| Grain boundary diffusion plays a very important role in solid state diffusion-controlled processes. It has been established that the counter-diffusion of oxygen and aluminum along these short-circuiting pathways is primarily responsible for the growth of alumina scales on alumina-forming alloys operating at high temperatures. Moreover, trace amounts of rare earth elements in alumina profoundly alter mass transport due to the segregation of these oversized ions to grain boundaries so as to minimize the size misfit strain energy. However, it is not exactly known what the relative contributions of the inter-diffusion species are, and how these reactive elements affect scale growth rate and transport properties. This work is part of a comprehensive program to conduct a systematic series of model experiments and theoretical analysis to obtain a fundamental understanding of the diffusion mechanisms responsible for alumina scale growth and the effect of reactive elements on associated transport rates.; In the experimental component of this work, cationic grain boundary diffusion was investigated using chromium (in place of aluminum), and boundary transport in both pure and Y-doped fine-grained alumina was studied over the temperature range of 1250°C--1650°C. From a quantitative assessment of the chromium diffusion profile, as obtained from electron microprobe analysis, it was found that yttrium doping retards cation diffusion in the grain boundary regime, leading to more than an order of magnitude decrease in the cation diffusivity. The results indicate that the activation energy of grain boundary diffusion mostly stays the same for the undoped and Y-doped samples, and the difference in diffusivity is embodied mainly in the prefactor of the Arrhenius equation.; In conjunction with the experimental efforts, analytical models and numerical procedures that incorporate the impact of a complex microstructure on grain boundary diffusion are employed to investigate diffusive behavior. For example, the temporal evolution of diffusant in a simplified microstructure, as represented by a Voronoi model resulting from homogeneous nucleation and growth, was obtained using the finite difference method. Finally, approximate analytical equations describing diffusant uptake in polycrystalline microstructural models were developed and found to agree well with the numerical results. |
