Papers by Author: Keijiro Hiraga

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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: Keijiro Hiraga, Yoshio Sakka, Tohru Suzuki, K. Nakano
Authors: Keijiro Hiraga, K. Nakano, Tohru Suzuki, Yoshio Sakka
Authors: Keijiro Hiraga, K. Nakano, Tohru Suzuki, Yoshio Sakka
Authors: Yoshio Sakka, Tohru Suzuki, Koji Morita, Keijiro Hiraga
Authors: Koichiro Morita, Yoshio Sakka, Keijiro Hiraga
Authors: Koichiro Morita, Keijiro Hiraga, Yoshio Sakka
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).
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