Electron Microscopic Images of Dislocation Ribbons

The image profiles of dislocation ribbons, as observed in diffraction electron microscopy, are calculated numerically and compared with observations on graphite. It is found that parallel dislocations with the same Burgers vector may exhibit quite different image profiles under identical diffraction conditions. This is due to the overlap of the strain field due to the two parallel dislocations; it is particularly large in graphite due to the large anisotropy of this material. Calculations using the kinematic theory are made for threefold ribbons. The contrast of the three dislocations is found to increase or decrease systematically, the sense of the variation depending on the sign of s.