Roughening and strain-field evolution at a grain boundary in α−Al2O3

Using a combination of high-resolution transmission electron microscopy (HRTEM), geometric phase analysis of HRTEM images, and scanning TEM, we study the structural transition and strain-field evolution at the grain boundary in an $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{A}{\mathrm{l}}₂{\mathrm{O}}₃$ bicrystal with a misorientation relationship of $[0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}0\phantom{\rule{0.16em}{0ex}}1]/{30}^{\ensuremath{\circ}}$. The specimens are annealed at 300, 500, and 700 \ifmmode^\circ\else\textdegree\fi{}C in the TEM. The grain boundaries in the specimens before annealing and annealed at 300 \ifmmode^\circ\else\textdegree\fi{}C are observed to be faceted, but the grain boundaries in those annealed at 500 and 700 \ifmmode^\circ\else\textdegree\fi{}C become defaceted, exhibiting meandering morphology. Interestingly, for the as-received specimen and that annealed at 300 \ifmmode^\circ\else\textdegree\fi{}C, there occurs a periodic strain fluctuation along the grain boundary. However, such a periodic fluctuation along the grain boundary is not observed in the specimens annealed at 500 and 700 \ifmmode^\circ\else\textdegree\fi{}C. These results suggest an unexplored possibility that the roughening transition of a grain boundary is enhanced by strain relaxation occurring at the grain boundary.