Early stage of high temperature oxidation of TiAl and FeAl at low oxygen partial pressures

The oxidation behavior of Ti-50 mol%Al and Fe-40 mol%Al at high temperatures and at low oxygen partial pressures was investigated mainly by means of X-ray photoelectron spectroscopy and scanning electron microscopy. The layer structure consisting of the outer TiO2 layer and the inner TiO and Al2O3 mixed layer was formed on the surface in the early stage of oxidation of TiAl in air, The existence of TiO was manifested in the oxide products of TiAl. At low oxygen partial pressures, the stacking of such layers was no longer present, but instead there was the mixture of Ti-oxides and Al2O3 on the surface, without forming any protective Al2O3 layer against oxidation. The grain size of the oxides formed on TiAl increased with increasing temperature and with decreasing oxygen partial pressure. The layer structure consisting of the outer Fe2O3 layer and the inner Fe3O4 and Al2O3 layer was formed in the early stage of oxidation of FeAl in air. In this case, the inner Al2O3 layer acted to be a protection layer against the oxidation. On the other hand, only Al2O3 was observed on the surface of FeAl oxidized at low partial oxygen pressures and high temperatures except 1473 K. At 1473 K, a little Fe3O4 was also observed on the Al2O3 layer. The oxide morphology in FeAl did not change with the oxidation conditions so largely as that in TiAl.