Enhanced strength-ductility synergy in medium entropy alloy via phase selective precipitation

Abstract

Precipitation strengthening is paramount in the development of high-performance medium/high entropy alloys (M/HEAs). In this work, we showcase a phase-selective precipitation design applied to a (Ni_67.2V_32.8)₉₀Ti₅Al₅ MEA to enable enhanced strength-ductility synergy. Upon annealing at 950 °C, multiple precipitates form in this MEA, including L2₁, σ and hexagonal close packed (HCP) phases. However, an increase of 50 °C in annealing temperature removes most of the aforementioned precipitates except for the L2₁ phase. Density functional theory calculations are conducted to elucidate the formation mechanisms of phase-selective precipitation. Such selective approach to precipitation induces a brittle to ductile transition, increasing tensile elongation from 4 % to 43 % in our MEAs. Remarkably, the ultimate tensile strength of 1000 °C annealing MEA is maintained at ∼1.4 GPa, surpassing that of the precipitation-free Ni_67.2V_32.8 base alloy (∼1.1 GPa), but with a comparable tensile elongation. Analytical models suggest that the increase in strength is attributed to both precipitation strengthening and grain refinement strengthening due to the pinning effect of precipitates. In particular, we investigate the complex deformation response of the L2₁ phase, which includes the formation of slip steps and a phase transformation from body-centered cubic (BCC) to body-centered tetragonal (BCT) structures, with the underlying mechanisms revealed through experimental characterization and molecular dynamics simulations. This co-deformation of matrix and L2₁ precipitates alleviates stress concentration at phase boundaries during straining and further maintains the microband-induced plasticity in the matrix till later deformation stage. All these result in the excellent strain hardening and thus, markedly enhancing ductility. Our findings pave new ways to craft strong and ductile M/HEAs by selecting hard-yet-deformable intermetallic precipitates.