Papers by Author: Hideo Nakajima

Abstract: Lotus-type porous magnesium ingots were fabricated in pressurized hydrogen atmosphere through a mold casting technique. The mold consists of two cooling blocks placed at the bottom and one lateral side. It was found that the pores started to grow upwards and horizontally and the both directional pores collapsed and then shifted to the direction. Such anisotropic growth of pores is in good agreement with the map of temperature gradient predicted by two-dimensional finite differential analysis.

Abstract: Changes in morphology during the oxidation of iron nanoparticles and nanowires at 473~ 873 K have been studied by transmission electron microscopy. Iron nanoparticles and wires become hollow nanoparticles and nanotubes of Fe3O4 at temperatures below 673 K as a result of vacancy aggregation in the oxidation process. On the other hand, the hollow magnetite transforms into duplex porous structures with an interior nanopore and additional nanovoids at higher temperatures above 673 K, where the shrinkage of hollow nanoparticles and nanotubes starts and the phase transformation from Fe3O4 to -Fe2O3 occurs. Transition in porous structure seems to be related to the outward diffusion of vacancies from interior pore and the phase transformation in the shrinkage process.

Abstract: Dynamic and quasi-static compression tests were conducted on lotus-type porous iron with porosity of about 50% using the split Hopkinson pressure bar method and universal testing machine, respectively.　In the dynamic compression parallel to the pore direction, a plateau stress region appears where deformation proceeds at nearly constant stress, while the plateau stress region does not appear in the quasi-static compression. The plateau stress region is probably caused by the buckling deformation of matrix iron which occurs only in the dynamic compression. In contrast, the compression perpendicular to the orientation direction of pores exhibits no plateau-stress regions in the both dynamic and quasi-static compression.

Abstract: Al foams whose matrix contains dispersed AlN particles (Al/AlN composite foams) were prepared by a melt foaming method, and the effect of foaming temperature on the pore morphology of the prepared foams was investigated. First, Al/AlN composites were prepared by non-compressive infiltration of Al powder compacts with molten Al alloy in nitrogen atmosphere. Next, the prepared composites were melted by induction heating and foamed at various temperatures using TiH2 powders as blowing agents. The porosity of prepared Al/AlN composite foams slightly decreases with increasing foaming temperature, and the pore morphology of the foam becomes homogeneous simultaneously. When the foaming temperature is 1123 K, homogeneous pores are formed in all over the ingot. This pore homogeneity is probably achieved by the stabilization of the foaming behavior due to the formation of Al3Ti particles in the melt and dispersion of AlN particles.

Abstract: The elastic and plastic deformation behaviors of lotus-type porous copper (lotus copper) with cylindrical pores oriented in one direction were investigated using two acoustic methods (resonant ultrasound spectroscopy and acoustic emission method). All the independent components of elastic stiffness were determined by resonant ultrasound spectroscopy combined with electromagnetic acoustic resonance method, which revealed that the Young’s modulus exhibits the anisotropy originating from the anisotropic porous structure and anisotropic matrix texture. The porosity dependence of the anisotropic Young’s modulus can be calculated by the micromechanics modeling based on effective-mean-field theory. The tensile deformation behavior of lotus copper was analyzed by acoustic emission method, which revealed that many burst acoustic emission signals are detected during the tensile deformation. This implies that many cracks are formed during the tensile deformation.

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.

Abstract: Lotus-type porous metals with long cylindrical pores aligned in one direction are fabricated by unidirectional solidification through thermal decomposition method (TDM). The pores are evolved from insoluble gas when the molten metal dissolving the gas is solidified. In the conventional pressurized gas method, hydrogen pressurized in a high-pressure chamber is used to dissolve hydrogen in the melt. However, the use of high-pressure hydrogen is not desirable because of its inflammability and explosive nature. This is particularly true when scaling up to mass production of lotus/Gasar metals. In order to overcome this shortcoming, the thermal decomposition method was developed. Gas-forming compounds such as hydrides were added into the molten metal to fabricate lotus/Gasar metals. The porosity and pore size were controlled by the amount of gas-forming compounds, solidification rate, atmospheric pressure, etc. TDM method is applied to fabricate lotus copper, aluminium and iron.

Abstract: The lotus-type porous carbon steel with cylindrical pores has been fabricated by continuous zone melting technique in pressurized mixture of hydrogen and helium gases. In order to investigate the mechanical properties, the tensile and compression tests were carried out. The ultimate tensile strength of the specimen with cylindrical pores parallel to the solidification direction is lower than the estimated value assuming that the strength is decreased in proportion to decreasing cross section area of the specimen, while the yield strength is higher than that estimated. The compressive yield strength is also higher than that estimated. The increase in yield strength is attributed to the precipitation strengthening. The tensile strength is increased by quenching and tempering, while the elongation decreases. Such mechanical properties are discussed in terms of microstructureal analysis. Furthermore, simulations of the mechanical properties by FEM analysis were carried out.

Abstract: The formation mechanisms of hollow metal oxide through the oxidation of several metal nanoparticles have been studied by transmission electron microscopy. For Zn, Al, Cu, Ni and Fe nanoparticles, hollow oxide nanoparticles were obtained as a result of vacancy aggregation in the oxidation processes. The formation of the hollow morphology is attributed to the faster outward diffusion of metal ions through the oxide layer in the oxidation processes. Further changes in morphology during the annealing of hollow Cu, Ni and Fe oxides at higher temperatures in air were examined.

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.