Oxidation behavior of two-phase γ+σNb-Ti-Al alloys

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