Papers by Author: Tatsuya Okada

Abstract: Residual stresses of a copper bicrystal were measured by X-ray diffraction and synchrotron radiation. A copper bicrystal specimen with a 90-degree tilt boundary was fabricated by the Brigdman technique. After the plastic extension of 30%, kink bands developed in a deformed matrix along the grain boundary. In this study, we focused on the residual stress distribution along the transverse direction of the specimen surface and the residual stresses in deformed matrix and kink band near the grain boundary. Residual stresses were evaluated by the X-ray single crystal measurement method. Stereographic projections were used to determine crystal orientations of deformed regions. It was found that crystal orientations were different between the deformed matrix and the kink band. Residual stresses in the direction along the grain boundary were compressive in the vicinity of the boundary and tensile in the region apart from the boundary. Residual stresses in the kink band were large in compression in compared with those in the deformation matrix. The difference in the results between X-rays and synchrotron radiation suggests that there is a depth variation in the deformation and therefore the residual stress development.

Abstract: Residual stresses near the grain boundary of a bicrystal were measured by synchrotron radiation of SPring-8 at Japan Synchrotron Radiation Research Institute. A copper bicrystal specimen with a 90-degree tilt boundary was deformed 30% in tension. After the plastic extension, kink bands developed in a deformed matrix along the grain boundary. In this study, we focused on the residual stresses in the deformed matrix and the kink band. Residual stresses were evaluated by the X-ray single crystal measurement method. Stereographic projections were used to determine crystal orientations of deformed regions. Our observation showed that crystal orientations were different between the deformed matrix and the kink band. Residual stresses in the direction along the grain boundary in the deformed matrix and kink band were compressive. Residual stresses in the direction vertical to the grain boundary were seen opposite between the deformed matrix and the kink band.

Abstract: We carried out cross-sectional transmission electron microscopy (TEM) investigation of femtosecond laser-induced ripples formed on 4H-SiC single crystal surface. Here, we paid attention to the crystal structures underlying the coarse and fine ripples and the three-dimensional distribution of amorphous phase. Conventional and high-resolution TEM analyses made clear that a continuous amorphous layer approximately of 50 nm thick exist at the topmost region of both coarse and fine ripples. The result strongly suggests that the fundamental surface deformation process is common for coarse and fine ripples, even though the factors which determine their periods are different.

Abstract: It is shown using aluminum single crystals with kink bands like a plate through the depth that the nucleation and growth of recrystallized grains (RGs) on/in the surface precede those in the interior.

Abstract: Plan-view transmission electron microscopy (TEM) was applied to investigate the origin of surface defects in 4H-SiC homoepitaxial films. Their origin existed at the substrate/epi-film interface. Hence, almost the entire thickness of the epi-film was removed by plasma etching leaving a very thin film. Then, the etched epi-crystal was thinned from the substrate side so that we could observe the crystallographic imperfections at the substrate/epi-film interface in plan-view TEM. Morphological features of the epi-film surface remained unchanged after the plasma etching process. Hence, one-to-one correspondence between surface defects and crystallographic imperfections was confirmed by comparing optical micrographs and TEM images. Crystallographic imperfections associated with “carrot defects” were observed. They were composed of stacking faults on the (0001) plane and partial dislocations bounding them. These imperfections originated from foreign particles at the interface. From X-ray energy-dispersive spectrometry (XEDS), it was confirmed that particles contained zirconium (Zr). Selected area diffraction patterns showed that the particles were crystalline.

Abstract: Deep-ultraviolet (DUV) micro-Raman spectroscopy was applied to study the micro structures of surface defects in a 4H-SiC homoepitaxially grown film. From DUV Raman spectrum, inclusions of 3C-SiC was found in comet defects. The shape of 3C-structure in comets was investigated and it was found that 3C inclusions in comets can be classified into two types. In addition, spectrum broadening due to the coupling of nonfolded longitudinal optical phonon mode and the photo-excited carriers was also found. The formation mechanisms of 3C inclusion in comets were discussed.