Superhard Mixed Transition Metal Dodecaborides.

Solid solutions of mixed metal dodecaborides of ZrB12, YB12, and ScB12 were prepared by arc-melting and studied for their mechanical properties. Zr1–xYxB12 formed an essentially perfect solid solution, closely following Vegard’s law. Zr1–xScxB12 and Y1–xScxB12 undergo a face centered-cubic to body-centered tetragonal transition at 90–95 at. % Sc as determined by powder X-ray diffraction and transmission electron microscopy. The compounds Zr0.5Y0.5B12, Zr0.5Sc0.5B12, and Y0.5Sc0.5B12 are superhard (Vickers hardness ≥ 40 GPa) and demonstrate an increase in hardness to 45.8 ± 1.3, 48.0 ± 2.1, and 45.2 ± 2.1 GPa under a load of 0.49 N, respectively, compared to 40.4 ± 1.8, 40.9 ± 1.6, and 41.7 ± 2.2 GPa for pure ZrB12, YB12, and ScB12, respectively. In addition, Zr0.5Y0.5B12, Zr0.5Sc0.5B12, and Y0.5Sc0.5B12 solid solutions show a substantial increase in oxidation resistance to approximately 630, 685, and 695 °C, respectively, when compared to other superhard metal borides (e.g., ∼400 °C for WB4) and their all...