Shuhei Nogami

Helium-bubble formation behavior of SiCf/SiC composites after helium implantation

Abstract Helium-bubble formation behavior in SiC-fiber-reinforced SiC-matrix (SiCf/SiC) composites was studied using helium implantation. Microstructural observation of the He-implanted specimens was carried out after post-implantation annealing at 1673 K for 1 h. Microstructural observation revealed small cavities in the SiC matrix only. No cavities were observed in the SiC fibers or in the carbon coating layers or their interfaces.

Effect of dual-beam-irradiation by helium and carbon ions on microstructure development of SiC/SiC composites

Microstructural changes of SiC-fiber reinforced SiC-matrix (SiCf/SiC) composites after simultaneous helium-ion and carbon-ion irradiations were studied. SiCf/SiC composite specimens were irradiated up to 10 dpa and 1000 at. ppm He at 1073 and 1223 K. Cross-sectional microstructural observation and irradiated surface analyses were performed. Dimensional changes in SiC-fibers and pyrolytic graphite layers were observed but a dimensional change was not observed in the SiC-matrix. Microstructural features such as voids and dislocation loops were not observed in these experimental conditions.

Effect of simultaneous ion irradiation on microstructural change of SiC/SiC composites at high temperature

Abstract The effect of simultaneous triple ion irradiation of He, H and Si on microstructural evolution of two kinds of SiC/SiC composites (HNS composite (using Hi-Nicalon type S SiC fiber) and TSA composite (using Tyranno SA SiC fiber)) at 1000 °C has been investigated. The microstructure observations of SiC/SiC composites irradiated to 10 dpa were examined by transmission electron microscopy. He bubbles were hardly formed in matrix of TSA composite, but many helium bubbles and some cracks were observed at grain boundaries of matrix of HNS composite. He bubbles and cracks were not, on the other hand, observed in the both fiber fabrics of HNS and TSA composites. Debonding between fiber and carbon layer following irradiation region was not observed in the both composites. Under these irradiation conditions, TSA composite showed the better microstructural stability against ion beams irradiation than one of HNS composite.

Anisotropy in the Mechanical Properties of Potassium and Rhenium Doped Tungsten Alloy Plates for Fusion Reactor Applications

Abstract The effects of K-bubble dispersion and 3 wt.% Re addition on the tensile properties and their anisotropy in W were investigated in this work. K-doped W and K-doped W-3%Re showed ∼45 and ∼65% higher tensile strengths than pure W, respectively. The ultimate tensile strength and its temperature dependence in pure W, K-doped W, and K-doped W-3%Re showed anisotropy. However, the effects of K-bubble dispersion and 3% Re addition on the anisotropic tensile strength were not clearly observed. K-doped W and K-doped W-3%Re showed better deformation abilities than pure W. K-doped W-3%Re showed better tensile properties than pure W under non-irradiation conditions used in this work. Since irradiation hardening is suppressed by adding 3% Re, K-doped W-3%Re is expected to be more advantageous as a plasma facing material in a fusion reactor than pure W and K-doped W.

High-Temperature Helium Embrittlement of 316FR Steel

The helium embrittlement behavior of 316FR austenitic stainless steel was investigated by a tensile test at 750°C using miniature tensile specimens, which were helium-implanted below 100°C up to 5, 30, and 100 appm using a cyclotron accelerator, and were post-implantation-annealed at 750°C for 10 and 100 h. The helium-implanted specimens showed a fully intergranular fracture regardless of the helium concentration and annealing time. No microstructural changes in the as-implanted specimen up to 30 appm and formation of a small number of helium bubbles due to the post-implantation annealing were observed. The gradual release of the helium during the tensile test started after the yielding, and a sharp peak of the helium release was detected in the final fracture phase. The total number of helium atoms released was strongly dependent on the implanted helium concentration, rather than on the annealing time.

Improvement of Surface Exfoliation Behavior by Helium-ion Bombardment of a Tungsten Alloy Fabricated by Mechanical Alloying

Blistering and exfoliation of several tungsten alloys, which cause surface damage, were investigated using 3-MeV He-ion bombardment at room temperature (RT), 400, and 550°C. The alloy W-0.3TiC, which was fabricated by the mechanical alloying method and had an ultrafine grain structure, a K-doped W alloy, and pure W metal were examined to explore a way of suppressing the surface damage. In RT irradiation, surface exfoliation occurred at a fluence of (1–2) × 1022 He/m2 in all the tested specimens. In the case of 550°C irradiation, surface exfoliation was observed above 2 × 1022 He/m2 irradiation in pure W and K-doped W, but no surface exfoliation was observed in W-0.3TiC up to a fluence of 2 × 1023 He/m2. The results showed that W-0.3TiC showed a higher resistance to surface exfoliation by He-ion bombardment and the level of resistance was temperature-dependent. The surface morphology, cross-sectional morphology, and microstructure were characterized by transmission electron microscopy. Helium thermal desorption spectrometry was carried out to determine the mechanism whereby the surface attained resistance to the damage through He-ion bombardment. The improvement in the resistance to the surface exfoliation could be attributed to the ultrafine grain structure and the intergranular enhanced He diffusion behavior of the MA-processed material.

