Modelling the influence of high diffusivity paths on diffusion controlled high-temperature oxidation processes

During high temperature exposure, technical components like gas-turbine blades, furnaces, or exhaust systems, are operating in corrosive atmospheres. The ongoing processes are diffusion‐controlled, and corrosive species penetrate into the material leading to the formation of internal precipitates. Besides a significant mass transport, chemical reactions and phase transformations are occurring at elevated temperatures. Furthermore, the distribution and structure of high diffusivity paths like grain boundaries (grain boundary diffusion) and dislocations (dislocation pipe diffusion) play an important role for the kinetics of many high-temperature degradation processes since the transport of matter along these paths is by order of magnitudes faster than throughout the bulk. Therefore, reducing the grain size, i.e., increasing the fraction of fast diffusion paths, may result in both, a detrimental effect in oxygen-diffusion controlled oxidation or creep or a beneficial effect when the formation of a protective oxide scale depends on the solute diffusion.

Within this study, a simulation software is presented and discussed by which grain boundary diffusion and phase nucleation processes are treated by means of the Cellular Automata approach, which has been applied to diffusion-controlled transformation processes earlier. The approach has been extended to account for high diffusivity paths like grain boundaries and dislocation pipes. In the first case, the effect of grain size and grain boundary width is investigated as well as diffusion along and segregation into the grain boundaries. Secondly, faster diffusion along dislocations as well as their accumulation and the resulting dislocation pipe diffusion is regarded. Finally, the model is verified with experimental results by observation of high diffusivity path on the high-temperature oxidation behavior of commercial nickel-based alloys, and low alloy steel.