Papers by Author: Hiromoto Kitahara

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Authors: Kounosuke Nakamura, Hiromoto Kitahara, Shinji Ando
Abstract: {10 2} twin is common in pure hcp metals. In this study, initiation and development of {10 2} twin in hcp metal was simulated by molecular dynamics (MD) method. Two types of model crystals were stretched along the y-axis by applying displacement of 0.005a (a is lattice constant of the basal plane) every 1000 MD steps and relaxed atoms by molecular dynamics method. The Y-boundary of the model was applied fixed boundary condition and the X and the Z-boundary were free boundary condition. The Lennard-Jones type interatomic potential was employed in this simulation. In the single crystal, {10 1} pyramidal slip initiated at the crack tip and the pyramidal slip was stopped at Y-boundary atoms. After that, a {10 2} twin was initiated at a front of the slip. With increasing external strain, the twin grew with increasing external strain. In the bi-crystal, {10 0} prismatic slip occurred at the crack tip in Crystal 1 and a-dislocation of the slip stopped at a grain boundary between the Crystal 1 and 2. With increasing applied strain, {10 2} twin was initiated in the Crystal 2 at the grain boundary. From the simulation results, the ‘shuffling’ process of twin deformation was estimated.
1586
Authors: Takuro Nakamura, Hiromoto Kitahara, Jung Goo Lee, Nobuhiro Tsuji
Abstract: Pure Al (99%) and pure Fe (99.5%) sheets were mutually stacked and severely deformed up to equivalent strain of 16 by the accumulative roll bonding (ARB) process in an attempt to achieve bulk mechanical alloying. The deformation was carried out at RT. The Al/Fe sheets ARB processed by 1 cycle showed a number of shear bands penetrating the stacked layers. The Fe layers, which were harder than the Al layers, were subdivided by the shear bands into diamond-shaped regions. Dissolution of Fe into Al was observed and a supersaturated solid solution was formed in the specimen ARB processed by 10 cycles. It was also found that local amorphization occurred at interface regions via formation of Al5Fe2 intermetallic compound.
695
Authors: Naomi Iida, Shinji Ando, Masayuki Tsushida, Hiromoto Kitahara, Hideki Tonda
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Authors: Takahiro Sakamoto, Shinji Ando, Hiromoto Kitahara, Masayuki Tsushida, Hideki Tonda
Abstract: In this present study, the fatigue behavior of titanium single crystals containing cracks growing in different crystallographic orientations has been investigated. To investigate fatigue fracture behavior of small single crystals, plain bending fatigue test method for thin sheet specimen was developed. One end of the sheet specimen is fixed at a voice coil of the loudspeaker and the other end is set free. A bending mode resonance occurs in the specimen due to forced vibration at the fixed end. In A-specimen, the notch plane and the direction are (11-20) and [1-100], respectively, the crack propagated parallel to {1-100} plane and striation-like markings were observed on the fatigue surfaces. In B-specimen with a notch of (1-100) and [11-20], the crack also propagated parallel to {1-100} plane as similar to A-specimen. These cracks are deduced to extend by alternating shear on two intersecting prismatic slip systems at the crack tip. In F-specimen with a notch of (0001) and [11-20], the crack propagated parallel to (0001) and twins were observed near the crack. As a result, S-N curves of each specimen showed strong orientation dependence and the fatigue strength of F-specimen was the highest in these specimens.
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Authors: Shinji Ando, Masayuki Tsushida, Hiromoto Kitahara
Abstract: In general, deformation behavior of magnesium in compression is different from tensile. To investigate deformation behavior of magnesium single crystals by non-basal slips and twins, c-axis compression and a-axis tension tests were performed in the range of 77K-573K. The crystals were yielded by second order pyramidal slip, and the yield stress shows anomalous temperature dependence (increased with increasing temperature) between 203K and 293K. Yield stress of c-axis compression was bigger than that of a-axis tensile. In compression, fracture surface were (11 4) under 293K and were {30 4} above 373K, and fracture strain was smaller than the case of tension test. {10 1}-{10 2} double twin were activated at higher temperature and the crystal, therefore, fractured along the twin interface.
699
Authors: Takaaki Kitahara, Shinji Ando, Masayuki Tsushida, Hiromoto Kitahara, Hideki Tonda
Abstract: In general, deformation behavior of magnesium in compression is different from tensile. To investigate deformation behavior of magnesium single crystals, c-axis compression was performed. The crystals were yielded by second order pyramidal slip, and the yield stress shows anomalous temperature dependence (increased with increasing temperature) between 203K and 293K. Yield stress of c-axis compression is bigger than that of a-axis tensile. {10-13} twin and {11-24} twin occurred at 77293K and 77473K respectively. Fracture surface at 77293K was {11-24} and at 473K was {11-22}.