Effect of Specimen Shape on the Low Cycle Fatigue Life of Reduced Activation Ferritic/Martensitic Steel

The effect of specimen shape on the low cycle fatigue life of reduced activationferritic/martensitic steel, F82H IEA-heat, was investigated to develop a reliable smallspecimen test technique (SSTT) for fatigue. The low cycle fatigue test was carried out at room temperature in air at a total strain range of 0.8–3.0%. One round-bar-type and three kinds of hourglass-type miniature fatigue specimens were used. The relationship between total strain range (δεt) and number of cycles to failure (Nf) in one of the hourglasstype specimens, which is the current standard miniature fatigue specimen in Japan, was represented by the Manson-Coffin-type regression formula (δϵt = 376N 0.84 f + 2.16N −0.16 f ). The effect of specimen shape on the lowcycle fatigue life and fracture mode was almost negligible among the four specimen types, and their fatigue lives were almost the same. Similar stress distributions at the minimum cross sections of these specimens were shown above the total strain range of 0.8% by elasto-plastic finite element analysis. Therefore, the almost no effect of the specimen shape on the low cycle fatigue life and fracture mode was considered to be due to the similar stress distributions in all the test conditions in this study.

Mechanical Property Changes and Irradiation Hardening Due to Dissimilar Metal Welding with Reduced Activation Ferritic/Martensitic Steel and 316L Stainless Steel

Dissimilar metal electron beam welding with reduced activation ferritic/martensitic steel, F82H IEA heat, and SUS316L austenitic stainless steel was studied. Mechanical property evaluation at room temperature by bend test, tensile test, Vickers hardness measurement and charpy impact test, and evaluation of irradiation hardening by proton irradiation at 300°C up to 0.5 dpa were carried out. The mechanical properties of the dissimilar weld were improved by the optimization of the electron beam position in the welding (shifted 0.2 mm on 316L side) and the post-weld heat treatment (PWHT) (750°C x 1 hour). The improvement of the mechanical properties might be due to the fact that the weld metal consisted of the austenitic phase. Smaller irradiation hardening than 316L was observed in the weld metal of the F82H/316L dissimilar weld after PWHT at 750°C for 1 hour, where the electron beam was shifted 0.2 mm on 316L side, though the formation of voids and dislocation loops occurred in the grain matrix of the weld metal.

Synergistic Effect of Displacement Damage, Helium and Hydrogen of Silicon Carbide Composite

ABSTRACT The mechanical properties of advanced SiC/SiC composite and polycrystalline, monolithic β-SiC under dual- and triple-ions irradiation to 1 and 10 dpa at 800°C, 1000°C, and 1300°C were investigated by a Nano-indentation test. Preliminary microstructural analysis by transmission electron microscopy was performed. Hardness and elastic modulus changes in response to ion irradiation were observed, but synergistic effects on these mechanical properties were not significant. In contrast, microstructural observation of the composites after 10 dpa at 1000°C showed that cavity formation behavior was dependent on the material and the helium or hydrogen implanted mode. The effect of gas elements on cavity formation and the mechanical properties are discussed.

Mechanical property change and swelling behavior of SiC fiber after light-ion irradiation

Studies on the effects of helium or hydrogen gases on the mechanical properties and size stability of SiC fibers were examined using accelerator implantation techniques. Three types of Nicalon fibers (Nicalon-CG, Hi-Nicalon, and Hi-Nicalon type-S) were examined. Three levels of implanted gas concentrations from 50 to 5000 appm were conducted. Swelling of the fibers was observed after helium implantation. The amount of swelling depended on the helium concentration and the type of fiber. The swelling had vanished after the post-implantation annealing at 1000 °C. The slight decrease of tensile strength was observed after the He implantation. It remained after the post-implantation annealing up to 1400 °C. The significant effects of hydrogen implantation on the strength and swelling were not observed in this work.