Evolution of Heterogeneous 316LSS Microstructures Under Neutron Irradiation, Nuclear Science User Facilities

This project aims to quantify the impact of as-printed heterogeneity in laser powder bed fusion (LPBF) 316LSS on spatially dependent post-irradiation segregation and precipitation behavior. To accomplish this task, the research team proposes to use the Low Activation Materials Development and Analysis (LAMDA) lab at ORNL to perform site-specific nanoindentation on varying weld pool regions of neutron irradiated samples. targeted focused ion beam (FIB) liftouts, followed by on-zone scanning transmission electron microscopy (STEM) with coupled energy dispersive X-ray spectroscopy (EDS). The samples to be investigated were previously irradiated to 0.2 and 2 displacements per atom at a target irradiation temperature of 300°C in the High Flux Isotope Reactor (HFIR) and are readily available for the targeted microscopy and indentation proposed in this work. Due to the immediate availability of specimens, and existing repository of pre-irradiation specimen data available through the previous Transformational Challenge Reactor (TCR) program, only 2-weeks of instrument time is requested at the LAMDA facility. This work is expected to be completed within 3 months of the project award date, with plans for a publication disseminating project results. From the coupled STEM+EDS and nanoindentation results, the research team will identify how nanoscale features, including dislocation cellular structures, loops, and precipitate densities relate to nano hardness results in regions with observed heterogeneity. This project is particularly impactful since it is the first study on how intrinsic heterogeneity at the microstructure level in additively manufactured 316LSS parts may impact post-irradiation performance. If significant deviations in post-irradiation evolution are observed, any standard material qualification approaches may need revision as spatial variation in mechanical performance may need to be considered prior to part qualification.