Bimodal stress relaxation

Abstract

The plasticity of a metal results from dislocations moving through a crystal; this rate of motion is determined by barriers in the crystal, and the drag between barriers. At low temperatures, where dislocation motion is underdamped, inertia plays an important role in overcoming barriers. In contrast to this situation, at high temperatures where dislocation motion is overdamped, dislocations are held up at barriers and this determines the rate of plastic deformation of a crystal. A measure of this high temperature behavior is obtained in the stress relaxation behavior of many materials, where a single relaxation mode is observed. At low temperatures, however, the stress relaxation, as shown below, shows two distinct modes. Further, these modes are affected by the state of the electrons, a clear manifestation of drag processes. These observations also demonstrate an interaction between thermal processes and dislocation inertia. The experiment is carried out as follows: single crystals of lead are deformed at a temperature, T, well below where phonon damping is of importance. After a crystal is plastically deformed for a fixed time the tensile machine is stopped and the stress relaxation recorded. Lead, which has a superconducting transition at --7.18K, can be deformed in the superconductingmore » state or in the normal state. This allows the drag on moving dislocations to be independently varied. Data for lead show that for crystal deformed in the superconducting state at 6K, there are two stress relaxation rates. Likewise, for a crystal deformed at a temperature above 7.18K in the normal state, there are two distinct relaxation modes. Additional stress relaxation data for a dilute lead-tin crystal confirm that stress relaxation behavior is bimodal at low temperatures.« less