Papers by Author: Hideo Nakajima

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Authors: Shunkichi Ueno, Li Ming Lin, Hideo Nakajima
Abstract: A new fabrication method of porous ceramics using unidirectional solidification was proposed in this paper and the role of silica additive on the formation of porous alumina structure was discussed. A porous alumina was formed by unidirectional solidification process that was conducted using floating zone melt method in hydrogen atmosphere at normal pressure. The effect of silica additive on the porosity and the pore size of the solidified samples was examined. The porosity of the solidified samples increased with increase of silica contents. On the other hand, the pore size decreased with increase of silica content. The average pore size of 10% and 20% silica added samples were 0.396 mm and 0.184 mm, respectively.
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Authors: Takuya Tsumura, Taichi Murakami, Soong Keun Hyun, Hideo Nakajima, Kazuhiro Nakata
Abstract: Effects of pore directions on the profile of fusion zone for lotus-type porous magnesium by laser welding has been investigated by comparing the experimental observations and the results of numerical simulation. The three-dimensional finite element calculations were performed, which takes into account equivalent thermal properties and anisotropy of thermal conductivity. There is the pore anisotropy in the profile of fusion zone by laser beam irradiation and the good weldability was obtained when the growth direction of the original pore equaled to the direction of the laser beam irradiation. A good agreement was obtained between the calculated profile of the weld fusion zone and the experimental results.
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Authors: Masakazu Tane, Rika Okamoto, Hideo Nakajima
Abstract: The tensile deformation of lotus-type porous copper with cylindrical pores oriented in one direction was investigated. Deformation was occured homogeneously in the copper matrix for loadings parallel to the orientation direction of pores (pore direction), while deformation was localized in the matrix around pores for loadings perpendicular to the pore direction. In the case of parallel loading the decrease in cross section of tensile specimen was smaller than that of nonporous copper, because of the constant-volume law (i.e. incompressibility condition) for deformation was not applicable to the deformation of pores. In the case of perpendicular loading, the deformed regions were disconnected and constant-volume law holds only in the matrix around the pores, and thus, the cross section hardly decreases during the tensile deformation.
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Authors: Takafumi Yoshikawa, Hajime Ohgushi, Toshimasa Uemura, Y. Ueda, Hideo Nakajima, Y. Enomoto, Kunio Ichijima, Yoshinori Takakura, Tetsuya Tateishi
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Authors: Hajime Ohgushi, Y. Dohi, Toshiaki Noshi, M. Ikeuchi, Takafumi Yoshikawa, M. Okumura, Hideo Nakajima, Yoshinori Takakura
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Authors: Hiroshi Utsunomiya, Tsuyoshi Yukimoto, Tetsuo Sakai, Shinsuke Suzuki, Hideo Nakajima
Abstract: Although forming of porous metal is demanded for industrial applications, the deformation characteristics have not been investigated sufficiently. In this study, lotus-type porous copper is processed by multi-pass cold rolling. At the early stage of rolling, the elongation of the porous copper in the rolling direction is small, and the porosity decreases almost linearly with the total reduction in thickness. It is found that pass schedule with small rolls and with small reduction per pass is effective to suppress pore closure. Hardness of the porous copper increases almost linearly with total reduction. If the effective total reduction is considered, the hardness change is similar to that of a nonporous copper.
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Authors: Hideo Nakajima, Ryusuke Nakamura
Abstract: . In intermetallic compounds, random vacancy motion is not possible as it would disrupt the equilibrium ordered arrangement of atoms on lattice sites. In view of this limitation, various atomistic models have been proposed, which allow atom-vacancy exchanges to take place without concomitant long range disordering. For a L12 -type A3B structure, the major element A diffuses faster than the minor element B. The trend is attributed to the different diffusing paths; A atoms can diffuse through site exchanges with a neighbouring vacancy on its own sublattice, while the jump of a B atom to a neighbouring site always creates wrong bonds. For L10-type structures such as γ-TiAl, significant diffusion anisotropy is observed; Ti atoms diffuse on the Ti sublattice, while Al atoms also diffuse on the Ti sublattice. The formation of hollow metal oxide nanoparticles through the oxidation process has been studied by transmission electron microscopy for Cu, Zn, Al, Pb and Ni. The hollow structure is obtained as a result of vacancy aggregation, resulting from the rapid outward diffusion of metal ions through the oxide layer during the oxidation process. This suggests the occurrence of two different diffusion processes in the formation of hollow oxides.
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Authors: Hideo Nakajima, Y. Nosé, N. Terashita, Teruyuki Ikeda, Hiroshi Numakura
Abstract: The diffusion coefficient of In in TiAl has been measured using ion implantation technique and secondary ion mass spectrometry. The diffusion coefficients of Fe and Pd in FePt have been measured at two compositions by radioactive tracer method. In order to clarify diffusion anisotropy, single crystal of each alloy was used. The In diffusion perpendicular to the [001] axis is faster than that parallel to the [001] axis. Such trend is similar to Ti diffusion previously measured in our group. The diffusion of Fe in FePt perpendicular to the [001] axis is faster than that parallel to the [001] axis at each composition, while the anisotropy of the Pd diffusion is different with composition. The predominant process of the diffusion in perpendicular to the [001] has been discussed on the basis of the expressions of the diffusion coefficients.
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Authors: Hiroyuki Y. Yasuda, Hideo Nakajima, M. Koiwa
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Authors: Hideo Nakajima, W. Sprengel, K. Nonaka
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