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

Abstract

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.