Synergistic L12 and B2 co-precipitation enabled by high Ti and Al additions for strength-ductility synergy in a heterostructured CoNiCr medium-entropy alloy

In this study, we designed a novel CoNiCr-based medium-entropy alloy (MEA) with high additions of Ti and Al, with a nominal composition of Co 40 Ni 30 Cr 15.9 Al 7 Ti 7 B 0.1. Through a proper thermomechanical processing (CAA), a partially recrystallized heterogeneous structure with coherent L1 2 and B2 precipitates is obtained in the CAA alloy. High-density L1 2 precipitates of ∼390 nm and B2 precipitates of ∼25 nm are homogeneously dispersed in the recrystallized zones, while L1 2 precipitates of ∼130 nm are uniformly distributed in the non-recrystallized zones. The volume fraction of L1 2 and B2 precipitates reaches ∼36.7% and ∼21.8%, respectively. The L1 2 phase is identified as (Ni,Co) 3 (Al,Ti), and the B2 nanoprecipitates as Ni(Al,Ti). The CAA alloy achieves a high yield strength of ∼1137 MPa, an ultimate tensile strength of ∼1544 MPa, and a good ductility of ∼14.5%, demonstrating a good strength-ductility combination. Notably, precipitation strengthening by L1 2 and B2 precipitates makes the largest contribution (∼66%) to the total yield strength. The good ductility stems from the deformation mechanisms of dislocation slip, microband formation, and stacking fault generation. Therefore, the strategy of adding high Ti and Al to CoNiCr-based alloys represents a promising research direction to achieve exceptional strength-ductility synergy in advanced materials, thereby stimulating broad research interest for future exploration.