Effect of MgO addition on interface structure and grain growth in alumina-anorthite

Abstract

Part I Alumina grain with anorthite liquid has polyhedral shape with atomically flat structure. Therefore grains are expected to grow by defect-associated process and two-dimensional nucleation. To examine the relation between growth velocity and supersaturation, sapphire single crystals with (0001) faces are embedded into alumina-15-wt% anorthite powder. The growth of basal plane of sapphire is not continuous. At high supersaturation kinetic roughening occurs. Thus the growth rate is proportional to the supersaturation and does not depend on the dislocation densities of basal planes. As lowering the supersaturation, increasing the heat treatment time, the growth rate decreases depending on the dislocation density of each basal plane. This growth behavior is consistent with step growth mechanism and leads to abnormal grain growth Part II Alumina with 5-wt% anorthite sintered at 1620℃ for 48 h shows faceted grains with flat singular surfaces in contact with liquid pools at the grain triple junctions when observed under TEM. When this specimen is heat-treated again at the same temperature after packing in MgO powder, such singular grain surfaces as the $\{1bar{01}2\}$ and {0001} planes remain, but some grain surfaces with probably high index orientations are curved. Thus the MgO addition appears to cause the roughening of edges and corners of the equilibrium shape of alumina grains in anorthite liquid and hence reduce the step free energy of the singular surfaces. This surface change resembles surface roughening transition and produces a drastic effect on grain coarsening behavior.