Electron Microscope Image Contrast from Dislocation Loops

Experimental electron microscope images of dislocation loops in molybdenum have been compared to theoretical loop images displayed in the form of computer simulated micrographs. The exact displacement field around a circular edge dislocation loop has been calculated using isotropic elasticity theory; this displacement field together with the two‐beam dynamical theory of electron diffraction contrast have been used to compute image intensities. Specific results are given for loops of radius ≈ 250 Å and ≈ 25 Å. Their contrast has been studied over a narrow range of the numerous variables which are known to influence image features. It has been found that the general topographical character of certain prominent experimental images is qualitatively accounted for by the simple “elastic” defect model with the two‐beam theory of electron diffraction.