The influence of lattice friction on point defect hardening

Abstract

A simple model based on a square array of point obstacles is used to investigate the influence of localized lattice defects on the average dislocation velocity”. It is shown that the effective back stress on a dislocation segment, as it bows out and eventua11y breaks away from local obstacles, is equivalent to a periodic internal stress field. The following types of obstacles are considered. 1. Obstacles that cannot be overcome by thermal activation. These produce a hardening which is practically temperature independent. 2. Obstacles that can be overcome by thermal activation. The influence of this type of obstacle on the average dislocation velocity depends very markedly on the magnitude of both the Peierls friction of the lattice and the interaction energy between the dislocations and the point defects. In crystals with a large Peierls friction the point defects produce a hardening which is proportional to the applied stress and is therefore temperature dependent; however, for large obstacle spacings the temperature dependence can be small, and the presence of point obstacles does not change the deformation rate controlling mechanism. In crystals with a negligible Peierls friction, the point obstacles control almost entirely the average dislocation velocity.

These results are used to explain the point defect hardening behaviour of both b.c.c. and f.c.c. metals.