MATHEMATICAL-MODELING OF OXIDE-GROWTH MECHANISMS MEASURED BY O-18 TRACER EXPERIMENTS

A mathematical model was developed which allows the description of the growth mechanisms of oxide scales on high temperature alloys forming by oxygen grain boundary diffusion. The model uses a finite element technique both for the calculation of oxygen grain boundary diffusion as well as tracer exchange between grain boundary and oxide grains. For checking the validity of the model, FeCrAl-based ODS (oxide dispersion strengthened) alloys were oxidised at 1100-degrees-C in a two-stage oxidation technique using O-18 tracer. Oxygen isotope distributions in the so formed alumina scales, determined by SIMS or SNMS, were compared with calculated profiles. It was found, that the model, which accounts for experimentally observed changes in oxide grain size as a function of scale thickness, is able to simulate the in-scale transport processes with satisfactory accuracy. Comparison of experimental with calculated data revealed, that the assumption of grain boundary oxygen diffusion in a pure alumina scale on Fe-based ODS alloys cannot be applied to scales with a thickness of less than about 0.7 mum. This is due to the fact that in such scales the alumina contains small amounts of Fe- and Cr-containing oxides. In all cases, also for thicker scales, a small amount of outward scale growth occurs due to yttrium and titanium transport through the alumina scale.