The influence of microstructure on the isothermal and cyclic-oxidation behavior of Ti-48Al-2Cr at 800°C

The isothermal and cyclic-oxidation behavior of the intermetallic Ti48Al-2Cr (at. %) alloy were studied at 800°C in air. Emphasis was placed on the effect of microstructures, in a range relevant for practical applications; i.e., duplex, near gamma, nearly lamellar, and fully lamellar; obtained by various heat treatments. The oxidation kinetics of the intermetallic alloy showed initially the formation of a relatively protective oxide scale. After an exposure time of about 10 hr the oxidation rate increased significantly, due to a Loss of protectivity of the oxide scale. The growth rate of the oxide scale, as well as its composition, structure and morphology showed no major relation to the microstructure of the base material. The oxidation of Ti-48Al-2Cr in air, initially resulted in the formation of 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 the fast-growing TiO2, responsible for the rapid kinetics. Using18O/15N experiments some mechanistic aspects were discussed in relation to the existence of a nitrogen-rich Layer near the scale/alloy interface. Thermal-cyclic-oxidation experiments up to 3000 1-hr cycles showed that spallation of the oxide scale initiated after about 175 1-hr cycles. Also in this case the growth rate of the oxide scale as well as its composition, structure and morphology showed no major relation to the microstructure of the base material.