Deformation and phase transformations during the cyclic oxidation of Ni−Al and Ni−Pt−Al

The reversible high-temperature γ′ to β phase transformation may be critical to explaining the unusual high-temperature oxidation behavior of (Ni,Pt)Al alloys and coatings. During high-temperature, high-frequency (1 h) cyclic oxidation in dry, flowing O₂, unprecedented macroscopic deformation was observed in two-phase (γ′+β) cast specimens of Hf-doped Ni−Al at 1,150°C and Hf-doped Ni−Pt−Al at 1,100° and 1,150°C, Outside of this two-phase field or when the cycle frequency was decreased to 100h, no deformation was observed. Using high-temperaturex-ray diffraction in an inert environment, the β-to-γ′ phase ratio was observed to increase above 1,000°C, causing a 2.5% volume change. The addition of platinum appeared to lower the transformation temperature consistent with the deformation observed in castalloys and rumpling of simple and platinum-modified aluminide coatings.