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Yuichi Ikuhara and Taketo Sakuma Department of Materials Science, University of Tokyo, Bunkyo-ku Hongo 7-3-1, Tokyo, Japan (Received 8 September 1999; accepted 10 April 2000)

The thickness distribution of grain-boundary films during the superplastic deformation of fine-grained ␤–silicon nitride was investigated by high-resolution electron microscopy. In particular, grain-boundary thickness was considered with respect to the stress axis in two orientations; namely, parallel and perpendicular to the direction of applied stress. The results showed that the thickness distribution in boundaries perpendicular to the direction of applied stress was unimodal, whereas in parallel boundaries it was bimodal. Moreover, it was found that the majority of film-free boundaries were parallel to the direction of applied stress in the extremely deformed sample. The variation in spacing reflects distribution of stresses within the material due to irregular shape of the grains and the existence of percolating load-bearing paths through the microstructure.

I. INTRODUCTION

Many silicon-nitride–based materials have already been rendered superplastic.1–19 Regardless of the methods used to achieve large strains, a common feature of the materials is the presence of a residual glass phase, resulting from liquid-phase sintering and from being liquidlike at elevated test temperatures. This glass phase exists either as pockets at multiple-grain junctions or as thin films along grain boundaries.20 It is now wellestablished that intergranular phases possess films of even thickness, typically in the range 0.5–2.0 nm depending on material composition.21–25 These remaining phases are known to strongly influence the creep behavior and flow behavior of silicon-nitride–based materials.26–28 Grain-boundary sliding, which is believed to be the main mechanism of superplasticity, should be sensitive to the structure of the grain boundaries. Although many reports have addressed the issues of creep and superplastic behavior, very few have concerned themselves with the nature of grain boundaries, especially at the level of high-resolution electron microscopy (HREM).29–31 Changes in the thickness of intergranular films after creep have been reported by several researchers employing defocused Fresnel fringe imaging (FFI)29 a)

Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 15, No. 7, Jul 2000

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and HREM30 techniques. For example, Jin et al.29 investigated the thickness of grain-boundary films using FFI technique, and found that even though the mean values before and after deformation were almost the same, the standard deviation for film thickness in a given material was considerably larger after creep than before, indicating a significant redistribution of intergranular glass accompanying creep, which directly supports viscous flow mechanisms. They have subsequently also investigated redistribution in the glass phase at grain boundaries during creep deformation