Superplasticity in a fine-grained beta-silicon nitride ceramic containing a transient liquid

Compressive deformation in a dense fine-grained beta-Si3N4 sintered with 7 wt% cordierite was investigated over a wide range of temperatures (1450-1650 degrees C) and strain rates (1 x 10(-5) - 1 x 10(-3)/s). The superplastic characteristics and the results of microstructural examination are presented. These indicate that: (a) fine-grained beta-Si3N4 ceramics are capable of high rates of deformation (about 10(-5) - 10(-4)/s) at 1550 degrees C without strain hardening occurring; (b) remarkable phase and microstructural evolutions, such as formation of elongated and strongly textured Si2N2O grains occurred during deformation; (c) a transient liquid phase present in this material system which facilitated the formation of Si2N2O grains enhanced the superplastic flow, and well-aligned Si2N2O grains formed in situ did not have a detrimental impact on the superplasticity; and (d) the mechanism of deformation is grain boundary sliding accommodated by diffusion-controlled solution-precipitation creep. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.