Superplasticity can be generally achieved by grain boundary sliding (GBS). The GBS in polycrystalline materials sometimes accompanies with intergranular fracture because of stress concentrations at triple points and/or GB irregularities. To develop the superplastic flow, it is necessary to suppress the intergranular cracking. In the present study, therefore, polycrystalline molybdenum with distinct GB microstructures, such as grain boundary character distribution (GBCD), has been employed to clarify the relationship between fracture behaviour and GB microstructures. Microstructures were analyzed using a FE-SEM/EBSP/OIM system prior to 4-points bending tests at 77K, thereafter, crack propagation was observed. The main results obtained are as follows. Stress fluctuations on stress - strain curves were observed for specimens with random oriented grains, whereas such behaviour rarely occurred for ones with textured grains. Difference in the behaviour would result from difference in the fracture mode; while cleavage fracture was the predominant mode for the latter case, not only cleavage fracture but also intergranular fracture, particularly along random grain boundaries, took place for the former case. These evidences suggest that random GBs play a role as weak intrinsic defects, so that the connectivity of random GBs would become important. In addition, fracture strength depended remarkably on the frequency of low Σ GBs. When cracks propagated predominantly along GBs, the fracture strength increased with increasing the frequency of low Σ GBs up to cleavage strength. This tendency would probably be of great significance at high temperatures, because cleavage fracture would be expected to be limited at high temperatures. Accordingly, to introduce a large number of random grain boundaries leads to extensive microcracks along the boundaries, probably resulting in nonsuperplastic behaviour.
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Tsurekawa et al. (1999) studied this question.