Effects of Dodecaboride-Forming Metals on the Properties of Superhard Tungsten Tetraboride

Tungsten tetraboride alloys with Y, Sc, Gd, Tb, Dy, Ho, and Er were prepared by arc-melting and investigated for their thermal stability and mechanical properties. The phase composition and purity were confirmed by powder X-ray diffraction (PXRD) and energy dispersive X-ray spectroscopy (EDS). In all cases, except for Sc, a change to a "dendritic" morphology was observed. For alloys with Sc, the metal boride and boron grains became smaller; however, no patterning was observed. This shows that tuning the composition of the alloys of WB₄ with transition metals and lanthanides results in a modification of the morphology leading to patterning and smaller grains that enhance mechanical and thermal properties. For alloys of WB₄ with Y, Sc, Gd, Dy, Tb, Ho, and Er, an increase in hardness to 50.2 ± 2.4, 48.9 ± 2.5, 48.0 ± 2.1, 46.3 ± 2.6, 48.5 ± 2.3, 46.4 ± 3.4, and 47.2 ± 2.9 GPa at low load, respectively, compared to 41.2 ± 1.4 GPa for WB₄ is observed. Moreover, the alloys of WB₄ with Y, Sc, and Gd showed an increase in oxidation resistance from ∼450 °C for WB₄ to ∼470, ∼525, and 465 °C, respectively.