Disconnections, dislocations and generalized disclinations in grain boundary ledges

The structure of ledges in otherwise symmetrical tilt boundaries built from atomistic simulations is investigated in copper in terms of continuous dislocation and generalized disclination fields. A ″discrete-to-continuum” crossover method is used to build the relevant kinematic and defect density fields on the basis of discrete atomic displacements appropriately defined in the boundary area. The resulting structure of incompatibility is compared with the so-called disconnection model of boundary ledges. In addition to their dislocation content, which characterizes the elastic displacement discontinuity across the boundary, the ledges appear to be characterized by discontinuities of the elastic rotation and dilatation fields, which are reflected by non-vanishing generalized disclination density fields.