Oxidation behaviour of TiAl-based intermetallics—influence of heat treatment

The isothermal and cyclic oxidation behaviour of two TiAl-based intermetallic alloys, Ti–48Al–2Cr and Ti–48Al–2Cr–2Nb (at.%) were studied at 800°C in air. Emphasis was placed on the effect of microstructures, obtained by various heat treatments, in a range relevant for practical applications, i.e. near gamma, duplex, nearly lamellar and fully lamellar. Ti–48Al–2Cr–2Nb showed the highest corrosion resistance. Oxidation in air of both materials initially resulted in the formation of mainly α-Al2O3, TiO2 (rutile), Ti2AlN and TiN. After longer exposure times, the mixed oxide scale with an alumina rich layer at the outside was overgrown by TiO2. Thermal cyclic oxidation experiments showed that localised spallation of the oxide scale formed on Ti–48Al–2Cr initiated after about 175 cycles lasting 1 h, leading to significant material loss upon extended exposure. Spallation was limited in the case of Ti–48Al–2Cr–2Nb. For both materials the growth rate and spallation behaviour of the oxide scale, as well as its composition, structure and morphology showed no major relation to the microstructure of the base material.