Compositional factors affecting the establishment and maintenance of Al₂O₃ scales on Ni–Al–Pt systems

The beneficial role played by platinum addition in promoting the formation of a protective Al₂O₃ scale on representative γ′-Ni₃Al+γ-Ni coating compositions during high-temperature oxidation is discussed. 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 γ′. Related to the latter, Pt partitions almost solely to the Ni sites in the ordered L1₂ crystal structure of γ′, which has the effect of amplifying the increase in the Al: Ni atom fraction on a given crystallographic plane containing both Al and Ni. Such an effective Al enrichment at the γ′surface kinetically favors the formation of Al₂O₃ relative to NiO. A further contributing factor is that the Pt-containing γ′-based alloys show subsurface Pt enrichment during the very early stages of oxidation. This enrichment reduces Ni availability and can increase the Al supply to the evolving scale, thus kinetically favoring Al₂O₃ formation. This observed benefit of Pt addition promoting exclusive Al₂O₃-scale growth is inferred to be a special form of the third-element effect.