Papers by Author: Yoshihito Kawamura

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Authors: Michiaki Yamasaki, Keisuke Nyu, Yoshihito Kawamura
Authors: Yoshihito Kawamura, Akihisa Inoue
Authors: Mitsuhiro Matsuda, Shintaro Yoshimoto, Yoshihito Kawamura, Kazuhiro Ishikawa, Minoru Nishida
Authors: Shintaro Yoshimoto, Mitsuhiro Matsuda, Michiaki Yamasaki, Yoshihito Kawamura
Authors: Yoshihito Kawamura, Yasuhide Ohno, Akira Chiba
Authors: Masafumi Noda, Yoshihito Kawamura
Abstract: Lightweight Mg alloys are being widely adopted in electronic devices and automotive parts. However, the mechanical properties of Mg alloys are inferior to those of Al alloys, and their range of applications is limited. Recently, Mg-Zn-Y alloys have been found to exhibit superior mechanical properties to other Mg alloys. However, for use as industrial materials, it is necessary to improve their ductility while retaining their strength. We studied the mechanical properties of extruded Mg96Zn2Y2 alloy with long-period stacking order (LPSO) phases, in addition to the thermal stability of its structure during annealing. The elongation of the alloy improved from 5 % to 10 % with slightly loss of strength up to an annealing temperature of 623 K. Even at this temperature, it was possible to maintain good mechanical properties and a thermally stable microstructure.
Authors: Masafumi Noda, Yoshihito Kawamura
Abstract: Mg alloys are lightweight structural alloys that normally have a good castability and machinability as well as an excellent specific strength and rigidity. However, the mechanical properties of Mg alloys are inferior to those of Al alloys, and their range of industrial applications is limited. Recently, Mg–Zn–Y alloy has been found to show a high tensile yield strength with a good elongation. The alloy has a long-period stacking order (LPSO) phase as the secondary phase in an α-Mg phase. In general, the tensile yield strengths of LPSO-type Mg alloy are known to be markedly enhanced by the formation of kink bands in the LPSO phase and by microstructural refinement of the α-Mg phase during plastic deformation. The separate roles of the LPSO phase and the α-Mg phase in relation to the mechanical properties of high-strength LPSO-type Mg alloy were investigated at ambient and high temperatures. For high strengths at ambient and high temperatures, it was important that the α-Mg phase consisted of a fine-grain region and a nonrecrystallized region, and that the LPSO phase remained as a block-type phase. On the other hands, it was necessary to change the LPSO phase from a block-type phase into a plate-type phase by heat treatment before tensile testing to improve the ductility of the alloy while maintaining its tensile yield strength. Microstructural control of the LPSO phase and the α-Mg phase is necessary to obtained Mg–Zn–Y alloy with superior mechanical properties at ambient-to-high temperatures.
Authors: Minoru Nishida, Takao Yamamuro, M. Nagano, Yasuhiro Morizono, Yoshihito Kawamura
Authors: Akira Chiba, Yoshihito Kawamura, Minoru Nishida
Abstract: Recently, a number of amorphous alloys that possess high glass-forming ability and a wide supercooled liquid region before crystallization have been discovered. Especially, bulk metallic glasses, which are made in bulk form with a thickness of ~10 mm at slow cooling rates of the order of 1~100 K/s, have been noted as an industrial application. Hence the welding of bulk metallic glasses to other materials is very important. Explosive welding of most popular Zr41.2Ti13.8Cu10Ni12.5Be22.5 bulk metallic glass to crystalline pure Ti and SUS304 plates is investigated in this paper. The BMGs was found to retain the amorphous structure and the original mechanical properties. The sound bonding with other materials is expected to push forward the application of bulk metallic glass for industrial usage.
Authors: Shinji Ando, Kazuaki Toda, Masayuki Tsushida, Hideki Tonda, Yoshihito Kawamura
Abstract: Recently, Mg-Zn-Y alloys with superior performance, which have a long period order (LPO) phase, have been developed. Therefore, it is important to understand fundamental fatigue properties in such materials. In this study, the fatigue fracture behavior of Mg-Zn2-Y2-Zr0.2 alloy has been investigated using a plain fatigue bending testing machine, which was originally developed for thin sheet specimen. 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 250~700Hz was measured by a laser displacement gauge. For comparison, AZ31B alloy also has been investigated. S-N curve for the Mg-Zn2-Y2-Zr0.2 alloy was obtained using a stress ratio of R=-1, and the fatigue strength was estimated as about 200MPa at 106~108 cycles. The value corresponds to about 50% of 0.2% proof strength of the alloy. Two types of fatigue surface were observed in the alloy. One is striation like pattern and the other is relatively flat surface. The former is similar to fatigue surface of AZ31B. Therefore, these two types of fatigue surface correspond to crack passing through α-Mg phase and LPO phase, respectively.
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