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.