Abstract: Transition metal diborides (TMB_2s) are widely investigated for ultra-high-temperature applications due to their exceptionally high melting points, high strength, and good thermochemical stability. However, these ultrahigh temperature ceramics (UHTCs) are particularly vulnerable to high temperature oxidation during service. To address this issue, in this contribution, we pioneered the design of a novel Ta and Mo containing high entropy diboride with a nominal composition of (Ti_0.2Zr_0.2Hf_0.2Ta_0.2Mo_0.2)B₂ (HETMB₂) based on the defect chemistry theory. Surprisingly, during high-temperature oxidation, a dense and protective scale forms, which protects the HETMB₂ from further oxidation. To fully explore the oxidation protection mechanism, direct evidence of high valence Ta⁵⁺ substitution on the TM⁴⁺ site in TMO₂ (TM = Ti, Zr, Hf) lattice of the oxide scale was provided using atomic-resolution high-annular dark field (HAADF) and annular bright field (ABF) scanning transmission electron microscopy (STEM). The substitution of TM⁴⁺ with Ta⁵⁺ reduces the oxygen vacancy concentration and blocks the oxygen diffusion channels in TMO₂ (TM = Ti, Zr, Hf), which plays a crucial role in the formation of the protective oxide scale and the superior oxidation resistance of HETMB₂. Of particular interest, the molybdenum component in HETMB₂ preferentially forms high-melting-point Mo nanoparticles (≈ 2600 °C) embedded within the oxide scale, rather than undergoing oxidation to volatile MoO₃ by virtue of the low oxygen partial pressure in the oxide scale. Given that the formation of volatile MoO₃ poses a challenge to the long-standing paradigm regarding the poor oxidation susceptibility of molybdenum-based ceramics, concomitant incorporation of Ta and Mo in high entropy HETMB₂ holds promise to tackle this problem. The results of this work not only highlight the substantial advantages of Ta and Mo containing high-entropy diboride on the superior oxidation resistance but also open up a new avenue for the design of high-entropy boride and carbide ceramics with enhanced oxidation resistance.

Key words: High-entropy transition metal diborides, Oxidation behavior, Defect chemistry, Mo nanoparticles, Oxygen vacancy