Complementing In Situ Microscopy with Multimodal Synchrotron Experiments in the Characterization of Radiation Damage

The formation of gaseous defects during irradiation due to, e.g., transmutation or ion implantation, involves a transition from defect clustering to the nucleation of bubbles and its biasing to various microstructural sinks such as grain boundaries. In situ microscopy techniques are limited in their ability to resolve the behavior of sub-nanometer helium clusters during the incubation stages of bubble formation while common X-ray techniques such as X-ray diffraction (XRD) provide information specifically pertaining to changes in the lattice parameter in the presence of such defect clusters. In this study, a region of reciprocal space accessible via wide angle X-ray scattering (WAXS) is identified that allows for the direct probing of sub-nanometer helium clusters irresolvable through transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS) and only indirectly quantified by an XRD lattice parameter analysis. Using WAXS in a multimodal characterization campaign with TEM informed SAXS analysis and XRD of irradiated tungsten, our experiments build a more complete picture of the transition from helium defect clustering to the nucleation of bubbles and their subsequent biasing to grain boundaries. Specifically, nanoscale cavity formation is shown to accompany clustering with increasing fluence but with cavities homogeneously distributed between the grain matrix and boundary regions of the microstructure. An increase in temperature shifted the cavity population to a bimodal distribution containing small nanometer intragranular cavities and larger cavities biased to the grain boundaries, which was accompanied by a decrease in the sub-nanometer helium cluster population. Leveraging the defect distributions as a function of fluence and temperature, the mechanisms underpinning these transitions are discussed using insights from molecular dynamics simulations. Results demonstrate the utility of our multimodal approach in probing the incubation stages of cavity formation and its subsequent transition to biased cavity populations, thereby helping to close the analysis gap between nanoscale in situ TEM imaging and atomic-scale XRD measurements.