Localized strain and atomic structures of symmetrical tilt grain boundaries in Al₂O₃ bicrystals

Residual strain and atomic structures in the vicinity of the grain boundaries were investigated for (23̄10) [0001] ∑21, (45̄10) [0001]∑7 and (011̄2) [2̄110]R∑7 symmetrical tilt grain boundaries of Al₂O₃ bicrystals using convergent beam electron diffraction (CBED) and high-resolution electron microscopy (HRTEM). CBED measurement clearly showed that the R∑7 grain boundary has a relatively small strain across the boundary. On the other hand, there is strain of about 0.06-0.07% in the area 10 nm from the grain boundaries in both the ∑21 and ∑7 grain boundaries. It was found that signs of strain distribution in the vicinity of the grain boundary observed in the ∑21 specimen were opposite to those observed in the ∑7 specimen. The interplanar distances of (2̄110) a-plane and (0001) c-plane in the ∑21 boundary were confirmed to increase and decrease, respectively, in the vicinity of the grain boundary. In contrast, the respective interplanar distances in the ∑7 boundary decrease and increase, respectively. The difference in the change of interplanar distance observed in the ∑21 and ∑7 boundaries is considered to be closely related to the atomic density at the respective grain boundaries.