined by XRD, EBSD and TEM investigations. Experimental results demonstrate this Co-free MEA strengthened by nanosized coherent D0₂₂- (Ni, Cr, Fe)₃(Nb, Fe)-type precipitate (γ′′ phase) with disk shape. This γ′′-precipitation strengthened MEA exhibits the outstanding strength-ductility synergy, showing a high yield strength of ∼960 MPa, tensile strength of ∼1320 MPa and an excellent ductility of ∼34%. The nanosized γ′′ phases bought out effective precipitation strengthening in this Co-free MEA, contributing a majority yield strength enhancement of ∼660 MPa. For this γ′′-precipitation strengthened MEA, the dominant deformation model was the stacking-faults controlled mode, which resulted in a slight ductility sacrifice, in comparison with the solid solution treated MEA. Our work provides solid foundations for further development of high performance Co-free MEAs/(high-entropy alloys) HEAs.