Vacancy ordering during selective oxidation of β-NiAl

β-NiAl is widely used as a protective coating for high-temperature corrosion, where the selective oxidation of Al leads to the formation of a thin alumina scale to prevent further oxidation. However, understanding how the selective oxidation couples to the structural and chemical evolution of the base alloy remains largely unexplored. Here, by using in-situ environmental transmission electron microscopy and density functional theory calculations, we reveal at the atomic scale that the selective oxidation of Al is accompanied by injection of vacancies into the bulk lattice. In particular, we discovered that during the layer-by-layer oxidation process, the injected vacancies preferentially cluster along <1 1 0> directions on the (0 0 1) planes of β-NiAl, leading to the formation of band-like structural features. This finding sheds lights on the atomic mechanism of oxidation in β-NiAl, which may also extend to the selective oxidation of other ordered intermetallic alloys.