Oxidation of orthorhombic Ti2AlNb alloys in the temperature range 550-1000°C in air

Orthorhombic titanium aluminides offer the potential to substitute the current titanium alloys in aero engines, gas turbines and automotive turbo-chargers. The advantages are a wider range of thermo mechanical processing possibilities, a higher room temperature ductility, increased high temperature strength and better oxidation as well as titanium fire resistance. Nevertheless, uncertainties with regard to their oxidation behaviour presently impede their use in practical applications. In the present study a very detailed investigation of the oxide scales formed and of the resulting metal subsurface structures after oxidation for up to 1000 h was performed. The results show that both the oxide scale and the subsurface zone develop very complex structures which are responsible whether protective or non-protective behaviour is observed. In principle the oxidation behaviour of the three alloys investigated can be characterised by three types designated as type I, II or III. Each of these types has a characteristic scale structure and a different degree of protection and their occurrence depends on alloy composition and oxidation time. From a technical point of view the interstitial dissolution of oxygen in the metal subsurface zone plays an important role as this process can lead to severe embrittlement. The extent of the IAZ (interstitial affected zone) depends on the oxide scale structure. The paper presents the results from detailed metallographic and microprobe investigations which throw further light on the complex oxidation processes of this group of materials.