Papers by Author: Akira Kohyama

Abstract: Silicon carbide fiber reinforced silicon carbide composite (SiC/SiC composite) is expected as a potential replacement of the current Zircaloy fuel cladding in the light-water reactor because of its many superiorities, where it is necessary to develop the end-cap seal of SiC/SiC composite cladding. In this research, the applicability of diode laser irradiation for joining between Zircaloy and SiC/SiC composite tubes was examined based on the caulking method. As the method for fitting two tubes, the screw cutting was conducted for the inner face of Zircaloy tube and the outer surface of SiC/SiC composite. In addition, as a method to improve the joinability of Zircaloy and SiC/SiC composite, titanium nanopowder was inserted into a gap between two tubes. The laser beam irradiation was circumferentially applied to the outer surface of Zircaloy tube. Although the mechanical caulking joint was successfully produced, the airtightness was insufficient for the end-caps of SiC/SiC composite cladding. However, the good adhesion between Zircaloy and SiC/SiC composite was partially produced as the result of the generation of all proportional solid solution between titanium and zirconium. Namely, it can be concluded that the circumferential diode laser irradiation with inserting titanium nanopowder between Zircaloy and SiC/SiC composite tubes would have a good potential for producing the caulking joint of two tubes.

Abstract: To estimate the ion-irradiation effect on various types of SiCf/SiC composites, a silicon self-ion irradiation was performed at temperatures of 600 °C and 1200 °C and at doses of 5 dpa and 20 dpa, respectively. These SiCf/SiC composites were prepared by different processes such as CVI (chemical vapor infiltration), WA-CVI (SiC whisker assisted CVI) and hot-pressing (HP) method. Hardness was measured by a nano-indentation tester and microstructural changes were observed by TEM with SAD(selected area diffraction) technique for the specimens prepared by FIB (Focused Ion Beam) milling. The damage dose was calculated by the SRIM2003 code and then compared with microstructureal observation.

Abstract: SiC/SiC composites were fabricated by hot pressing (HP) via liquid phase sintering (LPS) using carbon coated 2D woven Tyranno SA fabrics as reinforcement. Both nano-SiC and micro-SiC powders with sintering additives were used for matrix. The effects of preparation conditions on the microstructure and mechanical properties of the composites were characterized. Highly densified composite was obtained at 1780°C under 20MPa with nano-SiC particles. The strength and elastic modulus of the composite were enhanced. When micro-SiC powder was used, higher strength revealed for the composite sintered at 1780°C under 15MPa, although it was not densified enough. Higher sintering temperature (1800°C) is beneficial for the densification of the composite, but is not obvious for the improvement of mechanical properties.

Abstract: Nano infiltration transient eutectic ceramic (NITE-SiC) was fabricated by hot pressing method using Al2O3 and Y2O3 as sintering additives. The ratio of the Al2O3/Y2O3 additives was changed between 6:4 and 4:6. The densification and mechanical properties were investigated for the sintered temperature. The starting powder was high purity β -SiC nano-powder with an average particle size of 30nm. The bending strength characterization and densification of NITE-SiC was investigated by the FE-SEM and three point bending test.

Abstract: Chopped fiber and a hybrid reinforcement of chopped and continuous fibers were used for fabricating SiC/SiC composites. Under the selected sintering pressure, the composite sintered at lower temperature (1820°C) had lower density. Increasing temperature to 1850°C, the density of the composite reached at a higher level. However, pores still existed and mainly distributed in the areas the fibers accumulated, especially inside of the fiber bundles. Densely sintered matrix still could be found in the composite sintered at 1820°C, 15 MPa. In the areas with the fibers accumulated, matrix was relatively weak so that the cracks were easily propagated leading to the delamination during bending test. When continuous fiber was included into the chopped fiber reinforced composite, a hybrid reinforcing mechanism was obtained. This kind of composite had obviously improved toughness and strength. On the fracture surface, the pulled out fibers that were perpendicular to the fracture surface were increased.

