Atomic structure of (001) twist boundaries in f.c.c metals Structural unit model

Abstract

The applicability of the structural unit model, developed for tilt boundaries by Sutton and Vitek (1983), to (001) twist boundaries in f.c.c. metals is investigated. The model is found to be useful in discussing the continuity of boundary structure with misorientation and the Burgers vectors of screw grain boundary dislocations. A surprising result is that the structures of low-angle boundaries (with misorientation, less than 22–62°) vary discontinuously with misorientation, in the sense that different boundary units appear in boundaries with slightly (in the limit infinitesimally) different misorientations, in accordance with a simple geometrical rule. At higher misorientations the boundary structure varies smoothly with misorientation and two non-equivalent decompositions into structural units can always be found. One of these decompositions is invariant with respect to the symmetry elements of the overall boundary structure, while the other is not. The continuity of boundary structure and grain boundary dislocation Burgers vectors, in particular misorientation ranges, is recognized by selecting the same set of structural units, even though the pattern of these units is not always invariant with respect to the symmetry operations of the overall boundary structure. Alternatively, one can always ensure that the decomposition pattern displays the same symmetry as the boundary structure, but the problem then is that the structural units and dislocation Burgers vectors vary discontinuously with misorientation. These results are discussed in relation to experimental observations of dislocation networks in (001) twist boundaries.