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 follo wing are considered: (al the effects of high density electric current pulse on the now stress at low to intermediate homologous temperatures; and (b) the effects of an external electric field on superplasticity at high temper atures. The major effect of the current pulses was to reduce the thermal co mponent of the flow stress. This resulted from the combined action of an el ectron wind force, a decrease in the activation enthalpy for plastic deform ation and an increase in the pre-exponential, the last making the largest c ontribution. Besides giving a reduction in the flow stress during superplas tic deformation, an external electric field reduced cavitation and grain gr owth. The influence of the external field appears to be on the migration of vacancies or solute atom-vacancy complexes along grain boundaries to the c harged surface. In the case of ceramics, the effects of an internal electri c field on the plastic deformation of polycrystalline NaCl at 0.28-0.75T(M) and on the superplasticity of fine-grained oxides (MgO, Al2O3 and ZrO2) at T > 0.5T(M) are considered. Regarding NaCl, at T less than or equal to 0.5 T(M) an electric field E greater than or equal to 10 kV cm(-1) is needed to enhance dislocation mobility in single crystals. However, a held 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 > 0.5T(M), ther e occurred a decrease in the flow stress of polycrystalline NaCl along with a reduction in the rate-controlling diffusion activation energy. Regarding the hue-grained oxides at T > 0.5T(M), an internal electric held E less th an or equal to 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 cavit ation in ceramics. (C) 2000 Elsevier Science S.A. All rights reserved.