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 sameextended γ + σ tie-line, was investigated at1200°C in air. The single-phase σ alloy suffered from extensive internal oxidation andoxidized at a much higher rate than the single-phaseγ alloy. In a two-phase γ + σmicrostructure, the γ phase was preferentiallyattacked to form “internal” alumina andTi-rich nitride. This preferential attack of γlimited the extent to which the σ phase wasinternally oxidized, but also interrupted the formationof a continuous alumina scale. The single-phase γ alloyalso did not form a continuous alumina scale. Theinability of the γ phase to form continuousalumina was attributed to a combination of nitrideformation and internal oxidation. The oxidation behaviorof the two-phase γ + σ Nb-Ti-Al alloys isdiscussed in terms of mechanisms developed for theoxidation of binary, two-phase alloys.