The Effects of Electric Currents and Fields on Deformation in Metals, Ceramics, and Ionic Materials: An Interpretive Survey

Abstract

The experimental and theoretical results of 50 years of research on the effects of electrical fields on deformation at high and low temperatures are reviewed. Application of currents to metals above a threshold in the range of about 3 × 102 to about 3 × 103 amp/cm2 (depending on material) increases the strain rate (i.e., decreases the stress). Above that threshold, the relationship between current density and strain rate is log-linear with a slope of about 3, independent of material. Many of the experiments were done in a “pulsed” mode to reduce Joule heating. The amount of Joule heating was measured and analytically eliminated from the results. Careful analysis of the data showed that the major effect of a field was to increase the frequency term . Reductions in the activation enthalpy and energy reductions due to the wind force were modest in terms of their effect on increasing the strain rate. The wind force on dislocations may be viewed as resulting from drift of the electron cloud; at low dislocation velocities the effect is to push the dislocation, while at high dislocation velocities the electron cloud acts as a drag. Thus the concept of a “push” and “drag” coefficient arises. Analysis is done in this paper to compute the electron wind force as a function of the orientation between the current density, Burgers, and unit normal vectors. The effects of fields in the range of 1–100 kV/cm are considered on ceramics, halides, and metals. The effect of a field on the deformation of ceramics and halides at high temperatures is generally to increase the strain rate. This is accomplished in a number of ways, such as changing the rate-controlling diffusion mechanism (e.g., from grain boundary to bulk in the case of Al2O3) or affecting the Peierls force (NaCl). The application of a field to metals might be expected, based on Gauss’ Law, to be nonexistent. However, significant effects are in fact observed. In the superplasticity regime for 7475 Al, a reduction of ∼20% in the flow stress, an increase of ∼20% in the strain hardening exponent, a significantly reduced cavitation, reduced grain growth, increased dispersoid-free zone (DFZ) thickness along the grain boundaries, and an alteration of the DFZ chemistry are all observed. Analysis of the data has revealed that the activation energy corresponds to that of bulk diffusion without the field and that there is a reduction of about 20% in the activation energy, which implies that there is either significant assistance from the field or that the field has changed the mechanism of diffusion. Understanding the basic mechanisms of deformation in the presence of fields and currents will allow these factors to be used intelligently in manipulating processes for improved structures and properties.