Mechanisms of Ni 3Al oxidation between 500°C and 700°C

Abstract

Oxidation of single- and polycrystalline Ni 3Al in air and at low (≤5.3 × 10 −3Pa) and high (2 × 10 4Pa) oxygen pressures in the temperature range from 500–700°C was studied by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). To study the influence of the initial condition, the surface composition was changed by annealing the samples at low pressures before oxidation in pure oxygen at high pressures. Preannealing at low pressures results in γ-Al 2O 3 layers with some metallic nickel. NiO particles grow on the Al 2O 3 scale after increasing the oxygen pressure. The initial stages of oxidation in air are characterized by the formation of NiO and Al 2O 3. Both oxides grow by outward diffusion of the metal cations until the faster growing NiO covers Al 2O 3. Subsequent oxidation occurs by outward diffusion of nickel and internal oxidation of aluminium to γ-Al 2O 3. Single crystalline Ni 3Al samples show a parabolic oxidation behaviour, but a variation by a factor of up to 5 of the oxide layer thickness after oxidation treatments at the same temperatures and times is observed. This is attributed to differences in the native oxide layers which consist of NiO and amorphous Al 2O 3. In some cases small amounts of γ-Al 2O 3 are found leading to a thicker oxide scale during the following oxidation.