Effect of heat treatment on the structure and hardness of high-entropy alloys CoCrFeNiMnV x (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 700-1100A degrees C. It has been found that the alloys consist of the fcc (gamma) solid solution and intermetallic sigma (sigma) phase. The volume fraction of the sigma phase increases with increasing vanadium content. As a result of annealing, phase transformations occur, including the precipitation of sigma particles from the gamma phase and, vice versa, the precipitation of gamma particles from the sigma phase. It has been shown that the change in the volume fraction of the sigma phase upon annealing occurs due to the changes in the total content of sigma-forming elements, chromium and vanadium, in accordance with the lever rule. With increasing temperature, the volume fraction of the sigma 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 sigma phase. The mechanisms of the phase transformations and quantitative relationships between chemical and phase compositions of the alloys and their hardness are discussed.