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.