Effect of TiO2-SiO2distribution on bimodal microstructure of TiO2-doped α-Al2O3ceramics

Distribution of TiO2 dopants and SiO2 impurities in the bimodal microstructure of Al2O3 with anisotropic and equiaxed grains is systematically analyzed using analytical electron microscopy (AEM). The TiO2-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. SiO2 and TiO2 exhibit a selective segregation behavior. The boundary between the equiaxed grains is segregated mainly by TiO2 but the boundary at the (0001) basal plane of anisotropic grains is covered with a thin amorphous film made up of mostly SiO2. Precipitation of Al2TiO5 occurs at high TiO2 doping levels. A bimodal microstructure develops in three stages, characterized successively by segregation, solution, and precipitation. The preferential adsorption of SiO2 to the (0001) basal plane initiates the anisotropic grain growth, starting at low TiO2 doping level. At higher TiO2 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 Al2TiO5 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.

Duke Scholars

Author Miaofang Chi Thomas Lord Department of Mechanical Engineering and Materia ...