Effect of TiO2-SiO2 distribution on bimodal microstructure of TiO2-doped α-Al2O3 ceramics

Distribution of TiO 2 dopants and SiO 2 impurities in the bimodal microstructure of Al 2 O 3 with anisotropic and equiaxed grains is systematically analyzed using analytical electron microscopy (AEM). The TiO 2 -doped ceramic materials were hot-pressed at 1500 °C in a reducing environment. Different amounts of Ti solutes in the anisotropic or equiaxed grains were observed after removal of the contamination signal stemming from Ti on the surface. SiO 2 and TiO 2 exhibit a selective segregation behavior. The boundary between the equiaxed grains is segregated mainly by TiO 2 but the boundary at the (0001) basal plane of anisotropic grains is covered with a thin amorphous film made up of mostly SiO 2 . Precipitation of Al 2 TiO 5 occurs at high TiO 2 doping levels. A bimodal microstructure develops in three stages, characterized successively by segregation, solution, and precipitation. The preferential adsorption of SiO 2 to the (0001) basal plane initiates the anisotropic grain growth, starting at low TiO 2 doping level. At higher TiO 2 doping level, bi-level Ti solution occurs, either as a result of equilibration between segregants and solutes, or incorporated as transient Ti solutes in the anisotropic grains due to fast-moving fronts. Further doping starts Al 2 TiO 5 precipitation, which may result in de-wetting of the basal boundary, possibly due to a change of interface energy. The correlation and competition between segregation, solution, and precipitation characterize and dictate the evolution of microstructure, as monitored by the aspect ratio of anisotropic grains.