In part Ⅰ, the effect of MgO on the structure of vicinal (0001) and (2243) surfaces of ＄Al_2O_3＄ has been examined by atomic force microscopy. When heattreated in air at 1600℃, the vicinal (0001) and (2243) surfaces are faceted with hill-and-valley structures indicating that the equilibrium shape ＄Al_2O_3＄ crystal is a polyhedron consisting of flat planes forming sharp corners and edges. However, when heat-treated in MgO atmosphere at the same temperature, both surfaces become defaceted with macroscopically flat shapes, and therefore atomically rough. This defaceting transition induced by MgO is reversible and indicates that at least some comers and edges of the equilibrium shape become rounded when MgO is added. The results are qualitatively consistent with the previously observed polyhedral equilibrium shape of ＄Al_2O_3＄ crystal and its chagne with MgO addition.
In part Ⅱ, the effect of additives on interface structures and grain growth of ＄Al_2O_3＄ with or without liquid phase has been studied. Firstly, the abnormal grain growth (AGG) in ＄Al_2O_3＄ with anorthite (＄Al_2O_3＄·CaO·＄2SiO_2＄) liquid phase and the effect of MgO addition have been investigated in the views of two dimensional nucleation theory for the AGG of polyhedral grains dispersed in a liquid matrix. When ＄Al_2O_3＄ specimens with anorthite liquid are sintered at 1620℃, come grains grow abnormally with elongated shapes along the basal. With increasing anorthite content from 0.05 to 1 wt.%, AGG begins earlier, the number of the abnormally large grains increases, and their size decreases. The number density of the abnormal grains and their growth rates increase with sintering temperature. When ＄Al_2O_3＄ specimens with 0.05 wt.% anorthite are sintered at 1500℃, the grains grow slowly without any indication of AGG up to 48 h, but if abnormal grains are produced by sintering at 1620℃, those grains continue to grow abnormally during sintering at 1500℃. The observations with transmission electrom microscopy shows that the grain surfaces at the liquid pockets of grain triple junctions and intergranular liquid films are either faceted or flat, indicating atomically flat structure. The observed AGG behavior appears to be qualitatively consistent with the growth of these flat surfaces by two dimensional nucleation of steps.
When MgO is added, the grains still grow abnormally, but the number of the abnormal grains increases with the amount of MgO added and the grains become nearly equiaxial. At high MgO contents, the number of the abnormal grains bacome so large that the grain size distribution can appear to be close to that of normal grain growth. Some grain surfaces at the liquid pockets of the triple junctions and intergranular liquid films are smoothly curved, indicating that those grain surfaces become atomically rough when MgO is added. The increasing number density of the abnormal grains with MgO addition is attributed to decreased step free energy at the surfaces which still remain atomically flat. The equiaxial grain shape is attributed to either the decreased surface energy of the non-basal planes or the decreased step free energy at the basal plane. These results also support the hypothesis that AGG occurs by two dimensional nucleation of steps.
Secondly, the effects of ＄SiO_2＄, CaO, and MgO addition on grain growth of ＄Al_2O_3＄ without liquid phase has been studied. When pure ＄Al_2O_3＄ is sintered at 1620℃, normal grain growth occurs with equiaxizl grains and curved grain boundaries. Under TEM all grain boundaries appear to be nearly straight indicating that they are atomically rough. When 100 ppm ＄SiO_2＄ together and 50ppm CaO is added, abnormal grain growth occurs with the large grains elongated in the directions of the basal planes. About 10% of the grain boundaries are faceted when observed under TEM and no frozen liquid pocket is found at the grain boundary junctions. When 600 ppm of MgO is added together with 100 pm ＄SiO_2＄ and 50 ppm CaO, normal growth occurs. Under TEM all grain boundaries appear to be nearly straight indicating that they become atomically rough. The same results are obtained when either ＄SiO_2＄ up to 500 ppm or MgO together with ＄SiO_2＄ is added and sintered at 1900℃. The results indicate that the occurrence of abnormal grain growth in alumina with ＄SiO_2＄ or together with CaO is correlated with the faceting of at least some of the grain boundaries. The MgO addition causes the grain boundary roughening and thus normal grain growth.
In part Ⅲ, the effect of non-linear relationship between the criving force and the velocity of grain boundary migration in polycrystalline materials has been examined by a new computer simulation method considering only the nearest-neighbor interaction of grains. When the velcoty of grain boundary migration is linearly proportional to the driving force, normal grain growth (NGG) occurs while AGG occurs when the velocity is nearly 0 at small driving forces and increases abruptly at a certain critical driving force. As the initial mean grain size increases, the number density of abnormal grains decreases and eventually all grains grow slowly without AGG above a certain value of initial mean grain size. The existence of the critical initial grain size for the occurrence of AGG is consistent with previous observations in real systems such as 316L stainless steel. As the fraction of rough grain boundaries with linear relationship between the driving force and velocity increases, the growth behavior is gradually changed from abnormal to normal growth. Theses results support the hypothesis that AGG in ＄Al_2O_3＄ without liquid phase may occur by the step growth of faceted grain boundaries, and MgO suppresses the AGG by roughening those facted grain boundaries.
