Atomic structures and energies of Σ7 symmetrical tilt grain boundaries in alumina bicrystals

Symmetrical Sigma7 tilt grain boundaries of alumina (Al2O3) were studied using bicrystals. Three types of Sigma7 boundaries were successfully fabricated, that is, rhombohedral twin (Sigma7{1 (1) over bar 02}) and two types of [0001] symmetrical tilt grain boundaries with grain-boundary planes {4 (5) over bar 10} and {2 (3) over bar 10} (Sigma7{4 (5) over bar 10} and Sigma7{2 (3) over bar 10}). Their atomic structures and grain-boundary energies were investigated using high-resolution transmission electron microscopy (HRTEM) and a thermal grooving technique, respectively. HRTEM observations showed that the Sigma7{1 (1) over bar 02} boundary had a completely symmetrical atomic arrangement with respect to the grain-boundary plane. In contrast, Sigma7{2 (3) over bar 10} and Sigma7{4 (5) over bar 10} boundaries exhibited asymmetrical atomic structures, which were confirmed by analyzing the atomic configurations using static lattice calculations. Thermal grooving experiments showed that the grain-boundary energies strongly depended on the properties of the grain-boundary planes.