The deformation mechanisms of superplastic flow in fine-grained beta-silicon nitride ceramics

The superplastic deformation of fine-grained beta-Si3N4 materials containing 5 wt% yttria and 2 wt% magnesia was investigated for the temperature range 1623-1973 K under compression. A strain rate of 5 x 10(-5)/s was achieved for the first time in a fine-grained material at the relatively low deformation temperature of 1623 K. The stress exponent, n, and the grain-size exponent, p, were calculated for 1823 K to be close to 1 and 3, respectively. The activation energy was found to be 298 kJ/mol for temperatures below 1823 K and 547 kJ/mol for temperatures equal to and above 1823 K. These results suggest that the rate-controlling mechanism for superplasticity at temperatures below 1823 K is grain-boundary sliding controlled by the viscous flow during the glass phase, whereas at temperatures of 1823 K and above the mechanism is solution-reprecipitation, which is controlled by liquid-phase-enhanced diffusion. An explanation of superplasticity is proposed, based on an existing model of viscous flow, in terms of squeezing out and replenishing the grain-boundary phase. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.