Analysis of Grain Boundary Microstructure and Grain Boundary Control

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Development of superplasticity is a key technology to overcome poor workability of intrinsically brittle materials such as refractory metals and ceramics. The goal of this project was to obtain a principle of grain boundary control and design to achieve superplasiticity. For this purpose, we quantitatively analyzed grain boundary microstructures such as the grain boundary character distribution (GBCD) in polycrystalline materials, and examined the relationship between superplastic behaviour and grain boundary microstructures. The chief results obtained are as follows :(1) Improvement of workability of brittle polycrystalline materials by controllinng grain boundary : Relationship between intergranular fracture behaviour and grain boundary microstructures was examined using systematically the grain size and the GBCD controlled molybdenum polycrystals. We have found that fracture strength depends on the grain size with the similar manner to the Hall-Petch relation. A particularly importa … More nt finding is that the observed Hall-Petch relation depends on the GBCD, in trun, the grain size dependence of fracture strength becomes more significant as the frequency of low Σ coincidence boundary decreases. In addition, we examined the evolution of grain boundary microstructures and cavitation at grain boundaries during superplastic deformation of Al-Li alloys. In the course of superplastic deformation, grain boundaries change their character towards increase in the frequency of random grain boundaries that slide more readily, and cavities preferentially nucleate at the grain boundary triple junctions where random grain boundaries interconnect. From these results, we provided insights on an optimized grain boundary microstructure not only to develop superplasticity but also to reduce cavitation damage.(2) Grain boundary structure and superplasticity in ceramic materials : Grain boundary structures in fine grained silicon carbides with addition of 7mol% AlィイD22ィエD2OィイD23ィエD2, 2mol% YィイD22ィエD2OィイD23ィエD2 and 1mol% CaO were observed by transmission electron microscopy. Then, development of superplasticity in SiC was examined in connection with the grain boundary structure. TEM observation revealed that the frequency of low angle and coincidence grain boundaries in SiC showing superplastic flow was ca. 25% and tended to decrease during deformation. We also investigated superplasticity in AlィイD22ィエD2OィイD23ィエD2/TiC composites that were sintered with different temperatures and with different amounts of TiC. A particular attention was paid to the role of TiC in superplastic deformation of AlィイD22ィエD2OィイD23ィエD2/TiC composite. A maximum superplastic elongation was obtained with the specimen containing 10 mol% TiC, and TiC particles were found to be responsible for retardation of grain growth in AlィイD22ィエD2OィイD23ィエD2. Less