Early-stage oxidation behavior of γ'-Ni3Al-based alloys with and without Pt addition

The early-stage oxidation behavior of gamma'-NbAl-based alloys of composition (in at.%) Ni-22Al and Ni-22Al with 10, 20, and 30Pt was investigated in terms of oxidation kinetics, scale evolution and resulting composition profiles during heating to 1150 degrees C in air. Platinum addition did not appear to affect the nature of the native oxide layer present on the gamma'-based alloys at room-temperature; however, it was found that the presence of increasing Pt content aided in promoting the establishment of a continuous Al2O3 scale during heating the gamma'-based alloys through to about 700 degrees C. This beneficial effect can be primarily ascribed to the fact that Pt is non-reactive and its addition decreases the chemical activity of aluminum in gamma'. Related to the latter, Pt partitions almost solely to the Ni sites in the ordered Ll(2) crystal structure of gamma', which has the effect of increasing the AI:Ni atom fraction on a given crystallographic plane containing both Al and Ni. Such an effective Al enrichment at the gamma' surface would kinetically favor the formation of Al2O3 relative to NiO. A further contributing factor is that the Pt-containing gamma'-based alloys showed subsurface Pt enrichment during the very early stages of oxidation. This enrichment would reduce Ni availability and increase the Al supply to the evolving scale, thus kinetically favoring Al2O3 formation.