Oxidation behavior of two-phase γ+σ Nb-Ti-Al alloys

The oxidation behavior of a series of single-and two-phase Nb-Ti-Al alloys, selected from the same extended γ+σ tie-line, was investigated at 1200 °C in air. The single-phase σ alloy suffered from extensive internal oxidation and oxidized at a much higher rate than the single-phase γ alloy. In a two-phase γ+σ microstructure, the γ phase was preferentially attacked to form `internal' alumina and Ti-rich nitride. This preferential attack of γ limited the extent to which the σ phase was internally oxidized, but also interrupted the formation of a continuous alumina scale. The single-phase γ alloy also did not form a continuous alumina scale. The inability of the γ phase to form continuous alumina was attributed to a combination of nitride formation and internal oxidation. The oxidation behavior of the two-phase γ+σ Nb-Ti-Al alloys is discussed in terms of mechanisms developed for the oxidation of binary, two-phase alloys.