Papers by Keyword: Crystal Deformation

Abstract: If the crystal grain size of a metal is made smaller, its strength is higher. So, many methods of grain refinement have been proposed. In this study, from the viewpoint of basic plastic working, the variations of static ultimate tensile strength and fatigue strength after the application of plastic torsional deformation on face centered cubic crystal metals, that are, aluminum and copper, were investigated. Tensile test, Vickers harness test and Rotating bending fatigue test were performed. The hardness of the materials varied from surface layer to center section in cross sectional area. In the case of aluminum, the tensile strength and fatigue limit were improved after application of torsional deformation. However, in the case of copper, the fatigue limit was not improved. This is strongly related to hardness distribution around the surface layer of the specimen. Also, it was found that the crack growth mode was changed by applying the pre-strain. From these results, one of an idea for improvement of material strength will be considered.

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: Microscopic deformation of each crystal of duplex phases of Ni-Ti-Nb alloy due to hydrogen absorption was investigated by X-ray diffraction technique. Ni30Ti30Nb40 which is hydrogen permeation alloy and consists of the primary phase, NbTi, and the eutectic phases, NiTi + NbTi, was used as a specimen. The change of lattice spacing of the specimen during hydrogen absorption was measured by Cu-Kα characteristic X-ray. As a result, the lattice spacing of crystal of NbTi phase increased extremely, while that of NiTi phase increased slightly. It was pointed out that the NbTi phase is responsible for hydrogen absorption in the Ni-Ti-Nb alloy. When hydrogen gas was released from the specimen at high temperature, both lattice spacing returned nearly to those of them before hydrogen absorption, and the specimen kept its original shape. Therefore, it was confirmed that the volume expansion of crystal of the Ni-Ti-Nb alloy due to hydrogen absorption was elastic deformation.