Electroplasticity in metals and ceramics

The influence of an electric field or corresponding current on the plastic deformation of metals and ceramics is reviewed. Regarding metals, the following are considered: (a) the effects of high density electric current pulse on the flow stress at low to intermediate homologous temperatures; and (b) the effects of an external electric field on superplasticity at high temperatures. The major effect of the current pulses was to reduce the thermal component of the flow stress. This resulted from the combined action of an electron wind force, a decrease in the activation enthalpy for plastic deformation and an increase in the pre-exponential, the last making the largest contribution. Besides giving a reduction in the flow stress during superplastic deformation, an external electric field reduced cavitation and grain growth. The influence of the external field appears to be on the migration of vacancies or solute atom-vacancy complexes along grain boundaries to the charged surface. In the case of ceramics, the effects of an internal electric field on the plastic deformation of polycrystalline NaCl at 0.28–0.75TM and on the superplasticity of fine-grained oxides (MgO, Al2O3 and ZrO2) at T>0.5TM are considered. Regarding NaCl, at T≤0.5TM an electric field E≥10 kV cm−1 is needed to enhance dislocation mobility in single crystals. However, a field of only 1 kV cm−1 significantly reduced the flow stress in polycrystals, which is concluded to result from an enhancement of cross slip. At T>0.5TM, there occurred a decrease in the flow stress of polycrystalline NaCl along with a reduction in the rate-controlling diffusion activation energy. Regarding the fine-grained oxides at T>0.5TM, an internal electric field E≤0.3 kV cm−1 gave an appreciable, reversible, reduction in the flow stress by an enhancement of the rate-controlling diffusion process. Limited work suggests that a field may also retard grain growth and cavitation in ceramics.