A new class of high-entropy M3B4 borides

Abstract A new class of high-entropy M 3 B 4 borides of the Ta 3 B 4 -prototyped orthorhombic structure has been synthesized in the bulk form for the first time. Specimens with compositions of (V 0.2 Cr 0.2 Nb 0.2 Mo 0.2 Ta 0.2 ) 3 B 4 and (V 0.2 Cr 0.2 Nb 0.2 Ta 0.2 W 0.2 ) 3 B 4 were fabricated via reactive spark plasma sintering of high-energy-ball-milled elemental boron and metal precursors. The sintered specimens were ∼98.7% in relative densities with virtually no oxide contamination, albeit the presence of minor (4–5 vol%) secondary high-entropy M 5 B 6 phases. Despite that Mo 3 B 4 or W 3 B 4 are not stable phase, 20% of Mo 3 B 4 and W 3 B 4 can be stabilized into the high-entropy M 3 B 4 borides. Vickers hardness was measured to be 18.6 and 19.8 GPa at a standard load of 9.8 N. This work has further expanded the family of different structures of high-entropy ceramics reported to date.