Papers by Author: Kenji Kaneko

Abstract: Y_1-xGd_xBa₂Cu₃O_7-y film with BaZrO₃ was fabricated on CeO₂ buffered LaMnO₃/ion beam assisted deposition MgO/Gd₂Zr₂O₇/Hastelloy C276^TM substrates by the trifluoroacetates metal organic deposition process, whose microstructural and elemental analyses were performed by transmission electron microscopy. Y_1-xGd_xBa₂Cu₃O_7-y film with the thickness about 700 nm was found composed of c-axis oriented grains and large numbers of randomly oriented precipitates, such as (Y,Gd)₂Cu₂O₅, CuO and BaZrO₃. (Y,Gd)₂Cu²O⁵ and CuO precipitates were heterogeneously dispersed in the Y_1-xGd_xBa₂Cu₃O_7-y matrix with their sizes ranging between 100 and 200 nm, and BaZrO₃ precipitates were uniformly dispersed with their sizes ranging between 10 and 20 nm. Electron tomography with elemental information was performed further to reveal the three-dimensional information of BaZrO3 precipitates.

Abstract: A systematic series of experiments was performed to determine the optimum firing schedule of Y2O3. A two-step sintering with subsequent profile succeeded to fabricate Y2O3 ceramic with the relative density of 97.6% and the average grain size of 0.46 μm: first step and second step temperature of T1 = 1490 °C and T2 = 1350 °C, respectively and second step holding time of t2 = 20 h. After the firing schedule of pure Y2O3 was determined, two types of TiO2/Y2O3 and ZrO2/Y2O3 composites were fabricated for the further improvement of mechanical properties. According to results, ZrO2/Y2O3 composite showed a dramatic increase in strength and toughness as 417 MPa and 4.4 MPa.m1/2, respectively.

Abstract: Crack tip dislocations and dislocations introduced by three point-bending tests at high temperature are observed by combinating scanning transmission electron microscopy and computed tomography (STEM-CT). Commercially available P type (001) single crystal silicon wafers were employed. A series of STEM image was acquired from -60º to +60º in tilt range with 2º in tilt step. The diffraction vector was maintained close to g(hkl) = 220 during the acquisition by adjusting the [110] direction of the sample parallel to the tilt axis of the holder. Reconstructed images of dislocations revealed dislocation structures in three-dimension.

Abstract: Structures and morphologies of Ge precipitates in an Al-Ge alloy were characterized by a combination of transmission electron microscopy and three-dimensional electron tomography. Faceting of the precipitates was clearly seen using transmission electron microscopy and varieties of precipitate morphologies were identified by three-dimensional electron tomography.

Abstract: Photochromic material Ag-TiO2 thin films are fabricated on quartz substrate by dual -target helicon magnetron sputtering. The phototchromic behavior is investigated for the sample loaded with 90% Ag. Spheres, ellipsoids and polyhedra shape of Ag particles with wide range size (5∼100 nm) are dispersed in the TiO2 amorphous matrix observed by transmission electron microscopy. The spectral hole burned by the irradiation of laser at the wavelength 532 nm can be explained by a particle-plasmon-assisted electron transfer from Ag nanoparticles to TiO2 and subsequent trapping by adsorbed molecular oxygen. Moreover, the mechanism of the slow recovery after photochromism is suggested as a slow thermal release of electrons from oxygen trapping centers and subsequent capture into the Ag nanoparticles.

Abstract: Severe plastic deformation (SPD) makes it possible to refine grain size in many metallic materials. Recently, we have developed a new SPD process designated the severe torsion straining process (STSP). This process requires no die but one side of a rod is rotated with respect to the other while producing a local heated zone in the rod and cooling both sides of the heated zone. Torsion strain is then introduced in the local heated zone. The STSP can be a continuous process because the rod is moved in the longitudinal direction while introducing torsion strain through the rotation. For grain refinement using the STSP, various factors may affect, which are the rotation speed, moving speed, straining temperature, cooling rate and diameter of the rod. In this study, the STSP is applied to grain refinement of an A5056 Al-Mg commercial alloy and the factors affecting the grain refinement are optimized. STSP was conducted at a temperature in the range from 573K to 723K. Microstructure was observed by optical microscopy, scanning electron microscopy with an orientation imaging system, and transmission electron microscopy. Microscopy observations revealed that the grain size was reduced to ~0.9 μm, when STSP was conducted at 573K with a rotation speed of 10 rpm and moving speed of 50 mm/min. There is a critical ratio of rotation speed to moving speed above which the rod breaks. The grain size tends to be finer as the straining temperature is lower, the cooling rate is faster and the ratio of rotation speed to moving speed is closer to the critical value.

Abstract: Grain refinement is attempted using severe plastic deformation (SPD) through the severe torsion straining process (STSP) which we have developed recently. The STSP is a continuous process for grain refinement without requirement of any die. In this study, an AZ61 Mg alloy was subjected to STSP at a temperature of 573 K with a rotation speed of 10 rpm and a moving speed of 200 mm/min. With this process, an initial grain size of ~20 μm was reduced to ~2~3 μm. Room temperature compression tests revealed that there were no cracks after 15% of compression for the STSP sample whereas fracture occurred for a conventionally extruded sample. For compression tests at 473 K, no cracks occurred in the STSP samples after 80% compression but compression was feasible without cracking only up to 20% for an extruded sample. It is shown that the STSP can be useful for grain refinement and ductility improvement of the AZ61 Mg alloy.

Abstract: A significant change in microstructure occurs during the application of severe plastic deformation (SPD) such as by equal-channel angular pressing (ECAP). In this study, intense plastic strain was imposed on an Al-10.8wt%Ag alloy by the ECAP process. The amount of strain was controlled by the numbers of passes. After 1 pass of ECAP, shear bands became visible within the matrix. With increasing numbers of ECAP passes, the fraction of shear bands was increased. In this study, the change in microstructures was examined by three-dimensional electron tomography (3D-ET) in transmission electron microscopy (TEM) or scanning transmission electron microscopy (STEM). With this 3D-ET method, it was possible to conduct a precise analysis of the sizes, widths and distributions of the shear bands produced by the ECAP process. It is demonstrated that the 3D-ET method is promising to understand mechanisms of microstructural refinement using the ECAP process.

Abstract: This study presents a new rapid continuous process for grain refinement in metallic materials through severe plastic deformation (SPD). The new process, designated in this study the severe torsion straining process (STSP), is applicable to a wide range of alloys based on aluminum, magnesium and copper including carbon steel. This process consists of producing a local heated zone in a rod and cooling both sides of the heated zone by spray water while rotating one end with the other. Thus, torsion strain is introduced in the local heated zone. The STSP can be continuous because the straining is achieved while the rod is shifted along the longitudinal axis of the rod. Furthermore, the process requires no die, suggesting a potential for commercialization of grain refinement through SPD. In this study, STSP was applied to an Al-Mg alloy and a Mg-Al-Zn alloy. It is shown that STSP is effective for both alloys so that the grain size is reduced to ~1.5 μm for the Al alloy and ~0.9 μm for the Mg alloy. Tensile testing showed that the strength is increased with a minimal decrease in uniform elongation. There is a critical ratio between rotation speed and moving speed, which defines the feasibility of STSP operation without breaking the rod. The grain size tends to be lowered as the ratio is close to the critical value.