Papers by Author: Takeo Muroga

Abstract: Since there are no fusion reactors generating high-flux 14 MeV neutrons, it is necessary to evaluate materials’ performance in fusion reactors based on a correlation of fission neutron and charged particle irradiations. However, the irradiation tests involve various issues which prevent simple correlation and evaluation. In this paper, the issues related to irradiation temperature control and dose rate effects are pointed out and analyzed, and proposals regarding future irradiation tests are given.

Abstract: National Institute for Fusion Science (NIFS) launched in 2014 a research program for developing Dispersion Strengthened (DS) Cu alloys for application to the heat sink materials of divertors of fusion reactors, using newly installed ball-milling, encapsulation, and Hot Isostatic Pressing (HIP) facilities. A unique feature of these facilities is that the entire process can be performed without exposing the materials to air, enabling precise impurity control. Cu-Al, Cu-Zr and Cu-Y alloys have been produced in this program. Various technological advancement has been made for the fabrication, such as suppression of powder adhesion to the wall of containers during the ball milling, and encapsulation technology including development of small volume tubular capsules.

Abstract: Liquid lithium lead (LiPb) eutectic is considered as one of the promising candidates of tritium breeder materials for fusion reactors. Series experiments on compatibility of LiPb with candidate structural materials such as CLAM steel and SiC f/SiC composites have been done in DRAGON serious experimental devices in FDS team such as DRAGON-RTand stirred pot device in NIFS at 500 oC and 600oC, respectively. The weight loss of CLAM specimens exposed in flowing LiPb with the velocity of 0.17m/s increased with temperature, and the morphology and composition of the corroded surfaces were done by SEM observation and EDX analysis. The coating specimens including Al 2O3 and FeAl/Al2O3 coatings prepared on the CLAM specimens were also exposed in the DRAGON-RT device, the results revealed that there was no obvious thinning observed on the outer surface of the protective coating. Preliminary analysis of SiC f/SiC composites specimens indicated that the mullite coating with plasma spray method on the SiC f/SiC composites specimen corroded in the high temperature LiPb, but no obvious corrosion attack was observed on the specimen surface, while the matrix and fiber of reaction-sintered composites showed slightly corrosion attack after exposure in static LiPb at 800°C for 200 hrs. Further corrosion experiment will be carried out in the near future.

Abstract: V-4Cr-4Ti alloy is an attractive candidate low activation structural material for nuclear fusion reactors. The properties of V-4Cr-4Ti during its operation strongly depend on initial microstructure, especially size and density of Ti-rich precipitates and their interaction with dislocations. This paper focuses on the effects of the precipitate states and other microstructural variables on mechanical properties of V-4Cr-4Ti alloys and the potential methods to improve the properties by microstructural control, based on Transmission Electron Microscopy.

Abstract: Laser welded high purity V-4Cr-4Ti alloy (NIFS-HEAT2), fabricated by National Institute for Fusion Science (NIFS), was used for this study. Copper ion irradiation was carried out with the tandem accelerator at Kyushu University. The TEM samples were sliced from the welded materials and irradiated at 573 and 873K up to the dose of 12 dpa. The microstructure before irradiation showed that relatively large precipitates, which were commonly observed in NIFS-HEAT2, disappeared in the center of the weld metal. After the ion irradiation, fine titanium oxides with {100} habit planes were detected even at the dose of 0.75 dpa. However, less number density of these oxides were observed in the base metal after the same irradiation conditions. This means that the behaviors of oxygen atoms, which dissolved from the large precipitates during the laser welding, is essential to the microstructural evolution of welded V-4Cr-4Ti alloys.

Abstract: Potential enhancement of mechanical properties of V-4Cr-4Ti by controlling precipitates of interstitial impurities (C, O and N) was investigated by means of various thermal and mechanical treatments. The increase in the cold working degree resulted in band structure of coarse Ti-CON precipitates having enhanced impact properties. Solid solution heat treatment followed by re-heating resulted in the formation of high density Ti-O-C precipitates enhancing high temperature strength. Combination of re-heating and cold rolling enhanced further the strength. Improvements in thermal and mechanical treatments are discussed for application of V-4Cr-4Ti to fusion structural materials.

Abstract: A V-4Cr-4Ti alloy and pure V were oxidized in flowing argon for 0.5-8h and subsequently annealed in vacuum for 16h at 973K. The oxygen of 1000-9000ppm was introduced into the V-alloys with little nitrogen, where oxygen was concentrated in region near the surface. By the annealing, oxygen was homogeneously diffused into depth of 150µm. The diffusion depth of oxygen in V-4Cr-4Ti is much smaller than that of pure V, because of Ti-O precipitates formed as oxygen trap. The study indicated that it is feasible to control oxygen level and distribute in the surface region of the vanadium alloys.

Abstract: To investigate the effect of impurites, such as O, N and C, on migration behavior of vacancies in several types of V-4Cr-4Ti vanadium alloy with different amount of impurites from 207wppm to 866wppm were irradiated with 28 MeV electrons using an electron linear accelerator at 100 K. After irradiation, positron lifetime measurements were carried out. Single vacancies were produced in all samples. The vacancy clusters were formed at 348 K and 433 K in samples with lowest and highest impurites, respectively. The temperature forming vacancy clusters incresased with increaisng amount of impurites. The vacancy migaration is depended on the amount of impurities rather than the kind of impurity.