An existing model, which took account of the combined grain-boundary and lattice diffusion of O (with exchange), was extended to cases where the grain size of the oxide varied over its thickness. The model predicted the tracer profile throughout the entire oxide for the case of scale growth via inward O diffusion. A computer program was developed for the easy generation of simulated tracer profiles. The sensitivity of simulated profiles to variations in the volume and grain-boundary O diffusivities in the oxide scale, as well as to variations in the oxide grain size, was studied. The results indicated that the normalized tracer profiles were sensitive to variations in the grain boundary diffusivity, but were more sensitive to variations in the bulk diffusivity. The incorporation of a variable grain size led to profiles which were very different to those which were obtained by using a constant oxide grain-size assumption.

N.Appannagaari, S.N.Basu: Journal of Applied Physics, 1995, 78[3], 2060-9