Oxidation of TiAl₃ and L1₂ Coatings on Ti-45Al-5Nb Alloy at 1173K

Abstract

Oxidation behavior of TiAl₃ and L1₂ coated Ti-45at%Al-5at%Nb alloys at 1173K in air was investigated using mass gain measurement, field emission scanning electron microscopy, transmission electron microscopy, electron-probe microanalysis, glow discharge optical electron spectroscopy, and glancing angle X-ray diffraction analysis. The TiAl₃ formed meta-stable alumina θ-Al₂O₃, while the oxide scale on the L1₂ consisted of a duplex structure with an outer rutile TiO₂ and an inner α-Al₂O₃. During the very initial stages of oxidation both Ti and Al in the L1₂ coating could be oxidized, and then the faster diffusing Ti goes out to form an outer, continuous Ti-rich oxide which covers the slow growing Al₂O₃. In case of the TiAl₃ an outer, continuous TiO₂ layer was not observed because of the smaller amount of Ti in the TiAl₃. It could be concluded that the outer, Ti-rich oxide enhanced a phase transformation of Al₂O₃ from θ to α. To elucidate the Ti effect, Ti-vapor treated TiAl₃ and Ni-50at%Al were oxidized at 1173K in air, and showed formation of an α-Al₂O₃, in contrast to θ-Al₂O₃ on their bare alloys.