Papers by Author: Noriyuki Takano

Abstract: Cracks are induced during hydrogen charging in Nickel. We have reported that the cracks initiate near the boundary between nickel matrix and nickel hydride formed near the surface by hydrogen charging. The driving force is considered to be residual stress caused by hydride formation. In the present paper, the stress distribution has been calculated near the boundary using Finite element method to clarify the crack initiation mechanism. As a result, the high-stress area corresponded to the area where the cracks were observed in the experiment.

Abstract: Magnesium has made an attention as implant material. Because it is decomposed and absorbed in the body, and its mechanical strength is stronger than that of polymers. It is, however, reported that the corrosion rate increases under the compressive elastic stress. In the other hand, it decreases in the specimen whose surface is applied to compressive residual stress by laser shot peening. This implies that compressive plastic deformation reduces the corrosion rate. In the present paper, the corrosion rate of magnesium that was plastically deformed by uniform high compressive stress was researched. As the result, the corrosion rate decreased as the compressive stress increased.

Abstract: It is known that type 310S stainless steel is insensitive to hydrogen embrittlement. Then, the effect of hydrogen on the mechanical properties has not been studied in detail. In the present work, SSRT was carried out under hydrogen cathodic charging condition. Elongation increased by hydrogen charging, while reduction of area decreased. A lot of micro-cracks were observed on the side surface of fractured specimens into which hydrogen was charged, while no cracks were observed on the side surface of fractured specimens without hydrogen charging. Micro-cracks were also observed on the surface of unloaded specimens that were annealed after hydrogen charging. Therefore, it is considered that cracks occur from the defects induced by hydrogen charging.

Abstract: Hydrogen embrittlement in iron and steels has been studied for long years. Although a few mechanisms of it have been proposed, it has not been clearly understood yet. Especially, the decohesion theory and the hydrogen enhanced localized plasticity theory have been discussed in the center of attention. Hydrogen is considered to induce a cleavage fracture in the former, but enhance slips in the latter. It is essential problem which hydrogen induces cleavage or slip in hydrogen embrittlement. The purpose of the present paper is to clarify the effects of hydrogen in iron single crystal on the fracture or deformation using first principles calculation. In the cleavage model, the changes of total energy were estimated with a distance of a pair of {001} planes in a unit cell composed of 12 iron atoms and a hydrogen atom. In the slip model, the changes of total energy were estimated with a displacement of a few {110} layers toward <111> direction. Hydrogen reduces the total energy in the cleavage model, but does not change that in the slip model.

Abstract: Notched tensile tests {orientation: tensile axis [001], direction of notching [010]or[110] } were performed to investigate the crack growth process in Ni-single crystal with hydrogen cathodic charging. Y-shaped hillocks and striation-like patterns were observed on the fracture surfaces of hydrogen embrittled specimens using the scanning electron microscope (SEM). The Y-shaped hillocks are exactly matched on both fracture surfaces of a specimen. In addition, cleavage fracture is occurred between a Y-shaped hillock and next one. It means that the cleavage fracture is occurred at first, and the Y-shaped hillock is secondary formed. It is considered that the striation-like pattern is formed by the localized slips and cleavage fracture between them. Peach-Koehler’s law leads that the dislocations on the equivalent slip systems with (111　)　[101] are subjected to the highest force of all dislocations at the crack tip. Furthermore, Ni-hydride was not observed on the surface of specimen just after charging hydrogen. Thus, it was concluded that Y-shaped hillock and striation-like pattern were occurred in Ni-single crystal with charging hydrogen under condition that Ni-hydride is not formed.