Dual precipitates and heterogeneous fine-grain structure induced strength-ductility synergy in a CoCrNi-based medium-entropy alloy

A novel (CoCrNi)93.5Al3Ti3C0.5 medium-entropy alloy was designed, in this work, based on the equiatomic surplus-ductility carrier to achieve multi-coupling strengthening. Dual precipitates of M23C6 carbides and L12 ordered phase were co-introduced to not only play strengtheners, but also play constructors for the grain architecture. A heterogeneous fine-grain structure (HGS) was produced under the interactions of the simultaneous precipitation and recrystallization behaviors. It consisted of the core-containing heterogeneous microdomains (CHMs) which acted as the functional motif with a continuous, yet uniform, spatial distribution. An excellent mechanical property combination of a yield strength of 1425 MPa, ultimate tensile strength of 1516 MPa, and a total elongation of 16% was achieved in the current alloy. It was revealed that the hetero-deformation induced (HDI) hardening and twofold precipitation hardening were primarily responsible for the high strength. Furthermore, the coupling deformation mechanisms of the studied alloy were unveiled, involving of the micro-scaled continuous hetero-deformation and the nano-scaled stacking-fault-mediated planar slip combining with coherency-induced shearing mechanism. These jointly contributed to the continuous strain partition, local stress relaxation, extra strain-hardening, and thereby the appreciable elongation. In addition, we demonstrated that the higher HDI hardenability dependent closely on the favorable CHM-HGS configuration factors. This study provides meaningful insights into the collaborative architecture between intermetallic compounds and grain structure, diversifying the microstructural design strategy of metallic materials.