A titanium aluminide alloy, Ti-46Al-8Nb (at %), with a lamellar structure (γ+α2) was tested for oxidation resistance in air at 700, 800, 850, and 900 °C in isothermal and thermal cycling conditions (1-hour and 20-hour hot-dwell time). The oxidation behaviour was assessed by means of mass change measurements as a function of time. The X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) were used to determine phase composition and chemical composition of the oxidation products. Surfaces of the oxide scales were examined by optical microscopy (OM) and scanning electron microscopy (SEM). Cross-sections of the selected samples were analysed using both secondary electron images and backscattered electron images (SEI and BEI) as well as transmission electron microscopy (TEM).
In isothermal tests the mass gains increased with temperature, the oxidation kinetics followed approximately a parabolic rate law. The calculated parabolic rate constants ranged from about 10-12 to 10-10 kg2/m4s. The oxide scales were built of Al2O3 and TiO2, the former being the main component of the outermost layer. The innermost layer contained oxygen and all three elements of the alloy with varying local concentrations. The oxidation behaviour of Ti-46Al-8Nb was compared with a commercial titanium alloy, WT4 (Ti-6Al-1Mn), and with selected oxidation-resistant alloys.