Improvement of superplasticity in ceramics due to grain boundary control

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The research results are summarized as follows.(1) Superplasticity in beta-spodumene glass-doped TZP exhibits that the high-temperature flow stress is reduced by the presence of grain boundary glass phase, and that the tensile ductility is not simply caused by flow stress but is determined by the nature of interfaces between ZrO_2 grain and glass phase.(2) Superplasticity in TZP is highly enhanced by SiO_2-doping. However the ductility is deteriorated by small dopant in SiO_2 phase. It is essential to analyze the change of grain boundary structure and chemical bonding state with dopant.(3) Amorphous SiO_2 phase is present in grain boundary corners or junctions of SiO_2-doped TZP but not in grain boundary faces. Si^<4+> ions segregate in grain boundary faces but does not form amorphous film.(4) A unique superplastic flow behavior is founf in TiO_2-doped TZP.TiO_2-doping results in grain size increment but in the reduction of flow stress and the improved ductility. A phenomenological analysis is made to explain the unique behavior.(5) New experimental evidence is found in superplastic flow in Al_2O_3-base ceramics. It is important to understand the grain boundary structure and chemistry to explain the results.