Manabu Satou

Transportation of a single object by two decentralized-controlled nonholonomic mobile robots

We propose a decentralized control algorithm for transporting a single object by two nonholonomic mobile robots driven by two wheels. We extend the leader-follower type control algorithm, proposed by Kosuge and Oosumi (1996) for holonomic robots, to mobile robots by introducing the dual caster action. The extended control algorithm is applied to two experimental tracked mobile robots, and the experimental results illustrate the validity of the control algorithm.

Microstructural development of neutron irradiated W–Re alloys

Abstract Tungsten (W) alloys are candidate materials to be used as high-heat-flux materials in fusion reactors. In our previous work, W–26 wt% Re showed drastic hardening and embrittlement after the neutron irradiation. In this study, to clarify the irradiation hardening and embrittlement behavior of W–26 wt% Re, from the viewpoint of microstructural development, the microstructure observation of the neutron irradiated W–26 wt% Re was carried out using transmission electron microscope (TEM). The specimens were irradiated at the materials open test assembly of the fast flux test facility (FFTF/MOTA-2A cycle 11) up to ∼1×10 27 n/m 2 , (En>0.1 MeV). The irradiation temperatures were 646, 679, 792, 873 and 1073 K. In all neutron irradiated W–26 wt% Re samples, sigma-phase precipitates and chi-phase precipitates were observed, while in the thermally aged specimen, only sigma-phase precipitates were observed. Irradiation effects on microstructural development are discussed.

Improvement in post-irradiation ductility of neutron irradiated V–Ti–Cr–Si–Al–Y alloy and the role of interstitial impurities

An attempt to maintain the uniform elongation (UE) of vanadium alloys after irradiation is described. Neutron irradiation was carried out at Advanced Test Reactor (ATR) and at Experimental Breeder Reactor II (EBR-II) to the fluence of 11 dpa. The UE was maintained about 6% after irradiation at 388°C. The UE became negligible after irradiation below 300°C for the alloy annealed at 1000°C and 1100°C. The specimens annealed at 900°C and 700°C still showed relatively large UE after such low-temperature irradiation. Comparing with the results in literature, it is proposed that oxygen concentration should be kept below 200 ppm to maintain the UE after irradiation below 400°C. The control of interstitial impurities, especially oxygen in solution, was important to maintain the UE of the alloy.

Tensile behavior of helium charged VTiCrSi type alloys

Abstract Helium effect on the mechanical properties of the alloy V5Ti5Cr1SiAl,Y (nominal) was studied, adopting various helium charging methods and helium-to-dpa ratio. The first method was helium ion implantation using a cyclotron accelerator at Tohoku University, where helium and displacement levels were 50 appm and 0.02 dpa, respectively. The second, helium was charged by tritium trick technique and following neutron irradiation in FFTF/MOTA-2A, associated with about 80 appm He and 43 dpa. The third was dynamic helium charging experiment (DHCE) conducted in FFTF/MOTA-2B, where helium was generated within specimens during neutron irradiation by tritium decay, and the helium-to-dpa ratio was adjusted to simulate the fusion reactor condition, that is, 177 appm He and 24 dpa. The effect of helium on tensile properties of the VTiCrSiAl,Y alloy depended on the helium charging methods. The uniform elongation of the alloy was 3.2% and total elongation was 8.3% at DHCE condition, which was the most fusion relevant condition of the methods. It is important that tensile properties of the present alloy could be acceptable for fusion reactor component materials.

High-temperature deformation of modified V-Ti-Cr-Si type alloys

Abstract Modified V-Ti-Cr-Si type alloys containing Al and Y were prepared in order to improve the oxidation resistance for fusion reactor applications. The fabrication procedure and the condition of heat treatment were determined for two alloys referred as V-5Ti-5Cr- and V-25Ti-15Cr-. Mechanical properties were measured using miniature-type tensile specimens at temperatures from 300 to 1273 K. The alloy V-5Ti-5Cr- showed the same level of strength and ductility as a reference alloy of V-15Cr-5Ti and the alloy V-25Ti-15Cr- was twice as strong as the reference alloy at temperatures from 300 to 873 K. Dynamic strain aging and work softening phenomena with large elongation were observed at intermediate and high temperature regions, respectively.

Tensile behavior and microstructure of neutron-irradiated Mo-5% Re alloy

Abstract This work reports the effect of heat treatment on the tensile behavior and and microstructure of neutron-irradiated Mo-5% Re alloy. Stress-relived and recrystallized specimens conditions were irradiated at five temperatures between 646 and 1073 K in FFTF/MOTA. The exposure levels were in the range of 6.8 to 34 dpa depending on the irradiation temperatures. Tensile tests were carried out at room temperature and 673 K and microstructures of the irradiated specimens were observed by TEM. The Mo-5% Re alloy irradiated at high temperatures shows ductile behavior even at room temperature. The total elongation of stress-relived specimens irradiated at 873 and 1073 K ranged from 5 to 10%, and that of recrystallized specimens irradiated at 1073 K was 5%. The fracture modes of these specimens were transgranular type. Voids were observed in all of the irradiated specimens, but precipitates were found only in specimens irradiated above 792 K. It is important for the Mo Re alloy to be used in high-heat flux components of fusion reactors that the alloy showed the ductility after neutron exposures of relatively high fluences.

