OUT-DIFFUSION OF THE NOBLE COMPONENT DURING THE INITIAL-STAGE OF THE OXIDATION OF GAMMA'-NI3AL

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies of the oxidation behaviour of single crystalline gamma'-Ni3Al revealed that Ni diffuses through the oxide layer to the surface of the material during oxidation at low oxygen partial pressure. Small pure nickel particles (diameter approximately 50 nm) form on top of the oxide scale. In this paper, this observation is explained on the basis of a micromechanical model. The model assumes that at intermediate temperatures the prevailing transport mechanism is the diffusion along the oxide metal interface caused by compressive stresses generated during oxide growth. Relaxation of internal stresses as well as the formation of the small Ni-particles on top of the oxide surface are controlled by interface diffusivity. Other possible stress relief mechanisms can be excluded by theoretical estimates.