Abstract
Characteristics of electron microscopic images of dislocations which lie in an inclined grain boundary are examined by theoretical computation. It is assumed that one side of the boundary is in a good diffracting condition and the other is not. The symmetry of the dislocation image varies with the relative values of g·b and g·(b×u), where g, b and u are the reflection vector, the Burgers vector, and the dislocation line vector, respectively. When g·b is large, the image is asymmetric across the dislocation line and the nature of asymmetry depends upon the sign of g·b. When g·(b×u) is large, the image is symmetric and it takes light or dark appearance depending upon the sign of g·(b×u). When the orientation of the crystal is deviated from the exact diffracting condition, symmetrical or asymmetrical bend of thickness fringes arises in the vicinity of the dislocation. The shape of fringes is also related to the values of g·b and g·(b×u).
It is shown that the Burgers vector can be identified from the knowledge of signs of g·b or g·(b×u) for several reflections. This method of identification is compared with others. Especially, it is pointed out that the conventional method of Burgers vector identification cannot be applied to boundary dislocations when only one side of the boundary is in a good diffracting condition. An edge dislocation in a boundary in such a case usually shows quite strong contrast in the micrograph even when g·b=0. A simple reasoning of the strong contrast is given.
