Papers by Author: Kenji Higashida

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Authors: Kenji Higashida, Masaki Tanaka, Sunao Sadamatsu
Abstract: Three-dimensional structure of crack tip dislocations were investigated by combining scanning transmission electron microscopy (STEM) and electron tomography (ET) in silicon single crystals. P-type (001) silicon single crystals were employed. <110> cracks were introduced from an indent on the (001) surface. The specimen was heated at 873K in order to introduce dislocations at the crack tips. The specimen was thinned to include the crack tip in the foil by an iron milling machine. STEM-ET observation revealed the three-dimensional structure of crack tip dislocations. Their Burgers vectors were determined by using an invisibility criterion. The local stress intensity factor was calculated using the dislocation characters obtained in the observation in this study, indicating that the dislocations observed were mode II shielding type dislocations.
Authors: Masaki Tanaka, Yumi Hoshino, Alexander Hartmaier, Kenji Higashida
Abstract: Two dimensional simulations of discrete dislocation dynamics were carried out to clarify a shielding effect due to dislocations at a crack tip. The configuration of dislocations around the crack tip was calculated under the conditions of mode I tensile load at high temperatures. The stress field around the crack tip due to dislocations was found to be compressive, accommodating mode I stress intensity at the crack tip. In order to experimentally confirm the stress accommodation, infrared photoelastic observation was also performed in a specimen pre-deformed at high temperatures. The experimental result is in good agreement with a simulated infrared photoelastic image derived from the stress field calculated.
Authors: Sunao Sadamatsu, Masaki Tanaka, Kenji Higashida, Kenji Kaneko, Masatoshi Mitsuhara, S. Hata, M. Honda
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.
Authors: Kyohei Kawamoto, Y. Aoki, Yasuji Oda, Takeshi Yoshimura, Hiroshi Noguchi, Kenji Higashida
Abstract: In order to clarify the effects of hydrogen on the fatigue characteristics of an austenitic stainless steel, bending fatigue tests were conducted in air, in a hydrogen gas and in a nitrogen gas. Main results obtained are as follows. Effects of hydrogen gas environment are not clearly seen on the strain range - fatigue life diagram, because there are opposite effects to crack propagation and to crack initiation; accelerates crack propagation, but retards crack initiation. Striation spacing or in-situ observation confirms the acceleration. The retardation seems to be attributed to the absence of oxygen or water vapor in the hydrogen gas.
Authors: Y. Aoki, T. Matsuyama, Yasuji Oda, Kenji Higashida, Hiroshi Noguchi
Abstract: In order to investigate the hydrogen gas effect on non-propagation phenomena of a type 304 austenitic stainless steel, fatigue tests with in-situ observation using a Scanning Laser Microscope were performed in air, in 0.18MPa hydrogen gas and in 0.18MPa nitrogen gas. A nonpropagating crack was observed during the fatigue test in air. At almost the same stress level of non-propagating in air, non-propagating cracks were also observed in fatigue tests in hydrogen and in nitrogen. Stress level of the non-propagation is not sufficiently different in the three environments. However, the process up to non-propagation differs from each other, for example, the crack path and debris.
Authors: Tatsuya Morikawa, Daisuke Kinoshita, Yoshihito Kawamura, Kenji Higashida
Abstract: Microstructures developed by warm extrusion for Mg97Zn1Y2 alloy including long-period stacking order (LPSO) phase have been investigated using SEM and TEM. The extruded magnesium alloy with LPSO phase exhibits high strength and sufficient ductility. Such superior mechanical properties appear by warm extrusion around the temperature of 623K. The microstructure of the extruded alloy consists of matrix of fine-grained hcp phase and elongated grains with fine-lamellae including LPSO phase. The grain size of hcp matrix was about 1μm, indicating that remarkable grain refinement was occurred by extrusion since the grain size of as-cast alloy was about 500μm. Special attention has been paid on the enrichment of solutes at stacking faults and grain boundaries in the fine-grained matrix, which would contribute not only to the strengthening but also to the stability of fine-grained structure because of its role of an inhibiter against grain coarsening.
Authors: Tatsuya Morikawa, Taku Moronaga, Kenji Higashida
Abstract: Fine-grained structures in Fe-36mass%Ni Invar alloy have been investigated by using transmission electron microscopy (TEM). Particular attention has been paid on the role of deformation twinning in the formation of fine-grained structures and its influence on tensile stressstrain behaviours of rolled specimens. In Fe-Ni Invar alloy with a moderate stacking fault energy, deformation twin did not appear in usual cold-rolling at room temperature (RT), where a kind of cell walls was formed and the Cu-type texture was observed. On the other hand, twinning was occurred by rolling when specimens were cooled to liquid nitrogen temperature (LNT) immediately before the rolling. In such case, a fine lamellar structure was developed and the brass-type texture appeared. However, the lamella boundaries did not coincide with any crystallographic planes, and they were intersected with the bands of deformation twin. Specimens rolled by 90% in thickness reduction exhibited tensile stressstrain behaviours similar to those observed in specimens with SPD structures. In particular, specimens rolled at LNT showed high yield strengths and non-uniform deformation.
Authors: Masaki Tanaka, Naoki Fujimoto, Tatsuo Yokote, Kenji Higashida
Abstract: The enhancement of toughness at low temperatures in fine-grained low carbon steel was studied, basing on the theory of crack-tip shielding due to dislocations. Low carbon steel was subjected to an accumulative roll bonding (ARB) process for grain refining. The grain size perpendicular to the normal direction was decreased to approximately 200nm after the ARB process. The fracture toughness of low carbon steel with the ARB process was measured at 77K by four-point bending, comparing with the fracture toughness of those without the ARB. It was found that the value of fracture toughness at 77K was increased by grain refining due to the ARB process, indicating that the ARB process enhances toughness at low temperatures and that the brittle-to-ductile transition (BDT) temperature shifted to a lower temperature. Quasi-two-dimensional simulations of dislocation dynamics, taking into account crack tip shielding due to dislocations, were performed to investigate the effect of a dislocation source spacing along a crack front on the BDT. The simulation indicates that the BDT temperature is decreased by decreasing the dislocation source spacing.
Authors: Kaveh Edalati, Z. Horita, Masaki Tanaka, Kenji Higashida
Abstract: High-pressure torsion (HPT) was conducted on commercial grade pure titanium (99.4%) by applying pressures in a wide range from 1.2 to 40 GPa. When the microhardness was plotted against equivalent strain, the hardness saturates to a constant level at each applied pressure. Such a level at the saturation depends on the applied pressure: for up to the pressure of 4 GPa, the saturation level is independent of the pressure but, for the pressures above 4 GPa, the hardness gradually increases with pressure because of the formation of an  phase. Bending tests showed that an excellent ductility as well as high bending strength was achieved for the sample processed at 2 GPa. The bending ductility was reduced for the sample at 6 GPa because of the  phase formation.
Authors: Kaveh Edalati, Z. Horita, Hiroshi Fujiwara, Kei Ameyama, Masaki Tanaka, Kenji Higashida
Abstract: Pure Ti powders were subjected to ball milling and subsequently high-pressure torsion (HPT) for consolidation. It is found that a fully dense (99.9%) disc with ultrafine grained structure (~50-300 nm) was produced. The strength and ductility were well comparable to those of ball-milled Ti-6%Al-4%V powders after hot roll sintering.
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