The oxidation characteristics of the γ-γ′ tie-line alloys of a nickel-base superalloy

A novel family of bond coats designed to be in EQuilibrium with their nickel-base superalloy substrates has been developed. These coatings have hence been termed 'EQ coatings'. The first step taken for EQ coating development was to examine the gamma-gamma' tie-lines of several superalloys. In these superalloys, gamma and gamma' are in equilibrium with each other, and elemental activities are thus equal in these two phases. With equal activities. there will be no chemical potential differences between the two phases. and there is thus no driving force for diffusion. Applying gamma, gamma'. or a combination of both as bond coat candidates will ensure that no inter-diffusion zone (IDZ) will form. This is desirable, as IDZ causes gross local mechanical property degradation. In this paper. the gamma' volume fraction was varied from 0 to 100% at 25% intervals along the gamma-gamma' tie-line of a nickel-base single-crystal alloy, TMS-82+. Cyclic and isothermal oxidation tests were conducted at 1100 degrees C in ambient atmosphere. The resultant oxide structures, and the element concentration profiles from around the oxide/substrate interface, through the depletion zone. into the sound substrate. were analyzed by SEM/EDX. All samples started with the same element activities hence, at the onset of oxidation, all samples were expected to form the same oxide structure. However, experiments showed that the specimens formed vastly different oxide structures. and that gamma' had the best resistance against high temperature oxidation.