Crystal-Lattice Images of End-On Dislocations in Deformed Aluminum

Of particular interest in the theory of dislocations is the magnitude and extent of the perturbation to the atomic structure in the center of a dislocation and in its immediate vicinity. In the present work, two-beam crystal-lattice images have been used to directly examine the perturbing influence of end-on dislocations on the 2.34 Å spacing of (111) planes in deformed aluminum. The crystal-lattice images were obtained using a high-vacuum, short-focal-length (2.1 mm) Siemens Elmiskop 1 operated at 100 kV with pointed filaments. The experimental observations very clearly illustrate several fundamental properties of dislocations in crystals. As for the core region of a dislocation, the strains were observed to be nonuniform and high with a maximum of ∼0.15. A detailed examination of two end-on edge dislocations in a low-angle tilt boundary revealed dislocation core widths of 6.8 and 8.5 b. These widths agree favorably with the 7.3 b width predicted by isotropic-elasticity theory. Outside the dislocation core, the observed strain field agrees with that expected from elasticity theory, except for the existence of small-scale expansions and contractions of the lattice which are observed over a distance of 20 to 30 Å from the center of a dislocation.