Tailored hierarchical L1₂ nano-precipitates achieve superior strength-ductility synergy in additively manufactured CoCrNi-based medium entropy alloys

A gain in strength is normally accompanied by the sacrifice of ductility in metallic materials, which has long plagued the development of advanced materials. New metallurgical processing and structural design strategies might offer the possibility to overcome this. Here, we controllably introduced novel hierarchical nano-precipitates in the medium-entropy alloy (MEA) CoCrNi to achieve strength enhancement without loss of ductility. Alternating in situ introduction of Al and Ti into the CoCrNi during the additive manufacturing process by pre-alloyed powders yielded a stepwise heterogeneous composition distribution in the fabricated MEA and then proper post-fabrication heat treatment was adopted to achieve hierarchical L1₂ nano-precipitates. Compared with the as-built CoCrNi, the ultimate tensile strength of the heterogeneous MEA with hierarchical L1₂ nano-precipitate (1186 MPa) is ∼ 2 times higher and the ductility (42%) is further improved. The diffusion region, the various deformation and hardening mechanisms evoked by the interaction of hetero-regions during deformation, are key to achieving excellent combination of strength and ductility. This study provides a promising paradigm for the design and preparation of strong yet ductile metallic materials by deliberate control of microstructural heterogeneity in additive manufacturing combined with suitable post-fabrication heat treatment.