Irradiation Damage Of Fe-based Structural Materials For Fusion Reactor

With the development of the international fusion reactor project, the irradiation damage of nuclear material has become a hot topic of materials science and nuclear energy research. Reduced activation ferritic/martensitic steel is considered as the primary structure materials of international thermal experimental reactor and fusion reactor, due to good resistance to radiation swelling properties, good processing performance and economy, etc. A good understanding on the irradiation damage mechanism and the interaction of irradiation defects with mechanical property is therefore important to develop structural materials in fusion reactors and for a safe design and operation of innovative nuclear systems.Low activation ferritic/martensitic steel and model alloys were irradiated by ions and electrons. By using high resolution transmission electron microscope (HRTEM) and scanning transmission electron microscope (STEM), the microstructures of the materials were observed and characterized in detail. The evolution of the microstructure of irradiatied materials has been investigated and in situ observed using ultra-high voltage TEM (HVEM). Combining nano-indentation and other mechanical properties testing means, mechanical properties of materials before and after ion irradiation also were investigated. Innovative achievements were stated as following:A large amount of defects such as dislocation loop has been induced in model alloys after deuterium ions implantation at room temperature (RT). High resolution TEM image and structural information of dislocation loops were achieved, which provides experimental data for damage research and theory calculation. The evolution of the dislocation loop at different temperature was in situ observed. Migration energy of the vacancy was determined as0.66±0.1eV from the growth speed of interstitial loops at different temperature. In pure iron with deuterium ion implantation at RT, interstitial dislocation loops formed firstly and then vacancy loops formed after aged30min at753K. The vacancy-type dislocation loops of Burgers vector [100] shrinked and disappeared under the subsequent electron irradiation. The shrink rate was related to the electron irradiation dose. Comparing to hydrogen implantation, vacancy loop formed at higher temperature in deuterium implanted iron, which suggested higher binding energy of deuterium with vacancy.In addition to the dislocation loops, the irradiation of deuterium ion at773K induced the precipitation of a new phase rich in Cr in the Fe-lOCr model alloy. These nanoscale precipitates arranged along the<100> direction. The results from series of EDPs combined with diffraction calculations revealed that the phase crystallizes in the cubic system, belongs to the space group of Pm3m, adopts a cube-on-cube orientation relationship with the Fe matrix and has the same lattice parameters as Fe. The1-D elongation of the precipitate was attributed to the distribution of strain fields around the precipitates. Under the subsequent2MV electron irradiation, the precipitates were highly stable and hindered the surrounding loop growing up.Specimens of China Low Activation Martensitic (CLAM) steel with the addition of Si improved its tensile strength mainly because large amount of fine MC particles spread within the matrix. After irradiation, irradiation hardening was observed in both CLAM steels with and without Si addition, which is attributed to the production of defect clusters induced by irradiation. However, the irradiation hardening ratio of the CLAM steel with Si addition was lower than that of the CLAM steel without Si because the MC precipitates with high interface-to-volume ratio could alleviate the effect of irradiation on hardness by decreasing the density of the dislocation loop induced during irradiation. And a large number of helium bubbles were induced in CLAM steel after high dose helium irradiation.Given the limitation on ion irradiation depth and sample size, conducting tensile tests was not practical. Therefore nano-indentation is widely used to evaluate the mechanical properties of ion-irradiated materials. The measured nanoindentation hardness needs to be converted to the bulk-equivalent hardness by using some computation models. We proposed a modified model based on the popular Nix-Gao model and Korsunsky film/substrate model for the evaluation of the composite hardness of the ion-irradiated materials. The model is expected to be able to be applied to coated systems and bulk materials generally. It fits well to the experimental data and has widely application for ion-irradiated materials nano-indentation in the further.