Effect of heat treatment on the structure and hardness of high-entropy alloys CoCrFeNiMnVx (x = 0.25, 0.5, 0.75, 1)

High-entropy alloys CoCrFeNiMnVKharkov Institute of Physics and Technology, ul. Akademicheskaya 1, Kharkov 61108 (Kharkov Institute of Physics and Technology, ul. Akademicheskaya 1, Kharkov 61108 = 0.25, 0.5, 0.75, 1) were prepared by vacuum arc melting. The structure and microhardness of the alloys have been studied in the cast state and after annealing at temperatures of 700–1100°C. It has been found that the alloys consist of the fcc (γ) solid solution and intermetallic sigma (σ) phase. The volume fraction of the σ phase increases with increasing vanadium content. As a result of annealing, phase transformations occur, including the precipitation of σ particles from the γ phase and, vice versa, the precipitation of γ particles from the σ phase. It has been shown that the change in the volume fraction of the σ phase upon annealing occurs due to the changes in the total content of σ-forming elements, chromium and vanadium, in accordance with the lever rule. With increasing temperature, the volume fraction of the σ phase varies nonmonotonically; first, it increases, then it decreases. The microhardness of the alloys correlates well with the change in the volume fraction of the σ phase. The mechanisms of the phase transformations and quantitative relationships between chemical and phase compositions of the alloys and their hardness are discussed.