129
Authors: Shinji Ando, Le Ping Bu, Shunsuke Tanaka, Hiromoto Kitahara, Hideki Tonda
Abstract: For development of high strength magnesium alloys, a melt stirring method was employed to Mg-Al and Mg-RE (rare earth metal) alloys. As reinforcement particle, B2O3 powder was added into melts and stirred at 953K in 10 or 20 minutes with argon atmosphere. The grain sizes of both alloys were decreased by 3wt% B2O3 addition. These results show that B2O3 have grain refinement effects to magnesium alloys. Micro Vickers hardness of Mg-Al alloy was increased by 3wt% B2O3 addition. On the other hand, the hardness of Mg-RE alloy was decreased by B2O3. Though addition of B2O3 into Mg-Al and Mg-RE systems make grain size to fine, the hardness of Mg-RE alloys decreased. The tensile properties of Mg-RE alloys with B2O3 were extremely improved by extrusion process.
271
Authors: Hiromoto Kitahara, Takuya Horike, Masayuki Tsushida, Shinji Ando, Nobuhiro Tsuji
Abstract: Fatigue crack propagation behaviors of ultrafine grained (UFG) Al sheets fabricated by the accumulative roll bonding (ARB) process were investigated. The ARB process was carried out up to 6 cycles (equivalent strain, eq.=4.8). The ARB processed sheet had lamellar boundary structure elongated to rolling direction of the sheet. The mean spacing of the boundaries was 182 nm. The tensile strength of the starting Al sheet increased after the 6-cycle of the ARB. Fatigue crack growth tests were performed to clarify the crack growth rate and threshold stress intensity factor range for crack growth (Kth). The fatigue crack profile in the ARB processed sheet differs from that in the starting Al sheet. The Kth of the ARB processed sheet was smaller than that of the starting sheet. The Kth of Al would decreased with decreasing the crack closure phenomena after the 6-cycle of the ARB. The fatigue crack growth rate test shows that the critical load for starting to propagate the fatigue crack and the fatigue crack growth rate decreased by ultrafine grain refinement.
2479
Authors: Masayuki Tsushida, Kazuaki Toda, Hiromoto Kitahara, Shinji Ando, Hideki Tonda
Abstract: Recently, Mg-Zn-Y alloys with superior performance, which have a long period stacking order (LPSO) phase, have been developed. Therefore, it is important to understand fundamental fatigue properties in such materials. In this study, the fatigue fracture behavior of the Mg96Zn2Y2 alloy has been investigated with a plain bending testing machine, which was originally developed for thin sheet specimen at room temperature and 523K. One end of the sheet specimen is fixed at a voice coil of the loudspeaker and the other end is set free. A bending mode resonance occurs in the specimen due to forced vibration at the fixed end. To estimate stress amplitude of bending, deflections at the free end of the specimen oscillating at a frequency of about 200~500Hz was measured by a laser displacement gauge. For comparison, AZ31B alloy also has been investigated. S-N curve for the Mg96Zn2Y2 alloy was obtained using a stress ratio of R=-1, and the fatigue strengths were estimated as 200MPa at room temperature and 120MPa at 523K at 106~107 cycles. These values correspond to about 50% of 0.2% proof strengths of the Mg96Zn2Y2 alloy. Two types of fatigue surface were observed in the Mg96Zn2Y2 alloy. One was striation-like-pattern and the other was relatively flat surface. Striation-like-pattern was similar to fatigue surface of AZ31B. Therefore, these two types of fatigue surface correspond to crack passing through α-Mg phase and LPSO phase, respectively. The feature of fracture surface at 523K was almost the same as that at room temperature.
267
Authors: Ayaka Kutsukake, Shinji Ando, Masayuki Tsushida, Hiromoto Kitahara, Hideki Tonda
Abstract: Generally, plastic deformation of magnesium alloys is difficult at room temperature. In order to improve formability of magnesium, impurity elements in magnesium were reduced by vapor deposition technique. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was applied to the determination of trace elements in refined magnesium. To investigate influence of impurity element to deformation behavior, high purity magnesium single crystals were prepared. When the magnesium single crystals are stretched in <11-20> direction, {11-22} <-11-23> pyramidal slips were activated just after yielding in the range of 77K to 293K. The yield stress of high purity magnesium was a half of the stress in raw magnesium.
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