The initial stages of oxidation of γ-TiAl: an X-ray photoelectron study

Abstract The initial stages of oxidation of γ-TiAl were studied by means of X-ray photoelectron spectroscopy (XPS). The samples were oxidised at low oxygen partial pressure in situ in the XPS equipment. Oxidation temperatures and times ranged from 30 to 900 °C and from 2.5 to 90 min, respectively. Angle-resolved XPS measurements and sputter-depth profiling were used to analyse the depth dependence of the sample composition in the surface region. The different metallic and oxidic components of Ti 2p and Al 2p spectra were unravelled by dedicated peak fitting procedures. For all oxidation conditions applied aluminium oxide and low valence titanium oxides were observed. With increasing oxidation time, oxidation at 600 °C results in a growing oxide-layer thickness and an increasing valence for the titanium oxides developing at the surface. After prolonged oxidation, TiO2 is dominant at the sample surface; and a sublayer consisting of a mixture of lower valence titanium oxides and aluminium oxide is formed adjacent to the substrate. Increasing the oxidation temperature from 30 °C up to approximately 600 °C for a fixed oxidation time leads to an increasing oxide-layer thickness, whereas upon further increase of the oxidation temperature, i.e. above 600 °C, a much thinner oxide layer occurs that consists predominantly of Al2O3 and low valence titanium oxides in association with a significant dissolution of oxygen in the bulk.