Phase equilibria of VCrMnFeCo high entropy alloys

The almost infinite compositional space is one of the most attractive aspects of High Entropy Alloys (HEAs), but finding promising compositions is usually a great challenge. The most studied families of HEAs include the 3d transition metals, in which promising face-centered cubic alloys have been found, but few single-phase body-centered cubic (BCC) alloys were designed and produced with these elements. In the present work, a systematic exploratory study was performed within the VCrMnFeCo system, a vast compositional space currently underexplored and prone to form BCC solid solutions. It is shown that the brittle and usually undesirable sigma phase is often observed in this HEA family and, therefore, predicting its formation is essential in alloy design. The phase equilibria were studied in a wide range of compositions by the CALPHAD method and the combination of two empirical methods proposed by Tsai et al. (2013; 2016) for predicting the sigma phase. Six compositions were produced and characterized in the as-cast and annealed condition (1150 ºC, 4 h), namely V42Cr41Mn17, VCrFe, VCrMnFe, V37Mn25Fe38, VCrMnCo, and VCrMnFeCo. The first three alloys presented a single-phase BCC microstructure, while the others were sigma dominant. The CALPHAD and Tsai criteria disagreed on some predictions, and both were partially accurate when compared to the experimental characterization. Considerations on alloy design of 3d transition metal HEAs were discussed, as well as the advantages and limitations of these predictive methods when applied in the proposed system. The simultaneous application of CALPHAD and Tsai criteria is recommended for the sigma-prone 3d transition metal HEAs. This work collaborates to a better understanding of the dimension of compositional fields in 3d transition metal HEAs.