OXIDATION BEHAVIOR OF TIAL COATED WITH A FINE-GRAIN CO-30CR-4AL FILM

The oxidation behavior of TiAl coupons coated with a fine-grain Co-30Cr-4Al (mass %) film of about 30-mum thickness has been studied at 1100-1400 K in a flow of purified oxygen at atmospheric pressure for up to 500 ks. Three oxidation stages were recognized: initial transient, parabolic, and accelerated stages. However, at 1100 K a parabolic stage continues for more than 800 ks. The activation energy for parabolic oxidation agrees with reported values for the oxidation of alumina-former alloys, although the mass gains during the parabolic stages are relatively small at 1200 and 1300 K Micropores developed mainly at the scale/coating and coating/substrate interfaces as oxidation proceeded This is attributable to recrystallization of the coating during oxidation and a Kirkendall effect due to preferential diffusion of Co into the substrate. The accelerated oxidation can be explained in terms of the formation of rutile mounds on the scale.