Influence of oxygen partial pressure on the oxidation behaviour of TiAl at 1300 K

The isothermal oxidation behaviour of γ-TiAl (Ti-39 mass % Al) coupons has been studied at 1300 K in a flow of purified O2, mixtures of Ar and O2 or Ar under atmospheric pressure to clarify the influence of oxygen partial pressure. The oxidation kinetics was followed by thermogravimetry and conventional metallographic examinations were performed for the specimens oxidised under specified conditions. The oxidation curve in O2 is the lowest and it becomes higher as the oxygen partial pressure decreases. Even in the Ar atmosphere which contains very small amounts of nitrogen and water vapour as impurity the oxidation rate is considerably high. During the heating to the test temperature and initial period of oxidation thin and flat scales consisting mainly of Al2O3 are formed. However, as the oxidation proceeds they become convoluted and TiO2 crystals grow at their roots where cracking took place, resulting in the formation of oxide mounds. The oxide mound consists of an outer TiO2 layer and a porous inner layer which is a mixture of TiO2 and Al2O3 grains. As the oxygen partial pressure decreases the transition from flat scale to oxide mound takes place earlier. The observed variation is attributable to change in mechanical nature of the initially formed Al2O3 scale which may depend on the oxygen partial pressure.