The application of weak‐beam imaging to studies of small dislocational loops

SUMMARY Coupled differential equations for calculating diffracted ands tranmitted intensities from thin crystals have been solved numerically for many beam diffracting conditions. This approach has been used to enable us to study weak‐beam dark‐field images from small circular edge dislocation loops. The exact displacement field around such loops has been calculated using isotropic elasticity theory. Crystal orientations have been chosen to obtain single systematic rows of low order reflections and loop images have been calculated for different combinations of strong and weak beams. Although emphasis has been placed on investigating the application of weak beam imaging to determine the nature of small loops (diam. < 10 nm) a preliminary study has also been made of g.b = 0 residual images. The calculated images have been compared with experimental weak beam images of small loops in neutron irradiated molybdenum.