Role of metal elements in oxidation resistance of high‐entropy diborides

Abstract The research on the oxidant behavior of high‐entropy diborides (HEBs) is essential to facilitate their application in extreme environments. In this work, (Ti 1/3 Zr 1/3 Hf 1/3 )B 2 , (Ti 1/3 Zr 1/3 Ta 1/3 )B 2 , (Ti 1/4 Zr 1/4 Hf 1/4 Ta 1/4 )B 2 , and (Ti 1/5 Zr 15 Hf 1/5 Ta 1/5 W 1/5 )B 2 were used for comparative study to reflect the effects of different elements on oxidation resistance, and ZrB 2 was used as a control. After oxidization at 1400°C for 30 min, the oxide layers thicknesses of ZrB 2 , (Ti 1/3 Zr 1/3 Hf 1/3 )B 2 , (Ti 1/3 Zr 1/3 Ta 1/3 )B 2 , (Ti 1/4 Zr 1/4 Hf 1/4 Ta 1/4 )B 2 , and (Ti 1/5 Zr 15 Hf 1/5 Ta 1/5 W 1/5 )B 2 were 380, 290, 250, 190, and 200 µm, respectively. The results showed that (Ti 1/4 Zr 1/4 Hf 1/4 Ta 1/4 )B 2 had the best antioxidant performance due to the formation of a special dense oxide layer. This was attributed to the formation of high‐entropy oxide solid‐phase skeleton dominated by Zr and Hf, which had good stability and was easy sintering. Liquid oxides of Ti had good mobility but low viscosity. Ta significantly improved the viscosity, mass transfer effect, and oxygen barrier ability of composite liquid through a synergistic effect. The escape of WO 3 destroyed dense scale lead to poor oxidation resistance of (Ti 1/5 Zr 15 Hf 1/5 Ta 1/5 W 1/5 )B 2 . The high‐entropy structure can strengthen solid and liquid phases, but not for all elements, and the introduction of destructive elements would reduce performance. This work proposed an optimization mechanism for the oxidation resistance of HEBs.