Investigation of the Effect of Annealing on Irradiated Alloy 718
Abstract
During normal accelerator operations at the Isotope Production Facility (IPF) within the Los Alamos Neutron Science Center (LANSCE) at Los Alamos National Laboratory (LANL), an annealed Alloy 718 proton beam window experiences a peak temperature of 120°C but can have up to 100°C fluctuations due to nonstandard runs. These fluctuations can anneal radiation damage and possibly cause precipitation of other phases. To this end, this study aimed to systematically determine the effects of deliberate temperature excursions on the microstructure and mechanical properties of irradiated Alloy 718. We characterized properties of a set of Alloy 718 samples irradiated to 15 dpa under three temperatures (room temperature, 100°C, and 200°C) and subsequently annealed at three conditions (none, 300°C, and 500°C). Additionally, we characterized samples irradiated to 0.5 dpa at 100°C and annealed under three conditions (room temperature, 100°C, and 200°C). Each condition was compared quantitatively and qualitatively with unirradiated annealed Alloy 718. Microstructural evolution, including determining the presence and prevalence of precipitates and dislocations was carried out with a Transmission Electron Microscope (TEM). Additionally, nanoindentation testing was completed on the samples to determine the influence of these treatments on mechanical properties. The study has shown via TEM that precipitates do not form under any of the investigated conditions. However, a strong trend of decreasing hardness with increasing annealing temperature was observed across all samples. In other words, annealing does indeed change the specimen back toward its initial state prior to irradiation.
Although further testing is necessary to provide certainty, we attribute this trend to a decrease in dislocation density in the samples during annealing, as observed through TEM analysis. Overall, these results indicate that in-situ annealing radiation damage from the IPF window is indeed feasible.
Citation
Turman, Haley Brooke (2019). Investigation of the Effect of Annealing on Irradiated Alloy 718. Master's thesis, Texas A&M University. Available electronically from https : / /hdl .handle .net /1969 .1 /187917.