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

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• 作者：Georgiy Akopov ; Michael T. Yeung ; Christopher L. Turner ; Reza Mohammadi ; Richard B. Kaner
• 刊名：Journal of the American Chemical Society
• 出版年：2016
• 出版时间：May 4, 2016
• 年：2016
• 卷：138
• 期：17
• 页码：5714-5721
• 全文大小：827K
• 年卷期：0
• ISSN：1520-5126

文摘

Alloys of tungsten tetraboride (WBb>4b>) 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 WBb>4b> alloys with 8 at. % Ti, 8 at. % Zr, and 6 at. % Hf gave hardness values, Hb>vb>, of 50.9 ± 2.2, 55.9 ± 2.7 and 51.6 ± 2.8 GPa, respectively, compared to 43.3 GPa for pure WBb>4b> under an applied load of 0.49 N. Each of the aforementioned compositions are considered superhard (Hb>vb> > 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 Wb>1–xb>Zrb>xb>Bb>4b> alloy. In addition, Wb>0.92b>Zrb>0.08b>Bb>4b>, a product of spinoidal decomposition, possesses nanostructured grains and enhanced grain hardening. The hardness of Wb>0.92b>Zrb>0.08b>Bb>4b> 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 WBb>4b> 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 WBb>4b>.