Electron microscope image contrast from small dislocation loops. I. Theoretical predictions for edge dislocation loops in a BCC crystal

The theoretical image contrast from a small finite edge dislocation loop in an elastically isotropic medium has been studied in the two-beam theory of electron diffraction. Computer image simulation has been used to investigate the influence which image plane normal, diffracting vector, foil thickness and dislocation loop Burgers vector, orientation, position and size have on so-called black/white intensity distributions. The choice and range of the parameters investigated have been dictated by experimental work on radiation damage in BCC molybdenum published previously by the authors. The present results have indicated that a variety of images are possible for this simple elastic defect model but that useful image classifications can be made in terms of modulus g.b where g is the reciprocal lattice vector of the operating reflection and b the dislocation Burgers vector.