Improving ductility by coherent nanoprecipitates in medium entropy alloy

Controlling precipitates in spatial density and size distribution is essential for tailoring the microstructure and mechanical properties through precipitation hardening. We herein obtained heterogeneous grain structures with coherent L12 nanoprecipitates in (CrCoNi)94Al4Ti2 medium entropy alloy (MEA) by annealing and aging. Additional pre-aging leads to a high spatial density and more random distribution nucleation sites of the coherent L12 nanoprecipitates. The preaging doubled the ductility without apparently sacrificing the strength. Transmission electron microscopy (TEM) revealed that, in pre-aged MEA, finely dispersed L12 nanoprecipitates with higher spatial density were sheared by dislocation, promoting planar slips, which favors geometrically necessary dislocations (GNDs) piling up to increased hetero-deformation-induced (HDI) stress and work-hardening. Stacking faults, Lomer-Cottrell locks, and 9R structures were formed in aged and pre-aged MEA after tensile deformation. The formation of these defects enormously enhanced strain hardening by blocking dislocation movements and accumulating dislocations. Moreover, a higher frequency of interactions between defects and coherent L12 nanoprecipitates can be observed in the pre-aged MEA due to the more randomly distributed L12 nanoprecipitates, substantially increasing ductility. This work demonstrates a new route to achieving a super strength-ductility combination of single-phase FCC high entropy alloys by nanoscale coherent precipitation strengthening.