On the phase constituents of three CoCrFeNiX (X = V, Nb, Ta) high-entropy alloys after prolonged annealing

In this work, the phase constituents, microstructures, and phase compositions of three equimolar CoCrFeNiX (X = V, Nb, Ta) high-entropy alloys (HEAs) after prolonged annealing were investigated. The results show that all the alloys are composed of two major phases - a dominating intermetallic (IM) phase and an FCC phase. These major phases do not change with annealing temperature. However, annealing at 500-900 degrees C can lead to the formation of new minor phases [i.e. mu, sigma, (Ni, Co)(3)Nb and (Ni, Co)(3)Ta phases]. Existing phase formation theories were applied to the alloys. It was found that simple solid solution (SSS) HEA prediction models can predict IM formation in CoCrFeNiNb and CoCrFeNiTa, but failed to do so in the case of CoCrFeNiV. This is mainly due to the IM phase type in CoCrFeNiV. On the other hand, phase-specific formation criteria successfully predicted the stability of Laves and s phases in the alloys. The phase formation trends in the alloys were also compared and discussed. (C) 2019 Elsevier B.V. All rights reserved.