Search

Ion beam implantations are widely performed to understand the effects of radiation damage such as He bubbles on nuclear structural materials. This work investigates near-surface changes caused by low energy ion implantations, as well as deep ion implantations leading to bulk-property changes.The near-surface interactions of low energy (25-60 keV) He ions with Ti and Cu targets were investigated by atomic force microscopy (AFM), nanoindentation, Transmission Electron Microscopy (TEM), and Positron Annihilation Spectroscopy (PAS). AFM showed a linear increase in swelling with respect to dose for all materials, and blistering onset doses of 5 × 1017 ions/cm2, 8 × 1017 ions/cm2, and 1 × 1018 ions/cm2 for Ti(0001), Ti(10¯10), and Cu(100), respectively. Cavities on the order of 287 nm diameter were observed in Cu, and surface blisters formed from the intersection of these large cavities. He bubbles observed in Ti were around 1 nm diameter, and surface blisters formed from inter-bubble fracture. PAS showed an increase in defect concentration with respect to depth and dose, agreeing well with AFM and TEM results. An additional PAS measurement determined that shock loading can lead to a decrease in vacancy concentration in a pre-implanted material.To fill the gap between widely available low energy ion implantations and difficult-to-achieve bulk implantations, this work aimed to establish the use of the 88-Inch Cyclotron for nuclear materials studies. Four HT-9 SS-J-geometry tensile specimens were irradiated with high energy (19-25 MeV) deuterons at the 88-Inch Cyclotron to doses of approximately 0.2 dpa prior to small scale tensile testing. The results from this study showed irradiation hardening characterized by black dot irradiation defects, and the tensile test results were in agreement with the available data. To support future deep ion implantation campaigns, a novel ion beam degrader capable of uniformly implanting bulk-scale materials with He ions was designed.

A molecular dynamics (MD) simulation study was performed to investigate the effects of helium (He) on the mechanical properties of nanocrystalline body-centered cubic iron (BCC Fe). Simulated X-ray diffraction (XRD) was used to explore the relationship between the generation of cracks and the change of the crystal structure in nanocrystalline BCC Fe during tensile deformation. It is observed that the peak stress and the elastic modulus decrease with increasing concentration of He atoms, which are introduced into the grain boundary (GB) region of nanocrystalline Fe. The generation and connection of intergranular cracks are enhanced by He atoms. Significant peak separation, which is associated with the generation of cracks, is found in the simulated XRD patterns of nanocrystalline Fe during the tensile process. The lower diffraction angle of the {211}′ peak suggests a more serious lattice distortion during loading. For all nanocrystalline Fe deformed to 6% strain, the degree and fraction of the lattice distortion increases with the increasing loading stress.

Helium (He) effect on the microstructure of nanocrystalline body-centered cubic iron (BCC-Fe) was studied through Molecular Dynamics (MD) simulation and simulated X-ray Diffraction (XRD). The crack generation and the change of lattice constant were investigated under a uniaxial tensile strain at room temperature to explore the roles of He concentration and distribution played in the degradation of mechanical properties. The simulation results show that the expansion of the lattice constant decreases and the swelling rate increases while the He in the BCC region diffuses into the grain boundary (GB) region. The mechanical property of nanocrystalline BCC-Fe shows He concentration and distribution dependence, and the existence of He in GB is found to benefit the generation and growth of cracks and to affect the strength of GB during loading. It is observed that the reduction of tensile stress contributed by GB He is more obvious than that contributed by grain interior He.

The micro-structural effects of different neutron irradiation temperatures and helium contents, for 16 dpa dose, have been investigated by means of small-angle neutron scattering (SANS) in B-alloyed ferritic/martensitic steel Eurofer97-1 (0.12 C, 9 Cr, 0.2 V, 1.08 W wt%, B concentrations up to 1000 ppm); due to B transmutations, fusion relevant He/dpa values are expected to be produced under neutron irradiation. SANS measurements have been carried out on a sample irradiated at 350 °C, with estimated helium content of 5600 appm, and compared to previous SANS results, obtained on two other irradiated samples of this same B-alloyed steel. These new measurements confirm that for such high helium contents the SANS cross-section increases in order of magnitude and the magnetic SANS component is strongly reduced, compared to lower helium content (400 appm). Such effects are attributed to increase in helium bubbles density and to the presence of micro-cavities, produced after dissolution of large B-carbides. The SANS data analysis procedure has been improved, also thanks to the additional information provided by the new measurements, and more accurate helium bubble size distributions have been obtained for all the investigated samples. For 5600 appm helium content, bubble volume fractions are found of 0.025 for the sample irradiated at 350 °C and of 0.041 for the previously investigated sample irradiated at 400 °C, significantly increasing with the irradiation temperature. These values are approximately one order of magnitude larger than the value of 0.003 previously found for the sample with 400 appm helium. The size distributions are compared with electron microscopy observations of these same samples. It appears that the occurrence of complex micro-structural changes in irradiated Eurofer97-1 steel should be taken in due account when considering its application under high He/dpa ratio values. Keywords: Helium effects, Neutron irradiation, Small angle neutron scattering, Electron microscopy