Thermodynamic and microstructural study of Ti 2 AlNb oxides at 800 °C.

The high-temperature structural applications of Ti2 AlNb-based alloys, such as in jet engines and gas turbines, inevitably require oxidation resistance. The objective of this study is to seek fundamental insight into the oxidation behavior of a Ti2 AlNb-based alloy via detailed microstructural characterization of oxide scale and scale/substrate interface after oxidation at 800 °C using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). The oxide scale exhibits a complex multi-layered structure consisting of (Al,Nb)-rich mixed oxide layer (I)/mixed oxide layer (II)/oxygen-rich layer (III)/substrate from the outside to inside, where the substrate is mainly composed of B2 and O-Ti2 AlNb phases. High-resolution TEM examinations along with high-angle annular dark-field (HAADF) imaging reveal: (1) the co-existence of two types (α and δ) of Al2 O3 oxides in the outer scale, (2) the presence of metastable oxide products of TiO and Nb2 O5 , (3) an amorphous region near the scale/substrate interface including the formation of AlNb2 , and (4) O-Ti2 AlNb phase oxidized to form Nb2 O5 , TiO2 and Al2 O3 .