TEM Observation of the Initial Stages of Oxidation on TiAl and TiAl-0.2Zr Intermetallic Compounds

The present study aims at clarifying the mechanism by which the oxidation resistance of TiAl intermetallic compound was much improved by a small addition of Zr. TiAl and TiAl-0.2 mass%Zr were oxidised at 1273 K in O2 of 0.013 Pa for up to 28.8 ks. The oxide scales and the layers beneath them were examined for the both specimens by TEM, EDS, AES, SEM and EPMA. The scale formed on the TiAl specimen consists mainly of a continuous α-Al2O3 layer (virtually Al2O3 scale) of varying thickness with a few TiO2 grains on it. An Al depleted layer is formed beneath the scale. With the progress of oxidation of TiAl, TiO2 grains are formed in the Al2O3 scale and grow. This was considered to be the onset of breakaway oxidation. The addition of Zr results in lower oxidation rates with the formation of Al2O3 scales of uniform thickness. Contrary to the TiAl specimen, there was no sign of formation of TiO2 grains in the scales, although a few TiO2 grains formed in the very initial stage remain on the scales. The Al-depleted layer found with the TiAl specimen is very similar to the Z-phase shown in the literature. However, the growth manner of this layer was much changed by the addition of Zr. The length to thickness ratio for TiAl is around unity and decreases gradually with time, while this ratio becomes significantly large for the TiAl-Zr specimen. The addition of Zr modifies the nature of the initially formed Al2O3 layer and the Al-depleted layer.