Improved Strength–Ductility Synergy of a CoCrNi Medium-Entropy Alloy by Ex Situ TiN Nanoparticles

Abstract

The introduction of ex situ reinforcement particles to increase the strength of alloys generally reduces ductility. Herein, a method to fabricate CoCrNi/TiN composite via spark plasma sintering (SPS) and rolling and annealing to achieve a superior combination of strength and ductility is presented. Under the as-SPSed condition, the CoCrNi/TiN composites exhibit the fracture strain of 41.9%, yield strength (YS) of 0.48 GPa, ultimate tensile strength (UTS) of 0.88 GPa, and hardness of 232.0 Hv. After rolling at 25 °C for the thickness reduction of 50%, the alloy presents fracture strain of 6.9%, YS of 1.24 GPa, UTS of 1.41 GPa, and hardness (408.9 Hv). After rolling at 25 °C for the thickness reduction of 50%, and annealing at 700 °C for 1 h, a good combination of YS of 0.77 GPa, UTS of 1.01 GPa, and fractured strain of 55.2% can be obtained in the samples. The superior strength–ductility synergy can be attributed to the refined structure, the formation of lattices defects (i.e., stacking faults [SFs] and Lomer–Cottrell Locks (LCs)), the interaction of nanotwin–TiN particles, and the concurrent process of potential grain boundary sliding accommodated by intragranular dislocation in the softer face-centered cubic (fcc) matrix.