Extrinsic Hardening of Superhard Tungsten Tetraboride Alloys with Group 4 Transition Metals

Alloys of tungsten tetraboride (WB₄) with the group 4 transition metals, titanium (Ti), zirconium (Zr), and hafnium (Hf), of different concentrations (0–50 at. % on a metals basis) were synthesized by arc-melting in order to study their mechanical properties. The phase composition and purity of the as-synthesized samples were confirmed using powder X-ray diffraction (PXRD) and energy dispersive X-ray spectroscopy (EDS). The solubility limit as determined by PXRD is 20 at. % for Ti, 10 at. % for Zr, and 8 at. % for Hf. Vickers indentation measurements of WB₄ alloys with 8 at. % Ti, 8 at. % Zr, and 6 at. % Hf gave hardness values, H_v, of 50.9 ± 2.2, 55.9 ± 2.7 and 51.6 ± 2.8 GPa, respectively, compared to 43.3 GPa for pure WB₄ under an applied load of 0.49 N. Each of the aforementioned compositions are considered superhard (H_v > 40 GPa), likely due to extrinsic hardening that plays a key role in these superhard metal borides. Furthermore, these materials exhibit a significantly reduced indentation size effect, which can be seen in the plateauing hardness values for the W_1–xZr_xB₄ alloy. In addition, W_0.92Zr_0.08B₄, a product of spinoidal decomposition, possesses nanostructured grains and enhanced grain hardening. The hardness of W_0.92Zr_0.08B₄ is 34.7 ± 0.65 GPa under an applied load of 4.9 N, the highest value obtained for any superhard metal at this relatively high loading. In addition, the WB₄ alloys with Ti, Zr, and Hf showed a substantially increased oxidation resistance up to ∼460 °C, ∼510 °C, and ∼490 °C, respectively, compared to ∼400 °C for pure WB₄.