Ultrahigh strength enabled by dual-scale L12 precipitation and back-stress strengthening in an LPBF-fabricated (CoCrNi)94Al3Ti3 medium-entropy alloy

Simultaneously achieving ultrahigh strength and useful tensile ductility in precipitation-strengthened medium-entropy alloys requires control over not only precipitate characteristics but also the recovery and recrystallization of metastable additively manufactured microstructures. In this study, we utilize the nonequilibrium microstructure inherited from laser powder bed fusion (LPBF) to regulate precipitation, local recrystallization, and mechanical heterogeneity in an (CoCrNi) 94 Al 3 Ti 3 medium-entropy alloy. The as-printed cellular substructures, high dislocation density, and solute segregation provide favorable sites and stored energy for subsequent microstructural evolution. Aging at 700 °C for 4 h produces a heterogeneous microstructure consisting of recrystallized fine-grain along with parent microstructure, and dual-scale L1 2 precipitates, resulting in a yield strength of approximately 1237 MPa, an ultimate tensile strength of 1565 MPa, and a total elongation of 12.3%. Fine spherical L1 2 precipitates are sheared by dislocations, whereas coarse rod-like L1 2 precipitates impede dislocation motion through bypassing and pile-up. Meanwhile, loading–unloading–reloading measurements yield an estimated average back-stress of approximately 971 MPa over the analyzed strain range. The back-stress response may be associated with deformation incompatibility within the evolving microstructure and may contribute to the flow stress during plastic deformation. These findings demonstrate that aging can transform the LPBF-inherited nonequilibrium microstructure into a precipitation-strengthened heterogeneous architecture, enabling ultrahigh strength while retaining useful tensile ductility.