Study on deformation and fracture behavior of tuicrystals and its application to design of grain boundaries in polycrystals

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The purpose of this project is to understand the structures of grain boundaries and boundary nodes (triple lines) necessary for strengthening of polycrystals. To achieve the purpose, we have studied roles of a grain boundary and a triple line in deformation and fracture from experiments employing tricrystals. Mains results obtained are summarized as follows.1. We have grown single, bi-and tri-crystals of metals and alloys, and studied deformation and fracture behavior of the crystals by tensile, creep and fatigue tests. Localized deformation and formation and propagation of cracks at and around the grain boundaries and triple lines have been quantitatively and systematically studied including the effects of orientations of the grain boundaries and triple lines. We have revealed contribution of the triple lines to deformation and fracture and its temperature dependence, and presented the structures of grain boundaries and triple lines necessary for the strengthening of materials.2. We have adopted tricrystals consisting of so called special boundaries as main specimens. The same triple lines with the triple lines in the tricrystals have been found in recrystallized copper bicrystals. We have shown the application of the results obtained using the tricrystals to the discussion of mechanical properties of polycrystals.3. Considering the importance of multi-phase materials as practical structural materials, effects of intrphase boundaries on mechanical properties have been studied. We have (gamma/alpha) stainess bicrystals by diffusion bonding and investigated the constraint of grain deformation and the superplastic flow at the interphase boundaries.