Grain boundary faceting transition and abnormal grain growth in oxides

Grain boundaries in metals and oxides are faceted at low temperatures and become defaceted above certain critical temperatures. The defaceted grain boundaries are expected to have an atomically rough structure and hence nearly isotropic properties. The grain boundary faceting and defaceting can be also induced by additives. When all or some of the grain boundaries are faceted in polycrystals, abnormal grain growth (AGG) occurs, and when all grain boundaries are defaceted, normal grain growth (NGG) occurs. In alumina, for example, all grain boundaries in a specimen prepared from a high purity powder and sintered at either 1620 degreesC or 1900 degreesC are defaceted with smoothly curved shapes, and the grains grow normally. When sintered at 1620 degreesC after adding 100 PPM Of SiO2 and 50 ppm of CaO, some grain boundaries become faceted and AGG occurs. When 600 ppm of MgO is added to this specimen, all grain boundaries become defaceted again and NGG occurs. Similar effects of SiO2 and MgO addition are observed when sintered at 1900 degreesC. In all of these specimens no liquid phase is found at the triple junctions under TEM. Such a correlation between the grain boundary faceting and AGG is found in a number of metals and some oxides. When the grain boundaries are faceted, some of the facet planes have ordered structures and may migrate by the movement of steps existing on such defects as dislocations or twins, or produced by two dimensional nucleation. The effective mobility of the larger grains will then be larger than that of the smaller grains, causing AGG.