Mechanisms of Ni3Al oxidation between 500°C and 700°C

Oxidation of single-and polycrystalline Ni3Al in air and at low (less than or equal to 5.3 x 10(-3) Pa) and high (2 x 10(4) Pa) oxygen pressures in the temperature range from 500-700 degrees 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 gamma-Al2O3 layers with some metallic nickel. NiO particles grow on the Al2O3 scale after increasing the oxygen pressure. The initial stages of oxidation in air are characterized by the formation of NiO and Al2O3. Both oxides grow by outward diffusion of the metal cations until the faster growing NiO covers Al2O3. Subsequent oxidation occurs by outward diffusion of nickel and internal oxidation of aluminium to gamma-Al2O3. Single crystalline Ni3Al 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 Al2O3. In some cases small amounts of gamma-Al2O3 are found leading to a thicker oxide scale during the following oxidation. (C) 1998 Elsevier Science B.V.