HIGH-RESOLUTION ELECTRON-MICROSCOPY OF GRAIN-BOUNDARIES IN NI-3(AL0.8TI0.2) BICRYSTALS .2. COINCIDENCE SITE LATTICE BOUNDARIES

The grain-boundary structure of Sigma = 5, [001] (36.8 degrees), Sigma = 25, [001] (16.26 degrees) and Sigma = 3, [011] (109.47 degrees) coi ncidence site lattice (CSL) tilt boundaries in the Ll(2)Ni(3)(Al0.8Ti0 .2) bicrystals prepared by hot pressing were investigated by high-reso lution transmission electron microscopy with the help of image simulat ion. An atomic arrangement in the Sigma = 5, [001] (36.8 degrees) boun dary is asymmetric with a small quantity of rigid-body translation; ac commodation of atoms is small and is localized on a near-grain-boundar y plane. An atomic arrangement in the Sigma = 25, [001] (16.26 degrees ) boundary is asymmetric without appreciable rigid-body translation; s ome atoms are withdrawn and it is interpreted as an array of dissociat ed (a/2)(110) superpartials. An atomic arrangement in the Sigma = 3, [ 011] (109.47 degrees) boundary is primarily symmetric, and local accom modation of atoms and rigid body translation vary with deviation from the grain-boundary plane. In the three CSL boundaries observed in this study, atomic-size cavities are commonly observed.