Papers by Author: Koji Morita

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Authors: Byung Nam Kim, Keijiro Hiraga, Koji Morita, Hidehiro Yoshida
Abstract: For steady-state deformation caused by grain-boundary diffusion and viscous grain-boundary sliding, the creep rate of regular polyhedral grains is analyzed by the energy-balance method. For the microstructure, the grain-grain interaction increases the degree of symmetry of diffusional field, resulting in a decrease of the effective diffusion distance. Meanwhile, the viscous grain-boundary sliding is found to decrease the creep rate. The present analysis reveals that the grain-size exponent is dependent on the grain size and the grain-boundary viscosity: the exponent becomes unity for small grain sizes and/or high viscosity, while it is three for large grain sizes and/or low viscosity.
Authors: Byung Nam Kim, Keijiro Hiraga, Koji Morita, Hidehiro Yoshida
Abstract: When the sliding of the grain boundary containing hexagonal particles is accommodated by grain-boundary diffusion, we evaluate the sliding rate and the stress distribution on the boundary, by taking the particle rotation and the intrinsic boundary viscosity into account. The sliding rate was obtained by the energy-balance method, and the particle-rotation rate by a condition of minimum energy-dissipation. With increasing boundary viscosity, the rotation rate increases and then decreases after a maximum. The sliding rate is enhanced by the particle rotation, and decreases with the boundary viscosity.
Authors: Yoshio Sakka, Tohru Suzuki, Koji Morita, Keijiro Hiraga
Authors: Koji Morita, Byung Nam Kim, Hidehiro Yoshida, Keijiro Hiraga
Abstract: The densification mechanism in park-plasma-sintering (SPS) processing was examined in MgAl2O4 spinel. As the relative density ρt increases, that is, as the effective stress σeff decreases, stress exponent n evaluated from effective stress-densification rate relationship continuously varies from n  4 to n  1. TEM observation shows that significant stacking faults caused by partial dislocations are frequently observed in the low ρt region. The results suggest that, for spinel, the predominant densification mechanism in SPS processing changes with ρt from plastic flow by a partial dislocation motion in the low ρt region (n  4) to diffusion-related creep in the high ρt region (n  1).
Authors: Koji Morita, Byung Nam Kim, Hidehiro Yoshida, Yoshio Sakka, Keijiro Hiraga
Abstract: In order to attain high-strain-rate superplasticity (HSRS) in ceramics, flow behavior was examined with ZrO2 reference sample. The results suggest that the enhancement of the accommodation processes of grain boundary sliding (GBS) is important in addition to the careful controlling the microstructural factors, such as stable fine grain structure, reducing residual pores and so on. The spinel particles dispersion can simultaneously provide the following positive factors to ZrO2: i) suppressed grain growth due to pinning effect of spinel particles, enhanced accommodation due to ii) accelerated relaxation of stress concentrations exerted by GBS through dislocation motion and iii) accelerated lattice diffusion caused by the dissolution of aluminum and magnesium into ZrO2 from the spinel particles. The positive factors due to spinel dispersion make it possible to attain HSRS in ZrO2 ceramics.
Authors: Keijiro Hiraga, Hidehiro Yoshida, Koji Morita, Byung Nam Kim
Abstract: In tetragonal zirconia, possibility is investigated of densification with finer grain sizes under the combination of doping and sintering in air. The materials used are CIP'ed compacts of 3-mol%-yttria-stabilized tetragonal zirconia (3Y-TZP) doped with a small amount of cations. For a given sintering temperature and initial density of the compacts, while the doped cations enhances densification in the latest stage of sintering, the effect is different in grain growth during densification: a doped cation tended to enhance grain growth, whereas the other cations tended to suppress grain growth. As a result, the doping of the latter cations brings about a grain size finer than that of the undoped 3Y-TZP for a given relative density.
Authors: Hidehiro Yoshida, Koji Morita, Byung Nam Kim, Keijiro Hiraga, Takahisa Yamamoto, Taketo Sakuma
Authors: Koji Morita, Keijiro Hiraga, Byung Nam Kim, Hidehiro Yoshida, Yoshio Sakka
Abstract: An average particle size of d ≈ 300 nm can be reduced to a nanocrystalline size of about 10 nm or less after 400 h HEBM process. Amorphous-like phase is also observed among the nanocrystalline particles. Using SPS, the nanocrystalline powder can successfully be consolidated into a dense nanocrystalline ZrO2-spinel composite of d ≈ 90 nm. As compared with the data for submicro-grain composite with d = 350 nm, nano-crystalization increased the strain rate by one order of magnitude or lowered the deforming temperature by about 100 K.
Authors: Keijiro Hiraga, Koji Morita, Byung Nam Kim, Hidehiro Yoshida
Abstract: In a high-purity 8Y-CSZ, the doping of 0.15 - 5 mass% pure silica introduces a glass phase dispersing uniformly along grain-boundary facets and at multiple junctions. For materials with grain sizes of 0.75 - 2.4 m, the dispersion of the glass phase decreases the elastic modulus, the Vickers hardness and the elastic modulus-to-hardness ratio, whereas it affects little in the fracture toughness measured by a Vickers-indentation method and a single-crack-precracked-beam method. Inspection of crack propagation paths shows that the glass phase with sizes smaller than those of the matrix grains is not a site for easy crack-propagation, but provides a site for a crack-deflection mechanism.
Authors: Hidehiro Yoshida, Koji Morita, Byung Nam Kim, Keijiro Hiraga
Abstract: High temperature creep and superplastic flow in high-purity, polycrystalline oxide ceramics is very sensitive to a small amount of doping by various oxides. The doping effect is attributed to change in grain boundary diffusivity owing to grain boundary segregation of the doped cations. The doping effect on the grain boundary diffusivity is caused mainly by change of chemical bonding state in the vicinity of the grain boundary segregated with the doped cations. In other words, controlling of grain boundary nanostructure based on the doping process will be a useful way to develop new high-performance functional ceramics in the near future.
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