Analysis and measurement of residual stress distribution of vanadium/ceramics joints for fusion reactor applications

Abstract Vanadium alloys are considered as candidate structural materials for fusion reactor system. When vanadium alloys are used in fusion reactor system, joining with ceramics for insulating is one of material issues to be solved to make component of fusion reactor. In the application of ceramics/metal jointing and coating, residual stress caused by difference of thermal expansion rate between ceramics and metals is an important factor in obtaining good bonding strength and soundness of coating. In this work, residual stress distribution in direct diffusion bonded vanadium/alumina joint (jointing temperature: 1400°C) was measured by small area X-ray diffraction method. And the comparison of Finite Element Method (FEM) analysis and actual stress distribution was carried out. Tensile stress concentration at the edge of the boundary of the joint in alumina was observed. The residual stress concentration may cause cracks in alumina, or failure of bonding. Actually, cracks in alumina caused by thermal stress after bonding at 1500°C was observed. The stress concentration of the joint must be reduced to obtain good bonded joint. Lower bonding temperature or to devise the shape of the outer surface of the joint will reduce the stress concentration.

Influence of heat-treatment on tensile behavior of neutron irradiated molybdenum

Abstract Tensile specimens of stress-relieved and recrystallized molybdenum were irradiated at five temperatures between 646 and 1073 K in FFTF/MOTA cycle 11. The exposure levels were in the range of 6.8 to 34 dpa depending on the irradiation temperatures. After irradiation tensile tests and hardness test were carried out at room temperature. The total elongation of stress-relived specimens irradiated at 792, 873 and 1073 K ranged from 9 to 13%, and those of recrystallized specimens irradiated at these temperatures were almost 0%. The fracture modes of the stress-relieved specimens irradiated at 646–873 K were transgranular type and those of recrystallized specimens irradiated at 646–1073 K were intergranular type. Grain growth and intergranular fracture surface were observed in the stress-relieved specimen irradiated at 1073 K but it still has 9% total elongation. It is important that the stress-relived molybdenum had enough ductility at room temperature after neutron exposures up to relatively high fluences.

Effects of doping elements on oxidation properties of V–Cr–Ti type alloys in several environments

The alloys examined in this study include V-4Cr-4Ti-0.5Si, V-4Cr-4Ti-0.5Al, V-4Cr-4Ti-0.5Y and V-4Cr-4Ti. Oxidation experiments were conducted in air. After oxidation, tensile tests, hardness measurements and scanning electron microscopy were performed. Surface oxidation layers were identified by X-ray diffraction (XRD) analysis. Based on this study, the alloy doped with Y was excellent in oxidation resistance at 600 and 700 °C. The XRD indicated that V 2 O 5 was the primary oxide phase for the alloys tested at 700 °C, except for the V-4Cr-4Ti-0.5Y alloy. The oxide identified at all temperatures in V-4Cr-4Ti-0.5Y alloy was VO 2 , that was also observed for the other alloys after testing at 600 °C. It was suggested that the formation of thin and dense VO 2 oxide layers had an important role for superior oxidation properties of the V-4Cr-4Ti-0.5Y alloy. From the results of this study, further optimization of the V-4Cr-4Ti alloy could be possible by controlling the small addition of yttrium.

Tensile properties of a series of V–4Ti–4Cr alloys containing small amounts of Si, Al and Y, and the influence of helium implantation

Abstract V–Ti–Cr type alloys containing Si, Al and Y have been developed to improve proof-oxidation properties and high temperature strength. In order to optimize the composition of Si, Al and Y, seven V–Ti–Cr type alloys containing Si, Al and Y up to 0.5 wt% were prepared. Tensile tests were carried out at temperatures from 300 to 1123 K. From the results of the tests, the dependence of yield stress and ultimate tensile strength on Si, Al and Y concentration is low at these temperatures. Total elongation of the alloys tested at 923 and 1123 K increase with increasing concentration of Si, Al and Y. Helium implantation up to about 50 appm by an accelerator was also carried out. The loss of total elongation at 923 and 1123 K depends on the concentration of Si, Al and Y. From the results of helium implantation, it is suggested that the appropriate concentration of Si, Al and Y for V–4Ti–4Cr alloys is between 0.1 and 0.5 wt%, respectively.