The Role of Pre-Implanted Helium and Carbon on Cavity Evolution in Ion-Irradiated T91

The objective of this thesis is to understand the role of pre-implanted helium, with and without the presence of excess carbon, on the cavity evolution of ion-irradiated T91. Alloy T91, heat C2269, was pre-implanted at room temperature with helium concentrations varying over 4 orders of magnitude (0, 1, 10, 100, and 1000 appm). These samples were then irradiated with 5.0 or 4.4 MeV Fe2+ ions at 460°C up to damage levels of 450 dpa at the Michigan Ion Beam Laboratory. An alumina coating was utilized to prevent carbon contamination on some samples during irradiation. Samples without an alumina coating experienced carbon uptake during irradiation, providing for a study on the effect of excess carbon. The swelling, precipitate, and dislocation evolution for the excess carbon and nominal carbon conditions for all helium concentrations was characterized. Scanning transmission electron microscopy (STEM) was used to characterize the microstructure of the irradiated specimens. In the nominal carbon conditions, swelling decreased with increasing helium concentration. At low helium levels (0, 1, and 10 appm), the cavity evolution was determined by the cavity sink strengths. Differences in density were observed at 50 dpa, however the three low helium conditions achieved very similar cavity distributions by 300 dpa. At high helium levels (100 and 1000 appm), bimodal cavity distributions were observed at all damage levels. High helium levels served to stabilize a population of bubbles with sizes below the gas-free critical radius. A substantial cavity sink strength, helium trapping, and a cavity interstitial bias contributed to reduced growth of larger cavities. In the excess carbon conditions, swelling was peaked at 10 appm He. The main role of carbon was to inhibit cavity nucleation, which reduced the cavity density at all damage and helium levels compared to the nominal carbon conditions. Additionally, excess carbon allowed for the formation of a very high density of M2X carbides. These carbides were strongly associated with helium bubbles and provided an interface for any emitted helium atoms. A bubble population was never observed in the 100 appm He condition, and bubbles in the 1000 appm He condition disappeared completely by 450 dpa. This work provides substantial insight into the complex evolution of cavities at various helium and carbon levels.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/143960/1/amonterr_1.pd

Investigating Helium Bubble Nucleation and Growth through Simultaneous In-Situ Cryogenic, Ion Implantation, and Environmental Transmission Electron Microscopy

Palladium can readily dissociate molecular hydrogen at its surface, and rapidly accept it onto the octahedral sites of its face-centered cubic crystal structure. This can include radioactive tritium. As tritium β-decays with a half-life of 12.3 years, He-3 is generated in the metal lattice, causing significant degradation of the material. Helium bubble evolution at high concentrations can result in blister formation or exfoliation and must therefore be well understood to predict the longevity of materials that absorb tritium. A hydrogen over-pressure must be applied to palladium hydride to prevent hydrogen from desorbing from the metal, making it difficult to study tritium in palladium by methods that involve vacuum, such as electron microscopy. Recent improvements in in-situ ion implantation Transmission Electron Microscopy (TEM) allow for the direct observation of He bubble nucleation and growth in materials. In this work, we present results from preliminary experiments using the new ion implantation Environmental TEM (ETEM) at the University of Huddersfield to observe He bubble nucleation and growth, in-situ, in palladium at cryogenic temperatures in a hydrogen environment. After the initial nucleation phase, bubble diameter remained constant throughout the implantation, but bubble density increased with implantation time. β-phase palladium hydride was not observed to form during the experiments, likely indicating that the cryogenic implantation temperature played a dominating role in the bubble nucleation and growth behavior

Multiple ion beam irradiation for the study of radiation damage in materials

The effects of transmutation produced helium and hydrogen must be included in ion irradiation experiments to emulate the microstructure of reactor irradiated materials. Descriptions of the criteria and systems necessary for multiple ion beam irradiation are presented and validated experimentally. A calculation methodology was developed to quantify the spatial distribution, implantation depth and amount of energy-degraded and implanted light ions when using a thin foil rotating energy degrader during multi-ion beam irradiation. A dual ion implantation using 1.34 MeV Fe+ ions and energy-degraded D+ ions was conducted on single crystal silicon to benchmark the dosimetry used for multi-ion beam irradiations. Secondary Ion Mass Spectroscopy (SIMS) analysis showed good agreement with calculations of the peak implantation depth and the total amount of iron and deuterium implanted. The results establish the capability to quantify the ion fluence from both heavy ion beams and energy-degraded light ion beams for the purpose of using multi-ion beam irradiations to emulate reactor irradiated microstructures

Solar wind contributions to Earth’s oceans

The isotopic composition of water in Earth’s oceans is challenging to recreate using a plausible mixture of known extraterrestrial sources such as asteroids—an additional isotopically light reservoir is required. The Sun’s solar wind could provide an answer to balance Earth’s water budget. We used atom probe tomography to directly observe an average ~1 mol% enrichment in water and hydroxyls in the solar-wind-irradiated rim of an olivine grain from the S-type asteroid Itokawa. We also experimentally confirm that H+ irradiation of silicate mineral surfaces produces water molecules. These results suggest that the Itokawa regolith could contain ~20 l m−3 of solar-wind-derived water and that such water reservoirs are probably ubiquitous on airless worlds throughout our Galaxy. The production of this isotopically light water reservoir by solar wind implantation into fine-grained silicates may have been a particularly important process in the early Solar System, potentially providing a means to recreate Earth’s current water isotope ratios