A study of dislocation mobility and density in metallic crystals

Abstract

This report summarizes the research accomplishments under the Atomic Energy Commission contracts, CALT-473 and CALT-767-P3b for the ten-year period, November 1, 1963 to October 31, 1973. The research was stimulated by technological advances which required improvements in our ability to predict the deformation behavior of materials. In the mid 1930's, theoreticians first recognized that crystal defects could play a central role in plastic deformation, and since that time a number of experiments have conclusively demonstrated the one-to-one correspondence between the motion of line defects (dislocations) and plastic deformation. Before the existence and significance of dislocations was recognized, theoreticians faced a puzzling problem: the predicted strength of crystals was several orders of magnitude larger than the strength actually observed. With the realization that crystal deformation is caused by the motion of dislocations, the theoretical problem reversed. The new problem became one of understanding the origin of the resistance to dislocation motion in order to explain the observed strength of crystals. The Atomic Energy Commission Sponsored Research on dislocation mobility and density in metallic crystals at the California Institute of Technology has focused on an understanding of the dynamics of dislocations. Important interactions between a moving dislocation and lattice phonons, conduction electrons, other dislocations, and point defects such as those introduced by neutron irradiation have been studied. The experimental phase of this research involved the introduction of isolated dislocations into a crystal, the observation of these dislocations by chemical or electrolytic etching, X-ray topography, and transmission electron microscopy (TEM); the application of appropriate stresses of controlled amplitude and duration, and finally determination of the stress-induced motion of the dislocations by observation of their new locations. The nature of the resistance to dislocation motion is deduced from these experiments.