The chemically induced interface migration(CIIM) and recrystallization(CIR), which can occur in most materials under chemical inequilibrium, have been investigated. This work consists of three parts: (1) chemically induced recrystallization(CIR) in alumina single crystal(Chap. 2 and 3), (2) liquid film migration(LFM) and its direction in alumina-anorthite (Chap. 4), and (3) chemically induced zigzag migration in alumina bicrystal(Chap. 5).
When $Fe_2O_3$ solute atoms dissolved into a $Al_2O_3$ single crystal with (0001) basal plane at 1500℃, chemically induced recrystallization(CIR) occurred. The CIR grains had mostly a plate-like and a hexagonal shape, and their nucleation sites were distributed randomly. The density of misfit dislocations formed by $Fe_2 O_3$ dissolution in a recrystallized region in alumina single crystal was calculated, based on the coherency strain theory with lattice misfit strain. The calculated dislocation density was $10^10cm^{-2}$, similar to that typically observed in cold-rolled metals. Subboundaries of dislocation walls was observed in the diffusion zone of solutes. Therefore the misfit dislocation wall mechanism, which describes a transition of low angle grain boundaries into high angle grain boundaries by rearrangement of misfit dislocations, appears to be operative in the nucleation of CIR. The nucleation process of CIR has also been compared with that of the primary recrystallization in cold-rolled metal.
In order to observe the growth characteristics of recrystallized grains in alumina, the first new grains nucleated were heat-treated successively at the interval of 30 min at 1500℃ under an $Fe_2O_3$ atmosphere. The recrystallized grains were formed to grow by chemically induced grain boundary migration(CIGM) mechanism. During annealing, the recrystallized grains were also refined by a repeated nucleation in the interior of the grains. The CIR grains thus formed by the repeated nucleation had a texture of (0001)[-1-120]. The growth characteristics of CIR grains can therefore be described in the following: (1) formation of nuclei, (2) production of misfit dislocations at grain boundaries and the interior of the first new grains nucleated, (3) subgrain formation by the rearrangement of misfit dislocations in the interior of the growing grains and (4) grain refinement by subgrain growth, that is, repeated recrystallization. During the subgrain growth, many slip lines formed. It is thus suggested that the growth of subgrains occurs by subgrain boundary dissolution by which individual dislocations at subgrain boundaries climb and glide to the neighbouring subgrain boundaries.
The migration behavior and the migration directions of liquid films between single crystal and polycrystal alumina plates have been observed on adding $Cr_2O_3$ solutes to diffusion couples of the plates. Various behaviors of the migration were observed: (1) oscillation of migration interface, (2) anisotropy of interface migration velocity, (3) saw-type migration, and (4) migration reversal. During the migration, most interfaces were faceted. The observed directions of migrating films were consistent with a prediction that the interface migrates toward the grain with higher coherency strain energy, as in the case of grain boundary migration (CIGM). The present result, however, was in better agreement with the prediction than the previous one obtained for grain boundary migration. The discrepancy between the predicted and previously observed migration directions of some grain boundaries in alumina may therefore be attributed to an effect of grain boundary structure and stress transmission across the boundary.
The effect of grain boundary structure on zigzag migration has also been studied. We first prepared five kinds of a-m diffusion couples with different twist angles by 30° from a [0001] common direction of each plane. When $Cr_2O_3$ was added into the above diffusion couples by a vapor phase, a zigzag migration of the grain boundary occurred. The fraction of zigzag migration did not essentially vary with the twist angle, but the magnitude and migration distance of individual migrating segment varied. Therefore the variation of CIGM morphology seems to result from the change in grain boundary mobility due to microscopic deviation of grain boundary structure out of a macroscopic grain boundary orientation.
