On the phase constituents of four CoCrFeNiX (X = Y, Ti, Zr, Hf) high-entropy alloys after prolonged annealing

Despite the large amount of literature on high entropy alloys (HEAs) in recent years, reports on the equilibrium/near-equilibrium phase and microstructure of HEAs are still very limited. Here, we investigate the phase constituents of four CoCrFeNiX (X = Y, Ti, Zr, Hf) HEAs after prolonged annealing. It is found that the alloys typically contain two major phases - a dominating intermetallic (IM) phase and a BCC phase. The major phases in general remain unchanged regardless of annealing temperature. Annealing at lower temperatures, however, typically leads to the formation of new minor phases. In particular, the formation of sigma phase during annealing is observed in all the alloys. Existing phase formation theories are applied to the alloys. The theories successfully predict that the present alloys do not form simple solid solution phases only. Phase-specific stability criteria also explain the prevalence of Laves and sigma phases quite well. (C) 2020 The Authors. Published by Elsevier B.V.