Direction of Black-White Contrast from Small Dislocation Loops

Abstract

Dislocation loops observed in the transmission electron microscope exhibit a characteristic black-white strain contrast under two-beam dynamical diffracting conditions. A simple concept of the nature of this contrast indicates that the black-white direction should lie parallel to the projection of the Burgers vector onto the image plane. Using the results of several contrast calculations for small loops, Wilkens and Riihle (1972) recognized that the black-white direction did not always lie parallel to the Burgers vector projection. For loops with an appreciable shear component, they concluded that a determination of the black-white direction would not be sufficient for analysis of the loop crystallography. However, for pure edge loops they predicted that the black-white direction would correspond (within a few degrees) to the projection of the Burgers vector. Numerous investigators have used this erroneous assumption to analyze the crystallography of loops.