Orientation dependant grain boundary diffusion in polycrystals

Abstract: Grain boundary diffusion in polycrystals is of importance in industrial applications as it often, to a large degree, dictates the effective rate of diffusion in polycrystals. For impurity diffusion the enrichment of impurities within the material will in turn change the material properties, such as ductility and fatigue rates. It is therefor beneficial to be able to quantify the effect of the grain boundaries on diffusion rates. This relates to the field of grain boundary engineering, whereby control of material parameters, such as the crystallographic texture, desired grain boundary properties and in extension material properties, are achieved. In this thesis a texture sensitive model for the rate of diffusion is derived and employed in finite element simulations of polycrystals. The developed model was found to correlate well with available data. Utilising the model, constant source diffusion is studied for different textures. The results show that the effective diffusion varies depending on the texture, consistent with previous studies. Furthermore, a classical slab model was compared to an exponential model for the distribution of the grain boundary diffusivity and a large difference in the resulting penetration profiles were found. The common experimental methods used to extract diffusion coefficients was examined using simulations and were found to give poor results for type B and C diffusion kinetics, whereas the results for type A diffusion were satisfactory. Difficulties were found regarding the lack of available experimental data, or the lack of some specific details in available data. The most prominent problem was the lack of accompanying texture data to that of polycrystal diffusion experiments. As such, a matching simulation to that of an experiment could not be made.