Abstract: Hot pressing is an effective way to densify powder compacts, especially during the inclusion of a second phase, such as particles, whiskers or fibers. In the present study, SiC/SiC and C/SiC composites were prepared by hot pressing via liquid phase sintering. Nano-SiC powder was used for matrix formation with sintering additives. The effects of preparation conditions such as sintering temperature, pressure and matrix composition, on the microstructural evolution and mechanical behaviors were discussed. Using micro-SiC powder and nano-SiC powder for matrix formation, the interaction between fiber and matrix was characterized. Because the matrix compositions can be easily modified in the CMC-processing from powders by hot pressing, the SiC-BN matrix was also studied in the present experiment. The inclusion of BN can either improve the machinability or provide oxidation resistance to the composite. BN was derived through an in-situ reaction between boron acid and urea by hot-pressing. Boric acid and urea were solved into the ethanol and mixed with nano-SiC particles, and then infiltrated into the fiber bundles. Correlations among microstructures, properties and compositions will be discussed.

Abstract: SiC has been extensively studied for high temperature components in advanced energy system and gas turbine because of its excellent high temperature mechanical properties and good thermal-chemical stability etc. However, the brittle characteristics of SiC such as low fracture toughness and low strain-to fracture still impose a severe limitation on practical applications of SiC materials. For these reasons, SiCf/SiC composites can be considered as a promising for various structural materials, because of their good fracture toughness compared with monolithic SiC. But, high temperature and pressure lead to the degradation of the reinforcing fiber during the hot pressing. Therefore, reduction of sintering temperature and pressure is key requirements for the fabrication of SiCf/SiC composites by hot pressing method. In the present work, monolithic Liquid Phase Sintered SiC (LPS-SiC) was fabricated by hot pressing method in Ar atmosphere at 1800oC under 20MPa using Al2O3, Y2O3 and SiO2 as sintering additives. The starting powder was high purity β-SiC nano-powder with an average particle size of 30nm. The characterization of LPS-SiC was investigated by means of SEM and three point bending test. Base on the composition of sintering additives-, microstructure- and mechanical property correlation, the compositions of sintering additives are discussed.

Abstract: SiC materials have excellent high temperature strength, low coefficient of thermal expansion, good resistance to oxidation and good thermal and chemical stability etc. However, the brittle characteristics of SiC such as low fracture toughness and low strain-to fracture still impose a severe limitation on practical applications of SiC materials. Therefore, in the interests of safety, we are required to measure fracture toughness of materials. In the present work, monolithic Liquid Phase Sintered SiC (LPS-SiC) was fabricated by hot pressing method under 20MPa using sintering additives at different temperature such as 1760oC, 1780oC, 1800oC and 1820oC. The starting powder was high purity β-SiC nano-powder with an average particle size of 30nm. Compositions of sintering additives were Al2O3 / Y2O3 = 0.7 and 1.5 (wt. %). Monolithic LPS-SiC was evaluated in terms of sintering density, hardness and fracture toughness through indentation fracture method by the Vickers hardness tester. Sintered density, hardness and fracture toughness of fabricated LPS-SiC increased with the increase of sintering temperature. They are higher than those of fabricated SiC by the chemical vapor deposition method.

Abstract: In order to apply a reduced activation ferritic (JLF-1) steel to the blanket/first-wall structure of a fusion reactor, its fracture toughness is very important for the strict estimation of material life. Fracture toughness testing of irradiated materials requires the use of miniaturised specimens and evaluation of TIG welding (tungsten inert gas arc welding) weldment properties is an important issue because necessary for production of nuclear fusion reactors. In this study, the fracture toughness tests were carried out according to the ASTM E1820-99. It was performed on various sizes (ligament and thickness) and various side-grooves of specimens and the TIG welding joint of JLF-1. The test results showed the standard specimen with side-groove of 40% represented valid fracture toughness. Fracture resistance curve (R-curve) increased with increasing specimen ligament and decreased with increasing specimen thickness. However, the R-curve of half size specimen was similar to that of the standard (1inch thickness) specimen. The fracture toughness test results of the TIG welded specimen showed a slight increase in the TIG welded specimen compared with JLF-1 base metal specimen.

Abstract: Irradiation damage produced by neutrons or energetic particles lead to changes of physical- and mechanical-properties of SiC. Radiation hardening and fracture toughness changing of SiC were clarified by indentation method previously. However, the mechanism studies have received little alteration. The purpose of this study is to improve the understanding of the mechanisms of mechanical property changes under irradiation. In this paper, the microstructural observation beneath and near an indentation will be used to infer mechanisms of radiation hardening and toughening. Indenting polycrystalline SiC creates deformation and cracking in the plastically deformed region. In the case of irradiated SiC, however, small-sized deformation zone was observed below contact indent, which resulted in the restricted size of residual impression. Additionally, the indentation cracks showed complex propagation behaviors such as deflecting, branching and microcracking.