High Temperature Microplasticity of Fine-Grained Ceramics

Abstract
Several fine-grained ceramics exhibit enhanced ductility or even structural superplasticity at high temperature. Grain boundaries play a dominant role in the deformation process of these materials which usually involves diffusion-accommodated grain boundary sliding. Sliding is either lubricated by an amorphous intergranular phase or takes place by glide and climb of grain boundary dislocations. At high temperature, anelastic deformation precedes plastic deformation and stems from the short range motion of lattice defects, such as dislocations and grain boundaries. The energy loss ("mechanical loss") associated with such motion can be measured by using the technique of mechanical spectroscopy. Moreover, at the onset of plasticity ("microplasticity"), long range irrecoverable motion of defects contributes to additional mechanical loss. Mechanical loss spectra may then give an insight into mechanisms operating at the transition between anelastic and plastic deformation. As an illustration, the spectra of three fine-grained ceramics (Si₃N₄, ZrO₂, Al₂O₃) are presented. In all cases, anelastic relaxation phenomena (peak and background) have been observed at high temperature (>1200K), bearing a close relation with creep behaviour. Their analysis permits to distinguish between different types of microstructural elements : bulk regions of amorphous intergranular phase at triple points, grain boundaries separated by a thin glassy film and "clean" grain boundaries.