Equimolar (Zr _, Hf _, Ta)B ₂ Solid Solution With Superior Oxidation Resistance Compared to Individual and Binary Constituents

ABSTRACT The development of multicomponent diborides is driven by the expectation that combining different transition metals within the AlB 2 ‐type structure may yield properties superior to those of the individual constituents. In this framework, the fabrication and oxidation resistance of Zr–Hf–Ta equiatomic diborides are systematically investigated in this work to demonstrate synergistic effects and possible benefits arising when moving from unary to ternary systems. All materials are synthesized through a combined self‐propagating high‐temperature synthesis (SHS) and spark plasma sintering (SPS) approach, producing dense single‐phase diborides except for (Hf 0.5 Ta 0.5 )B 2 , which retained ∼6.3 wt.% of a Ta‐rich secondary phase. Oxidation tests show that the behavior of such ceramics strongly depends on the specific oxide phases formed during their exposure. Higher protection is obtained when the scale is dominated by stable, single‐phase, oxides such as HfO 2 and A 6 Ta 2 O 17 (A = Zr, Hf), while the formation of detrimental oxides (ZrO 2 , Ta 2 O 5 , TaO 2 ) is minimized. In particular, when the complex A 6 Ta 2 O 17 phase becomes the predominant oxidation product, as observed for the (Zr 1/3 Hf 1/3 Ta 1/3 )B 2 composition, the resulting oxide scale is compact, stable, and significantly more resistant to oxygen transport. The critical role of compositional design in enabling synergistic oxidation behavior is discussed.