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

Abstract

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 α-Al₂O₃ bicrystal with a misorientation relationship of [0001]/30-. The specimens are annealed at 300, 500, and 700 °C in the TEM. The grain boundaries in the specimens before annealing and annealed at 300 °C are observed to be faceted, but the grain boundaries in those annealed at 500 and 700 °C become defaceted, exhibiting meandering morphology. Interestingly, for the as-received specimen and that annealed at 300 °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 °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.