Phase-dependent high-temperature oxidation behavior of multiphase Ti2AlNb alloy

Abstract

Environmental degradation of Ti 2 AlNb alloys is intrinsically coupled with their multiphase constitution, yet the local mechanisms by which individual phases mediate oxidation remain insufficiently resolved. Here, we reveal the phase-selective high-temperature oxidation behavior of a multiphase Ti 2 AlNb alloy exposed to air at 700-800 °C. By integrating oxidation kinetics with electron microscopy and nanoscale structural analysis, we establish a direct correlation between local phase constitution, oxide-scale architecture and defect-assisted transport. The alloy exhibits near-parabolic oxidation kinetics at 700 and 750 °C, whereas oxidation at 800 °C evolves towards near-linear kinetics, associated with degradation of scale protectiveness. The oxide scales are composed mainly of rutile TiO 2 , α-Al 2 O 3 , TiN and Nb 2 O 5 , but their spatial arrangement is strongly phase dependent. The α 2 phase develops a thin and compact bilayered scale, consisting of a continuous outer α-Al 2 O 3 layer overlying an inner rutile TiO 2 layer, with minor TiN formed near the scale/substrate interface. In contrast, the B2/O region forms a thicker TiO 2 -dominated scale with a discontinuous α-Al 2 O 3 layer and higher porosity. Quantitative analysis at 750 °C shows that the parabolic scale-growth constant of the B2/O region, 5.04 × 10 −3 μm 2 h −1 , is more than twice that of the α 2 phase, 2.17 × 10 −3 μm 2 h −1 . These distinct oxidation behaviors stem from the coupled effects of local elemental partitioning, crystallographic structure, interfacial architecture, and defect evolution. These findings demonstrate that oxidation resistance in Ti 2 AlNb alloys is governed not only by nominal alloy composition but also by local phase architecture, providing a microstructural basis for designing oxidation-resistant Ti 2 AlNb-based materials.