Formation of high entropy metal diborides using arc-melting and combinatorial approach to study quinary and quaternary solid solutions

High entropy metal diborides (HEBs) represent a radically new approach to extend the chemical composition window of ultra-high temperature ceramics (UHTCs). In this work, arc-melting was used to produce dense HEBs starting from UHTC powders. In order to understand the influence of each individual diboride within the quinary system (HfB, ZrB, TiB, TaB and CrB), we investigated five quaternary equimolar solid solutions e.g. Hf-Zr-Ti-Ta, Hf-Zr-Ti-Cr, Hf-Zr-Ta-Cr, Hf-Ti-Ta-Cr, Zr-Ti-Ta-Cr and the overall quinary equimolar combination. Arc-melting allowed a rapid screening of favorable and unfavorable combinations. The produced HEBs were free from undesired oxides and characterized by linear variation of lattice parameters typical of diborides and binary solid solutions. Because of evaporation during arc melting, CrB was hardly found in the solid solution, suggesting that vapor pressure should be taken into account when designing HEB compositions especially for operating temperatures exceeding 2000 °C. Finally, Vickers microhardness ranged between the typical values of starting diborides.