Observing the formation of high entropy metal diborides and studying the influence of single boride in the solid solution using arc-melting as consolidation technique

In the last few years, processing of high entropy metal diborides has been studied by many researchers, mainly through spark plasma sintering or hot pressing at temperatures over 2000°C. The biggest challenge is the formation of a homogeneous solid solution, and prediction of the multiple combinations that are more likely to result in a single phase. Arc-melting is a ultra-fast consolidation method, that allows to reach temperatures well above the melting point (>3000 °C) of UHTCs. In this work, arc-melting was used to directly synthesize/sinter high entropy metal diborides mixing HfB2, ZrB2, TiB2 TaB2 and CrB2. In order to understand the influence of each individual boride, 5 equimolar combinations of 4 out of 5 borides were studied; e.g Hf-Zr-Ti-Ta, Hf-Zr-Ti-Cr, Hf-Zr-Ta-Cr, Hf-Ti-Ta-Cr, Zr-Ti-Ta-Cr. Arc-melting allowed a fast discrimination of favorable and unfavorable combinations. For instance, Cr was hardly found in solid solution, as it preferably segregated at the grain boundaries. Vickers hardness and oxidation tests at 1600°C in air were carried out and results were compared with conventional monophasic borides.