In situ x-ray study of the γ- to α-Al₂O₃phase transformation during atmospheric pressure oxidation of NiAl(110)

The oxidation in air of NiAl(110) was investigated in the temperature range from 870 °C–1200 °C by in situ x-ray diffraction and transmission electron microscopy. Oxidation at 870 °C and 1 bar oxygen leads to the formation of an epitaxial layer of γ-alumina showing an R30° orientation relationship with respect to the underlying substrate. At oxidation temperatures between 950 °C and 1025 °C, we observed a coexistence of epitaxial γ- and polycrystalline δ-Al₂O₃. The α-Al₂O₃starts to form at 1025 °C and the complete transformation of metastable phases to the stable α-alumina phase takes place at 1100 °C. The fcc-hcp martensitic-like transformation of the initial γ-Al₂O₃to epitaxial α-Al₂O₃was observed. X-ray diffraction and cross-section transmission electron microscopy proved the existence of a continuous epitaxial α-Al₂O₃layer between the substrate and the polycrystalline oxide scale, having a thickness of about 150 nm. The relative orientation relationship between the epitaxial alumina and the underlying substrate was found to be NiAl(110) || α-Al₂O₃(0001) and [110] NiAl || [1120].