Influence of an electric field on the plastic deformation of fine-grained Al2O3

The influence of an electric field E = 300 V/cm on the plastic deformation at 1450 degrees C to 1600 degrees C of fine-grained alumina with similar to 300 ppm MgO and d(0) = 1.4 to 2.5 mu m was investigated. After removal of the effect of Joule heating, the field reduced the flow stress by similar to 4 MPa relatively independent of temperature, representing a 25 to 70 pet reduction in the flow stress. The field had no effect on the stress exponent n 2.2, nor on the grain size exponent p = 1.9. It did however have an appreciable effect on the combined constant AD(0) of the Weertman-Dorn equation and the activation energy Q. The latter increased from 492 kJ/mole without the field (typically that for Al3+ ion lattice diffusion) to 880 to 1070 kJ/mole with the field, which is more typical of sub-boundary or grain boundary diffusion of either Al3+ or O2-- ions in alumina. It therefore appears that the field changed the rate-controlling, grain boundary sliding accommodation mechanism from the lattice diffusion of Al3+ ions to either sub-boundary or grain boundary diffusion of Al3+ or O2-- ions. The electric field caused isotropic swelling of the specimen when applied for an extended period of time without application of stress. Pores responsible for the swelling occurred along the grain boundaries. They may have resulted from electrotransport of Al3+ ions, leaving behind an Al-deficient layer from which free oxygen bubbles formed along the grain boundaries. Electric field-enhanced gas reactions at the pores may have also contributed to the swelling. It is proposed that swelling due to electrotransport may possibly be avoided by the addition of solutes, which increase the electronic or oxygen ion conductivity transference numbers compared to the aluminum ions.