ATOMIC-STRUCTURE OF SIGMA-7(01(1)OVER-BAR2) SYMMETRICAL TILT GRAIN-BOUNDARIES IN ALPHA-AL2O3

High resolution transmission electron microscopy is used to study the atomic structure of a Sigma 7 grain boundary (Sigma is equal to the re ciprocal of the fraction of lattice sites in coincidence) in Al2O3. Th is grain boundary in Al2O3 has misorientation [0 (1) over bar 11]180 d egrees. The grain boundary plane is (01 (1) over bar 2)(1) parallel to (01 (1) over bar 2)(2). Four possible unrelaxed grain boundary structu res can be constructed from the constrained coincidence-site-lattice ( CCSL) patterns. The atomic structure of the boundary is determined fro m a through-focal series of high resolution images and computer image simulation. A DSC dislocation in the Sigma 7 grain boundary was identi fied. The Burgers vector of this dislocation is 1/14[05 (5) over bar 2 ] and it is a pure edge dislocation. Most of the boundary has a struct ure comprising oxygen terminating layers with an identical width of 0. 135 nm, known as the alpha-domain. There are no unsaturated Al-O bonds in the boundary core of the alpha domain, but the Al-O bonds are slig htly elongated. Coexistence of two interfacial domains, alpha and beta , is observed. The beta domain has the same atomic structure as that o f alpha domain, but the width of the interfacial layers is 0.17 nm. Th ese two interfacial structures show different contrast in the high res olution images and are separated by partial-DSC dislocations in the Si gma 7(01 (1) over bar 2) grain boundary. The Burgers vector of the par tial-DSC dislocations can be determined with reference to the interfac ial structures, CCSL patterns, step vector and close failure of the Bu rgers circuit. The partial-DSC dislocations in the Sigma 7(01 (1) over bar 2) grain boundary always appear as a pair which resembles a stack ing fault in the bulk of the crystal.