Dislocation structure and contrast in high angle grain boundaries

Abstract

The dislocation aspects of the earlier model of Bishop and Chalmers for the structure of high angle symmetric tilt boundaries in metals is extended to twist and more general boundaries having both tilt and twist components. The expected strain contrast in high angle grain boundaries is discussed. It is concluded that strain contrast effects will be observed whenever there is not a long-range cancellation of the strain field of the boundary because of perturbations in the primary dislocation array, provided that the perturbations are sufficiently strong and widely spaced. Two types of contrast are discussed. One will occur at near-coincidence orientations due to perturbations in the periodicity of the primary dislocation array. This can be described in terms of secondary dislocations. The second is the result of a small tilt or twist about a second axis superimposed on an otherwise simple tilt boundary.