L2₁-strengthened face-centered cubic high-entropy alloy with high strength and ductility

Face-centered cubic (FCC) high-entropy alloys (HEAs) strengthened by coherent L1₂-nanoparticles exhibit an excellent strength-ductility balance. However, the strength of previously studied HEAs remains inadequate for the requirements of high-performance structural applications, due to their emphasis on coherency strengthening and the suppression of the formation of incoherent precipitates, which could substantially increase the alloy strength but also leads to significant brittleness. In this study, we propose to employ incoherent precipitates as additional strengthening phases to further improve the mechanical performance of conventional L1₂-strengthened FCC HEAs without dramatic loss of their ductility. We achieve this using a prototypical (FeCoNiCr)₈₉Ti₆Al₅ (at.%) HEA, in which high-density, fine, and incoherent L2₁ precipitates were introduced and uniformly distributed at the recrystallized grain boundaries through proper thermomechanical processes, including large cold deformation, full recrystallization, and aging heat treatment. A superb combination of strength and ductility in the alloy is confirmed by the uniaxial tensile tests, with a yield strength of 1136 MPa, an ultimate tensile strength of 1597 MPa, and a ductility of 25.3%. This superior mechanical response is caused by the synergistic contribution of the fine and uniformly distributed L2₁ particles and the ultra-ductile and damage-tolerant FeCoNiCr matrix, which are responsible for increasing strength and maintaining ductility, respectively. These findings demonstrate the viability of a new method of using incoherent precipitates to strengthen other FCC HEAs by properly tuning their particle size and distribution.