On the phase constituents of three CoCrFeNiX (X = Cr, Mo, W) high-entropy alloys after prolonged annealing

Abstract

Equilibrium/near-equilibrium phase data is of great importance for the development and application of high entropy alloys. However, such results are still extremely limited. In this work, the phase constituent, microstructure and compositions of phases in three equimolar CoCrFeNiX (X = Cr, Mo, W) alloys after prolonged annealing were investigated. The as-cast alloys comprised two major phases. After homogenization, the BCC phase in CoCr2FeNi transformed to the sigma phase and the sigma phase in CoCrFeNiMo transformed to the R phase. Subsequent annealing at 500, 700 and 900 °C did not further change the major phases. Additionally, the formation of new minor phases during annealing is not as common as in other CoCrFeNiX alloys previously reported by the current authors. Existing phase formation theories were applied to the alloys and the results were almost entirely incorrect. This is associated with the type of intermetallic phases observed in these alloys – sigma and mu phases. The CALPHAD technique was also used to predict the phases in the alloys. The results show that the accuracy of CALPHAD prediction is directly associated with the fraction of assessed systems in the database. Together with previous and future parts of this systematic study, which investigated 22 CoCrFeNiX alloys with different X elements in total, we expect that the dataset can offer the foundation for new analyses and interpretations when looked at in its totality.