Papers by Author: Taketo Sakuma

Abstract: Superplasticity in fine-grained oxide ceramics has been generally elucidated on the basis of their experimental strain rate-flow stress relationship and phenomenological analysis of cavity nucleation and growth. It has been widely accepted that the high temperature superplastic flow and failure in ceramics is significantly influenced by the atomic structure and chemistry of grain boundaries. Such phenomenon cannot be explained based on the classical phenomenological analysis. Our research group has therefore proposed to establish a new research field, grain boundary plasticity, to describe the superplastic deformation related to the grain boundary atomic structure. This paper aims to point out the importance of the atomistic analysis of grain boundary to develop new superplastic ceramics.

Abstract: A new interpretation model, instead of classical Kohlrausch-Williams-Watt (KWW) equation, was applied to interpret stress relaxation behavior of Si3N4-Y2Si2O7 ceramics. Results revealed that the new model could obtain reasonable relaxation plastic viscosity and viscoelastic viscosity under testing temperature range of 1300°C~1575°C. From the plotted curve of viscosity vs 1/RT, an activation energy change occurred around 1500°C was found for both plastic viscosity and viscoelastic viscosity curves. This change, which indicated the microstructure change, was in good agreement with the significant decrease of high temperature strength retention property.

Abstract: High temperature creep and superplastic flow in high-purity oxide ceramics such as alumina and tetragonal zirconia polycrystals is very sensitive to a small amount of doping by various oxides. High-resolution transmission electron microscopy and an energy-dispersive X-ray spectroscopy analysis revealed that grain boundaries in high-purity oxide ceramics are free from amorphous phase, and that the doped cations tend to segregate along the grain boundaries. Since the high temperature plastic flow in oxide ceramics takes place mainly by grain boundary matter transport by atomic diffusion, the grain boundary nano-structure control must be a useful way to develop new high-performance functional ceramics in the near future.