The grain growth behaviors of ultra pure (>99.998 wt%) $α-Al_2O_3$ doped with a certain amount of anorthite and MgO have been investigated at 1650℃. In the 0.1 wt% anorthite + $Al_2O_3$, the abnormal grain growth (AGG) occurs, and all of the solid-liquid interfaces at the grain boundary junctions are observed to have faceted shapes and the singular structure. In the 0.1 wt% anorthite + 0.1 wt% MgO + $Al_2O_3$, the number of abnormally growing grains increases, and the bimodal distribution of grain size appears during the grain growth. In addition, some of the solid-liquid interfaces are observed to have rounded shapes and roughened structure, and the shapes of growing grains become quite isotropic compared to the specimen without MgO.
During the sintering of $Al_2O_3$, the impurities such as CaO and $SiO_2$ form a very small amount of liquid of which composition is similar to that of anorthite, so grain boundaries and grain boundary junctions are wetted by the liquid phase. In this situation, the growth rate is controlled by the movement of grain boundary junctions, and the junctions are moved by means of dissolution/precipitation reactions at the solid-liquid interfaces. Therefore, the growth behavior of grains varies according to the structure of solid - liquid interfaces at the grain boundary junctions.
If the interface energy is isotropic, the solid-liquid interfaces have rounded shapes and the atomically rough structure. The rough interface allows the continuous attachment of atoms from the liquid, and the interface mobility remains constant. Then all of the junctions can move by the dissolution/precipitation reactions, if there exist any driving forces; the normal grain growth (NGG) is expected under this condition. If the interface energy is highly anisotropic and there exist some cusps in the γ-plot of the interface energy, the solid-liquid interfaces have faceted shapes and the atomically singular structure. Without the ledge-generating sources such as screw dislocations, the grains with such singular interfaces can grow only by the 2-D nucleation. As the 2-D nucleation on a singular interface needs a driving force larger than a critical value, only the grains large enough to have capillary driving forces larger than the critical value for the 2-D nucleation can grow by dragging junctions. The growth of other grains is almost suppressed. Thus, the abnormal grain growth is expected when all of the solid-liquid interfaces at the junctions have the singular structure.
The faceted shapes of liquid-solid interfaces observed at the grain boundary junctions in 0.1 wt% anorthite + $Al_2O_3$ which shows the AGG behavior confirms this predicted relationship between the singular interface structure and the AGG. The increase in the number of growing grains after the MgO doping is supposed to be due to the roughening of some solid-liquid interfaces. The roughened interfaces can grow under the capillary driving forces smaller than the critical value for the 2-D nucleation. Thus, the fraction of movable junctions increases and more grains become to be able to grow as some interfaces are roughened.
Therefore, the effects of MgO are thought to increase the number of growing grains by roughening some of solid-liquid interfaces making the shapes of growing grains more isotropic. Due to these two effects, the final structure after the impingement of abnormally growing grains looks like the NGG structure.