High-Entropy Metal Diborides: A New Class of High-Entropy Materials and a New Type of Ultrahigh Temperature Ceramics

Abstract

Seven equimolar, five-component, metal diborides were fabricated via high-energy ball milling and spark plasma sintering. Six of them, including (Hf 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2)B 2, (Hf 0.2 Zr 0.2 Ta 0.2 Mo 0.2 Ti 0.2)B 2, (Hf 0.2 Zr 0.2 Mo 0.2 Nb 0.2 Ti 0.2)B 2, (Hf 0.2 Mo 0.2 Ta 0.2 Nb 0.2 Ti 0.2)B 2, (Mo 0.2 Zr 0.2 Ta 0.2 Nb 0.2 Ti 0.2)B 2, and (Hf 0.2 Zr 0.2 Ta 0.2 Cr 0.2 Ti 0.2)B 2, possess virtually one solid-solution boride phase of the hexagonal AlB 2 structure. Revised Hume-Rothery size-difference factors are used to rationalize the formation of high-entropy solid solutions in these metal diborides. Greater than 92% of the theoretical densities have been generally achieved with largely uniform compositions from nanoscale to microscale. Aberration-corrected scanning transmission electron microscopy (AC STEM), with high-angle annular dark-field and annular bright-field (HAADF and ABF) imaging and nanoscale compositional mapping, has been conducted to confirm the formation of 2-D high-entropy metal layers, separated by rigid 2-D boron nets, without any detectable layered segregation along the c-axis. These materials represent a new type of ultra-high temperature ceramics (UHTCs) as well as a new class of high-entropy materials, which not only exemplify the first high-entropy non-oxide ceramics (borides) fabricated but also possess a unique non-cubic (hexagonal) and layered (quasi-2D) high-entropy crystal structure that markedly differs from all those reported in prior studies. Initial property assessments show that both the hardness and the oxidation resistance of these high-entropy metal diborides are generally higher/better than the average performances of five individual metal diborides made by identical fabrication processing.