Oxidation of orthorhombic Ti2AlNb alloys in air

Different Ti2AlNb-based compositions with various amounts of beta-stabilising elements (Si, Mo, Ta) were investigated with respect to their oxidation resistance in air. A nonprotective AlNbO4/TiO2 multilayered scale developed on the orthorhombic alloys. This scale remained mostly adherent but porous below 700degreesC and showed complete spallation at 800degreesC. The metal/oxide interface consisted of a mixture of Al-rich oxide and Ti-rich nitrides followed by an interstitial affected subsurface. Oxygen inward diffusion led to severe embrittlement of the substrate. The oxidation kinetics and the depth of the subsurface embrittlement are discussed with regard to the influence of the P-stabilising elements.