Abstract

A modest dc electric field markedly reduced the tensile flow stress at high temperatures in three polycrystalline oxides, i.e. MgO, Al2O3 and yttria-stabilized tetragonal ZrO2 (Y-TZP). The reduction in flow stress ΔσE in Y-TZP consisted of three components: (i) ΔσT due to Joule heating, (ii) a rapid, reversible component obtained in on-off and electric field step tests and (iii) the cumulative effect of the field on microstructure. Only ΔσT and occurred in MgO and Al2O3. It is concluded that results from a reduction in the electrochemical potential for the formation of vacancies corresponding to the diffusion of the rate-controlling ion in the space-charge at the grain boundary. The calculated magnitude of the space-charge zone width and its temperature and solute composition dependence are in accord with theory and experiment; is attributed mainly to the retardation of grain growth by the field. The retardation could be due to one or more of the following effects of the field on the space-charge zone: (i) an increase in the segregated solute ions, (ii) a decrease in grain boundary energy and (iii) a decrease in solute ion mobility.