Achieving strength-ductility synergy in CoCrNi medium-entropy alloys via multi-step annealing induced heterostructuring

Enhancing strength usually comes at the expense of ductility, a trade-off commonly referred to as the strength–ductility dilemma in metallic alloys. In this work, we overcome this limitation by tailoring a thermally induced heterogeneous structure in the FCC CoCrNi MEA through a fine heat treatment design. This approach results in a cross-scale microstructure comprising fine-grained (FG), ultrafine-grained (UFG), and nanotwin (NT) bundles. The multi-step annealed sample exhibits a high yield strength of 1025 MPa and a uniform elongation of 20 %. Compared to single-step annealing, this sample retains over 95 % of its strength while exhibiting a 100 % increase in ductility. These optimized mechanical properties are attributed to the temperature-time gradient design, which promotes the formation of a tri-modal heterostructure, composed of FG grains (∼2.1 μm), UFG grains (∼0.6 μm), and NT bundle lamellae (λ ~ 38 nm). In particular, the formation of annealing twins in the FCC matrix increases the density of heterogeneous interfaces, which act as effective barriers to dislocation slip. This, in turn, synergizing with HDI hardening, defect-induced activation, and interactive strengthening mechanisms, collectively enabling a stable work-hardening rate across a wide range of strains.