Faceted Grain Boundaries in Al<inf>2</inf>O<inf>3</inf>

Different types of faceted grain boundaries in commercial poly‐crystalline Al2O3 were examined. Three distinct cases, namely the basal twin, the rhombohedral twin, and other special high‐angle grain boundaries, are discussed, comparing theoretical models and experimental observations. It is shown that the faceting of the boundaries can be associated with their migration. A mechanism for the separation of a σ= 13 grain houndary from a pinning void, small grain, or second‐phase particle is illustrated. It is also shown experimentally that faceting of high‐angle boundaries can occur on a scale of Σ 5 nm, and that, theoretically, faceting most likely also occurs on an atomistic scale. Throughout the theoretical analysis the basal plane is predicted to be a favored facet plane, which is confirmed by experimental observation. Copyright © 1984, Wiley Blackwell. All rights reserved