XPS study of the initial stages of oxidation of α2-Ti3Al and γ-TiAl intermetallic alloys

The initial stages of oxidation of α2-Ti3Al and γ-TiAl alloys at 650 °C under low oxygen pressure are shown to be characterized by three stages using XPS. Stage I is a pre-oxidation stage characterized by the adsorption and absorption of oxygen species. When the subsurface is saturated with dissolved oxygen (16 and 2 at.% on α2-Ti3Al and γ-TiAl, respectively), the selective oxidation of the alloy leads to the nucleation and growth of ultrathin (∼0.5 and ∼1.2 nm on α2-Ti3Al and γ-TiAl, respectively) alumina layers (stage II). The growth kinetics of the alumina layers is limited by the transport of Al in the alloy, leading to Al-depletion in the metallic phase underneath the oxide. When a critical concentration is reached (Ti82Al18 and Ti75Al25 on α2-Ti3Al and γ-TiAl, respectively), titanium oxidation occurs (stage III). Ti(III) and Ti(IV) oxide particles are formed at the surface of the still growing alumina layer.