Papers by Author: Tokuteru Uesugi

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Authors: Tokuteru Uesugi, Masanori Kohyama, Masahide Kohzu, Kenji Higashi
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Authors: Tokuteru Uesugi, Masanori Kohyama, Kenji Higashi
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Authors: Tokuteru Uesugi, Yorinobu Takigawa, Kenji Higashi
Abstract: We investigated the contribution to the high yield strength due to the solid solution strengthening in nanocrystalline Al-Ti alloys produced by a vapor quench method. The misfit strain due to solute Ti atom in aluminum was obtained from the first principles calculation. Then, the theoretical result of the contribution to the yield strength due to the solid solution strengthening was estimated from the misfit strain using the Friedel’s theory. In dilute Al-Ti alloy, the theoretical results of the solid solution strengthening from the misfit strain was in good agreement with the analytical result using the measured grain size and yield stress.
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Authors: Tokuteru Uesugi, Yorinobu Takigawa, Kenji Higashi
Abstract: Recently nanocrystalline Al-Fe alloys produced by a vapor quench method have been reported. These alloys are supersaturated solid solution and exhibit high strength with good ductility. It is postulated that the high strength of the Al-Fe alloys could be achieved by both the nano-grained structures and the solid solution strengthening. The contribution to the yield strength due to both the grain size strengthening and the solid solution strengthening were analyzed from the experimental data. Then the contribution to the yield strength due to the solid solution strengthening was estimated from the misfit strain calculated from the first principles in order to compare with analytical results estimated from the experimental data.
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Authors: Naoya Shirasawa, Ren Ito, Yorinobu Takigawa, Tokuteru Uesugi, Kenji Higashi
Abstract: New Ti-based ternary metallic glasses were designed and fabricated. A new parameter called effective atomic radius in the Ti solid solution from ab-initio calculation was used to design of the Ti-based metallic glasses. From the effective atomic radius, Ti-Zr-Mn, Ti-Zr-Fe and Ti-Zr-Co systems can be considered as a new Ti-based ternary metallic glass. And the reported scheme based on the concept of binary eutectic clusters is applied to predict alloy composition which shows glass transition.We prepared the Ti42Zr22Mn36, Ti43Zr29Fe28 and Ti44Zr30Co26 alloy sheets by a single-roller method. It showed that sharp diffraction peak corresponding to crystalline phases could be observed in the XRD spectra of Ti42Zr22Mn36 and Ti43Zr29Fe28. While for alloy with Ti44Zr30Co26 no sharp diffraction peak could be found except broad diffraction halos. This result suggests that a critical eutectic temperature in the phase equilibrium for forming glass phase is around 1000°C.
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Authors: Yuichi Ienaga, Kenshi Inoue, Tokuteru Uesugi, Yorinobu Takigawa, Kenji Higashi
Abstract: In order to develop a high strength and heat-resistant magnesium alloy, we focused on controlling microstructure of Mg96Zn2Y2 (at %) casting alloy by the addition of a 4th element. Initially, we investigated the effects of zirconium addition and cooling rate for grain refinement on microstructure and mechanical properties. Consequently, Mg95.8Zn2Y2Zr0.2 casting alloy contains fine equiaxed grains (approx. 0.01 mm), and it exhibits tensile and fatigue properties equivalent to or higher than those of commercial aluminum alloys at high temperature above 473 K. At 523 K, this alloy exhibited a tensile strength of 223 MPa nearly twice that of A4032-T6 alloy used in typical automotive pistons. The Mg95.8Zn2Y2Zr0.2 casting alloy also reveals sufficient ductility and good castability, characteristics not common in current heat-resistant magnesium alloys. Next, we focused on controlling microstructure of Mg96Zn2Y2 casting alloy by the addition of Ag. Mg96Zn2Y2 cast alloy is composed of alpha-Mg phase, long-period stacking ordered phase and Mg3Zn3Y2 phase; on the other hand, Mg-Zn-Y-Ag cast alloy had 4th phase by an addition of Ag. A substantial increase in yield strength at room temperature, without grain refining, was the result.
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Authors: K. Takeshima, Tokuteru Uesugi, Yorinobu Takigawa, Kenji Higashi
Abstract: Creep tests of ultra-high-purity (99.999%) Al and Al-22.2, 53.6, 101 at.ppm Fe solid solution alloys were conducted at 773 K in the stress range of 2-6 MPa in order to investigate effect of solute Fe on high temperature deformation of Al. Creep resistance was enhanced by addition of Fe in solid solution. The stress exponents of the samples exhibited values of about 5, which indicate that climb-controlled dislocation creep was dominant deformation mechanism. It could be suggested that Fe atoms segregating in dislocations due to the strong interaction between solute Fe atoms and the dislocation enhanced the creep resistance.
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Authors: M. Honda, Tokuteru Uesugi, Yorinobu Takigawa, Hiroshi Mabuchi, Kenji Higashi
Abstract: Dynamic recrystallization behavior that occurs during the hot extrusion was examined in two commercial Mg alloys, AZ31 and AZ80 alloys. The initial grain size, d0, and the dynamically recrystallized grain size, drec, after the extrusion was investigated using optical microscopy. The dynamically recrystallized grain size decreased with an increasing the Zener-Hollomon parameter.
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Authors: T. Noro, Tokuteru Uesugi, Yorinobu Takigawa, Masato Tsujikawa, Hiroshi Mabuchi, Kenji Higashi
Abstract: Dynamic recrystallization behavior of Mg-3.1wt.%Al-0.1wt.%Y alloy was investigated. During extrusion, dynamic recrystallization occurred and very fine microstructure was obtained. We compared the relationship between Zener-Hollomon parameter, Z, and the dynamically recrystallized grain size, drec, and between the normalized recrystallized grain size, drec/d0, where d0 is the initial grain size, and Z for Mg-3.1wt. %Al-0.1wt.%Y with commercial Mg-3Al-0.5Zn alloy. As the result, the dynamically recrystallized grain size of Mg-3Al-0.1Y was finer than that of Mg-3Al-0.5Zn alloy under the equivalent Z, while the initial grain size of Mg-3Al-0.1Y was coarser than Mg-3Al-0.5Zn alloy. The normalized grain size for Mg-3Al-0.1Y alloy was much smaller than that of Mg-3Al-0.5Zn alloy under the equivalent Z. We conclude that small addition of Y to Mg-Al alloys is effective for grain refinement by dynamic recrystallization.
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Authors: Ippei Takeuchi, Kinji Hirai, Yorinobu Takigawa, Tokuteru Uesugi, Kenji Higashi
Abstract: The effect of Ca and Sr content on the microstructure and mechanical properties of a cast AZ91 magnesium alloy is investigated. Ca and Sr additions in AZ91 magnesium alloy are expected high creep resistance. The microstructure of the alloy exhibits the dendritic α-matrix and the second-phases forming networks on the grain boundary. Tensile tests at elevated temperatures between 448 and 523K reveal that the creep resistance was improved with increasing the additional amount of Ca, especially more than 1.0wt%. From the perspective of grain refinement effect, it is expected that the additions of Ca and Sr to AZ91 magnesium alloy not only improve creep resistance but also improve mechanical properties at room temperature.
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