Papers by Author: Z. Horita

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Authors: Seung Chae Yoon, Do Minh Nghiep, Sun Ig Hong, Z. Horita, Hyoung Seop Kim
Abstract: Manufacturing bulk nanostructured materials with least grain growth from initial powders is challenging because of the bottle neck of bottom-up methods using the conventional powder metallurgy of compaction and sintering. In this study, bottom-up type powder metallurgy processing and top-down type SPD (Severe Plastic Deformation) approaches were combined in order to achieve both full density and grain refinement of metallic powders. ECAP (Equal-Channel Angular Pressing), one of the most promising processes in SPD, was used for the powder consolidation method. For understanding the ECAP process, investigating the powder density as well as internal stress, strain and strain rate distribution is crucial. We investigated the consolidation and plastic deformation of the metallic powders during ECAP using the finite element simulations. Almost independent behavior of powder densification in the entry channel and shear deformation in the main deformation zone was found by the finite element method in conjunction with a pressure dependent material yield model. Effects of processing parameters on densification and density distributions were investigated.
Authors: Seung Won Lee, Daichi Akama, Z. Horita, Tetsuya Masuda, Shoichi Hirosawa, Kenji Matsuda
Abstract: This study presents an application of high-pressure torsion (HPT) to an Al-Li-Cu-Mg alloy (2091). The alloy was subjected to solid solution treatment at 505oC for 30 minutes and was processed by HPT under 6 GPa for 5 revolutions at room temperature. The hardness increased with straining and saturated to a constant level at 225 Hv. Aging was undertaken on the HPT-processed alloy at 100, 150 and 190oC for the total periods up to 9.3 days. The aging treatment led to a further increase in the hardness to ~275 Hv. It is shown that the simultaneous strengthening of the alloy due to grain refinement and age hardening was successfully achieved by application of HPT and subsequent aging treatment. The enhancement of the strength is prominent when compared with the application of a conventional rolling process.
Authors: Daichi Akama, Z. Horita, Kenji Matsuda, Shoichi Hirosawa
Abstract: This research investigates simultaneous strengthening by grain refinement and fine precipitation in age-hardenable Al-Mg-Si alloys containing an additional element of either Ag, Cu, Pt or Pd. The alloys were solution-treated and processed by high-pressure torsion (HPT) at room temperature under a pressure of 6 GPa. They were aged at a temperature of 373 K for up to a total period of 6.7 hours. Vickers microhardness was measured after selected periods of aging and the microstructures were observed by transmission electron microscopy. It was found that, in all alloys, the grain sizes after HPT were refined to 300-400 nm and there were significant increases in the hardness through the HPT processing. The hardness was further increased by the subsequent ageing treatment, confirming the simultaneous strengthening by grain refinement and fine precipitation. However, the aging behavior was different depending on the alloying compositions.
Authors: Hirotaka Matsunaga, Z. Horita, Kazutaka Imamura, Takanobu Kiss, Xavier Sauvage
Abstract: An age-hardenable Cu-2.9%Ni-0.6%Si alloy was subjected to high-pressure torsion. Aging behavior was investigated in terms of hardness, electrical conductivity and microstructural features. Transmission electron microscopy showed that the grain size is refined to ~150 nm and the Vickers microhardness was significantly increased through the HPT process. Aging treatment of the HPT-processed alloy led to a further increase in the hardness. Electrical conductivity is also improved with the aging treatment. It was confirmed that the simultaneous strengthening by grain refinement and fine precipitation is achieved while maintaining high electrical conductivity. Three dimensional atom probe analysis revealed that fine precipitates with sizes of ~20 nm or smaller were formed in the Cu matrix and some particles consist of Ni and Si with no appreciable amount of Cu.
Authors: Z. Horita
Abstract: The process of severe plastic deformation (SPD) makes it possible to reduce the grain size to the submicrometer or nanometer range in many metallic materials. When the SPD process is applied to age hardenable alloys, it may also be possible to control aging behavior. In this study, a technique of equal-channel angular pressing (ECAP) is used as an SPD process and aging behavior is examined on the three selected Al alloy systems such as Al-Ag, Al-Mg-Si and Al-Si-Ge. The microstructures are observed using transmission electron microscopy and the mechanical properties including hardness are measured. It is shown that the SPD process introduces unusual phenomena in the precipitation process and there should be a potential for enhancement of strength over the conventional age-hardening process or for improvement of ductility while keeping the high strength.
Authors: Minoru Furukawa, Yukihide Fukuda, Keiichiro Oh-ishi, Z. Horita, Terence G. Langdon
Authors: Minoru Furukawa, Yukihide Fukuda, Keiichiro Oh-ishi, Z. Horita, Terence G. Langdon
Abstract: This paper describes experiments in which high purity copper single crystals of two different orientations were processed for one pass by equal-channel angular pressing (ECAP) and the deformed structures were examined using optical microscopy (OM), orientation imaging microscopy (OIM) and transmission electron microscopy (TEM). The first single crystal (0° specimen) was oriented within the entrance channel of the die so that the {111} slip plane and the <110> slip direction were parallel to the theoretical shear plane and shear direction, respectively. The second crystal (20° specimen) was oriented with the {111} slip plane and the <110> slip direction rotated by 20° in a clockwise sense from the theoretical shear plane and shear direction, respectively. For the 0° specimen, after passing through the shear plane there were two crystallographic orientations representing the initial orientation and an orientation rotated by 60° in a counter-clockwise sense from the initial orientation. For the 20° specimen, there was an orientation rotated by 20° in a counter-clockwise sense from the initial orientation after passing through the shear plane.
Authors: Patrick B. Berbon, Minoru Furukawa, Z. Horita, Minoru Nemoto, Nikolai K. Tsenev, Ruslan Valiev, Terence G. Langdon
Authors: Seung Won Lee, Z. Horita
Abstract: An Fe-50at %Ni alloy was processed by high-pressure torsion (HPT) and annealed at lower homologous temperatures. Vickers microhardness and microstructural evolutions were examined with respect to the annealing time. Disks with 10 mm diameters having 0.85 mm thicknesses were subjected to HPT under a pressure of 6 GPa for 1-10 revolutions at a rotation speed of 1 rpm. The annealing after the HPT processing was conducted at homologous temperatures of ~0.3 for up to 40 days. The hardness increases with straining and saturates to a constant level of ~400 Hv at large equivalent strain. Microstructure analysis using XRD revealed that there was a peak corresponding to an α (Fe, Ni) phase.
Authors: M. Watanabe, Z. Horita, Takayoshi Fujinami, T. Sano, Minoru Nemoto
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