Scale composition and oxidation mechanism of the Ti–46Al–8Nb alloy in air at 700 and 800 °C

Abstract

It is known that the oxide scale formed on TiAl alloys is generally composed of a mixture of alumina (Al 2O 3) and titania (TiO 2). The presence of niobium changes the activities of Ti and Al and influences the kinetics of oxidation and oxide layer composition. In this work, the Ti–46Al–8Nb alloy was subjected to cyclic oxidation in air at 700 °C (for 2 and 24 h) and 800 °C (for 300 h). Scale composition was analyzed by means of different techniques including X-ray photoelectron spectroscopy, X-ray diffraction and secondary ion mass spectroscopy. The scale consisted of several layers. The outer layer was built of alumina (amorphous or with very fine grains), whereas the inner layer – mainly of titania. After a longer exposure at a higher temperature (800 °C), niobium-rich precipitates and aluminum oxide grains were detected near to the alloy/scale interface and titanium nitride was found in the inner parts of the scale. Oxidation mechanism was studied by two-stage oxidation method using oxygen-18 and oxygen-16 isotopes combined with SIMS analyses. The distribution of oxygen isotopes over the alloy/scale interface indicated mixed inward/outward diffusion at the of reacting species. The experiments using Au markers showed that after longer oxidation time the inward diffusion was a predominant transport process.