Oxidation synthesis of Hf₆Ta₂O₁₇ superstructures

The microstructure evolution and phase formation sequence has been examined during the initial oxidation exposure of Hf-Ta alloys at 1500 °C for several alloy compositions. The oxidation reaction path involves the conversion of the initial β Hf-Ta BCC solid solution substrate phase into a two-phase monoclinic HfO₂ and a Ta rich BCC solid solution reaction layer that undergoes a further reaction between HfO₂ and Ta oxide to yield and outer Hf₆Ta₂O₁₇ superstructure. The overall reactive diffusion pathway is consistent with the calculated Hf-Ta-O ternary phase diagram. The oxidation reaction rate exhibited a minimum at Hf 26.7 at.%Ta where the outer oxide was single phase Hf₆Ta₂O_17. For other compositions the outer oxide was composed of Hf₆Ta₂O₁₇ and either HfO₂ (Hf-rich compositions) or Ta₂O₅ (Ta-rich compositions). The enhanced oxidation resistance for the Hf-26.7 at.%Ta alloy is attributed to the adherent Hf₆Ta₂O₁₇ superstructure.