Microstructural Characterization of Reactor Pressure Vessel Steels

Abstract

Ion irradiation is a promising tool to emulate neutron-irradiation effects on reactor pressure vessel (RPV) steels, especially in the situation of limited availability of suitable neutron-irradiated material. This approach requires the consideration of ion-neutron transferability issues, which are addressed in the present study by comparing the effect of ions with neutron-irradiation effects reported for the same materials. The first part of the study covers a comprehensive characterization, based on dedicated electron microscopy techniques, of the selected unirradiated RPV materials, namely a base metal and a weld. The results obtained for the grain size, dislocation density, and precipitates are put in context in terms of hardening contributions and sink strength. The second part is focused on the depth-dependent characterization of the dislocation loops formed in ion-irradiated samples. This work is based on scanning transmission electron microscopy applied to cross-sectional samples prepared by the focused ion beam technique. A band-like arrangement of loops is observed in the depth range close to the peak of injected interstitials. Two levels of displacement damage, 0.1 and 1 dpa (displacements per atom), as well as post-irradiation annealed conditions, are included for both RPV materials. Compared with neutron irradiation, ion irradiation creates a similar average size but a higher number density of loops presumably due to the higher dose rate during ion irradiation.

Microstructure-Informed Prediction of Hardening in Ion-Irradiated Reactor Pressure Vessel Steels

“…Table 3 summarizes the necessary unirradiated microstructure parameters that will be used to establish the model in Section 4. The results of the STEM experiments, in particular the number density (number per volume) and size (mean diameter) of irradiation-induced loops as a function of their distance from the surface of cross-sectional samples, were reported in detail and compared with the damage profiles (Figure 1) in our previous work [32]. The reported results are summarized in Figure 2 for the materials and irradiation conditions of interest.…”

“…Scanning transmission electron microscope (STEM) mode was used during imaging. More details on the STEM experiments and analysis were reported in [32].…”

“…The unirradiated microstructures were characterized comprehensively and reported in a previous work [32]. Table 3 summarizes the necessary unirradiated microstructure parameters that will be used to establish the model in Section 4.…”

“…Results of nanoindentation tests are reported, including modulus-based pile-up corrections. A previous study based on scanning electron microscopy (SEM) and transmission electron microscopy (TEM) [32] investigated the pre-existing microstructures, the irradiation-induced dislocation loops for the same materials and irradiation conditions. The current study briefly summarizes these published results, which are important for microstructure-informed predictions.…”

“…It was, therefore, decided to omit the electro-polishing step after mechanical fine polishing for all subsequently prepared samples. Comprehensive analyses of the microstructures and the resulting hardening mechanisms of the unirradiated materials were reported in [32].…”