  Five single-phase WB2- and MoB2-containing high-entropy borides (HEBs) have
been made via reactive spark plasma sintering of elemental boron and metals. A
large reactive driving force enables the full dissolution of 10-20 mol. % WB2
to form dense, single-phase HEBs, including (Ti0.2Zr0.2Hf0.2Mo0.2W0.2)B2,
(Ti0.2Ta0.2Cr0.2Mo0.2W0.2)B2, (Zr0.2Hf0.2Nb0.2Ta0.2W0.2)B2, and
(Zr0.225Hf0.225Ta0.225Mo0.225W0.1)B2; the successful fabrication of such
single-phase WB2-containing HEBs has not been reported before. In the
processing science, this result serves perhaps the best example demonstrating
that the phase formation in high-entropy ceramics can strongly depend on the
kinetic route. A scientifically interesting finding is that HEBs containing
softer WB2 and/or MoB2 components are significantly harder than
(Ti0.2Zr0.2Hf0.2Nb0.2Ta0.2)B2 (with harder binary boride components). This
exemplifies that high-entropy ceramics can achieve unexpected properties.
