Microstructure and mechanical properties of the as-cast and annealed CoNiV-based MEAs with Al addition

Lattice distortion, which is intended to increase the yield strengths of alloys, is central to alloy design. This study focuses on investigating the close connection between microstructure and mechanical properties of (CoNiV)₉₇Al₃ medium-entropy alloys (MEAs) in the as-cast and annealed states. The grain size and phase composition of the alloys were optimized by combining the large deformation cold-rolling and high temperature annealing treatment, and the toughening mechanism of the alloys was evolved. After annealing at 900 and 950°C, a small amount of diffuse B2 phase appears within the alloy, which is able to strengthen the alloy to obtain high yield strength. Since intermetallic compounds such as Ni3Al are extremely unstable, they will decompose during annealing at higher temperature (1000°C), the alloy transforms into a single FCC phase. Therefore, the as-cast and annealed (CoNiV)₉₇Al₃ alloy exhibited excellent mechanical properties, including yield strengths of 0.8–1.1 GPa, ultimate tensile strengths of 1.05–1.7 GPa, and an optimal elongation of 36.7 %. Combined with the mechanical properties and microstructure of the as-cast and annealed (CoNiV)₉₇Al₃ MEAs, the alloy strengthening was mainly attributed to the synergistic effect of dislocations, solid solution phases, and B2 brittle precipitated phases, while the good ductility was mainly due to the annealing twins. Thus, harmonizing multiple strength/toughness mechanisms can lead to a more optimal alloy strength–plasticity equilibrium point.