Relating grain-boundary complexion to grain-boundary kinetics II: Silica-doped alumina

This second paper in a series describes the relationship between grain growth kinetics and grain boundary complexions in silicadoped alumina. Dense high-purity silica-doped alumina samples were annealed for various times in the temperature range of 1300° and 1900°C and their grain growth behavior was quantified. Four different grain boundary complexions were observed in silica-doped alumina, all of which enhanced the kinetics relative to the intrinsic undoped alumina. These complexions included a thick crystallized film that was likely amorphous at high temperatures, a thin intergranular film, multilayer adsorption, and a type of boundary that showed no observable film by high-resolution transmission electron microscopy. A generational change in the population of grains occurred at 1500°C where all of the abnormal grains impinged and reestablished a new normal distribution. At higher temperatures a new set of abnormal grains containing different complexions formed in the microstructure. The activation energy of the normal and abnormal grains was approximately the same. The effects of varying dopant concentration were analyzed. The results for silica-doped alumina are compared with previous results for calcia-doped alumina in order to draw some generalized conclusions about the effect of complexions on grain growth.