Mechanism of Superplastic Flow in a Fine-Grained Ceramic Containing Some Liquid Phase

Several results pertaining to large deformations at fast strain rates in a fine-grained ceramic material are described. Results for strain-rate, grain size, and temperature dependence of the flow stress are presented. They show that (a) ultrafine-grained ceramics are capable of high rates of deformation (about 10−4 to 10−4 s−1) at quite low stresses (1 to 20 MPa); (b) the mechanism of deformation is the enhanced rate of matter transport through the liquid phase segregated in the grain boundaries; (c) either uniaxial compression or tension tests may be used to determine the flow properties, except that a correction must be implemented for friction in the case of compression tests; and (d) microstructural changes can occur during deformation which influence the flow behavior. The ceramic is almost infinitely ductile in compression, whereas in tension elongations as large as W5% in one material, and more than 400% in another, were obtained. A model material, β-spodumene glass-ceramic, was used for this study but the results are likely to hold for other materials with equivalent microstructures, e.g., liquid-phase-sintered or hot-pressed materials such as the nitrogen ceramics