Quaternary additions of 2 at. pct of Ta or Zr were made to the ternary Ti-44Al-xNb (X ϭ 9 and 11) alloys to study the oxidation behavior at 900°C, 950°C, and 1000°C for a period of 1 week. The Ta addition improves the oxidation resistance, while it is degraded by Zr compared to the ternary alloys. Identification of the oxides formed in the scale has been characterized by energydispersive atomic X-ray (EDAX) in a scanning electron microscope (SEM). The transmission electron microscope (TEM) analysis of the microstructures developed during oxidation has been compared with Ti-44Al-xNb alloys in order to determine the influence of quaternary additions of Ta and Zr on the phase transformations taking place during the extended period of heating. The formation of spotty ␣ 2 in the isolated ␥ grains appears to be associated with the inferior oxidation resistance of xNb2Zr alloys.

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