Dual enhancement in strength and ductility of Fe-rich medium-entropy alloys via an in situ formed heterogeneous multi-phase structure

In this work, we successfully achieved an exceptional strength-ductility synergy in the brittle Ni_0.6CoFe_1.4 medium-entropy alloy (MEA) via in situ formation of a heterogeneous structure by doping with Si. The newly developed Ni_0.6CoFe_1.4Si_x (x = 0.1, 0.2 and 0.3) MEAs exhibited a multi-phase heterogeneous structure consisting of face-centered cubic (FCC), body-centered cubic (BCC) and NiSi phases, with some B2 nanoparticles precipitated inside the BCC matrix. This multi-phase heterogeneous structure combined with precipitation hardening resulted in a dual enhancement in the strength and ductility of the alloys. Notably, the tensile strength and strain of the Ni_0.6CoFe_1.4Si_0.3 MEA were significantly increased to 1096.9 ± 39.0 MPa and 31.7 ± 0.3%, respectively, which were about 39.9% and 456.1% higher than those of the Ni_0.6CoFe_1.4 base alloy. Furthermore, density functional theory (DFT) results indicated that the charge redistribution caused by the addition of Si led to the presence of local charge enrichment and depletion regions, which facilitate plastic deformation and increase the ductility. The current study would provide valuable guidance to produce low-cost yet high-performance BCC-structured MEAs.