Papers by Author: Takuya Nagasaka

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: Liquid phase diffusion bonding between ODS steel and pure W was carried out and its joint strength was investigated for fusion applications. A block of high-Cr ODS ferritic steel and a W plate were diffusion bonded at 1240 °C for 1h with/without an insert material under an uni-axial compression load and a high vacuum atmosphere. Cross sectional microstructures of joint region were observed by scanning electron microscope and the mechanical properties of the joint region were evaluated by hardness test and torsion tests. Microstructure analysis revealed that high Cr ODS ferritic steel block and W plate with insert material was successfully diffusion bonded with free of voids. Shear strength of liquid phase diffusion bonded ODS steel and W was higher than that of directly solid state diffusion bonded ODS steel and W. This was attributed to residual strain which is resulted from the difference of thermal conductivity between the ODS steel and W.

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.