High Entropy Borides under Extreme of Pressure and Temperatures*

High-entropy materials represent a paradigm shift in materials science where five or more constituent elements are incorporated in a variety of alloys, oxides, nitrides, and borides. The high-entropy materials are thermodynamically stable at high temperatures and provide tunability in physical and mechanical properties that is only possible in compositionally complex systems. We have synthesized a series of high-entropy transition metal borides, e.g., (HfMoNbTaZr)B 10 at high-pressures and high-temperatures starting from a ball-milled metal oxide precursors and boron powder. A single hexagonal AlB 2 -type phase of (HfMoNbTaZr)B 10 has been synthesized and studied to 10 GPa and 2273 K in a Paris-Edinburgh press. The synthesized materials are recovered and studied in a diamond anvil cell by both axial and radial diffraction techniques. The hexagonal AlB 2 -type phase of (HfMoNbTaZr)B 10 is stable to 220 GPa pressure (30% compressions). The hardness, shear strength, and thermal oxidation resistance data will be presented for use of these materials in hypersonic applications.