Development of New Multiscale STEM-Based Techniques to Characterize Defects

Extreme nuclear reactor environments require materials to maintain their integrity all while a range of processes act in unison to degrade their performance. These processes are directly limited or accelerated by defects produced under irradiation. However, the trends associated with varying salt chemistry, temperature, and irradiation dose are not intuitive, and a mechanistic understanding of the associated thermodynamics and kinetics remains unknown. A close inspection and tracking of point defects such as vacancies and interstitials is essential for understanding material behavior in the coupled irradiation and corrosion environment. Recent developments in 4D-STEM with high-speed direct electron detectors and atomic resolution STEM has provided an opportunity for mapping defect distributions at the nanoscale and their associated strains, both of which have far-reaching implications for detailed analysis of complex irradiation / corrosion damage.<br/>Measurement of lattice parameters and defect distributions were studied <i>in-situ </i>as a function of increasing temperature (up to 1000°C) on suspended Au thin films, used as a model system. The 4D-STEM data sets were collected at varied temperatures and processed using the strain mapping protocol provided in the py4DSTEM software package. Furthermore, the 4D-STEM strain mapping methodology was implemented to study Au films which were exposed to low dose levels (100 – 2000 ions/nm²) of irradiation via He-ion microscope (HIM). Throughout irradiated zones, nanoscale defects in the form of vacancies, interstitials, dislocation loops and additional "black spot" damage were identified and were characterized via 4D-STEM and atomic resolution HAADF-STEM. At a minimum probe size of 1nm, the 4D-STEM strain maps reveal a sharp increase in the density of defects in the irradiated zones and can resolve the distribution and strain morphology of 1 – 5 nm point defects. Moreover, this technique is relatively insensitive to subtle changes in orientation and can be collected over a large (100 – 500 nm) field of view. However, the precise morphology and magnitude of strain associated with individual defects exceeds the capabilities of 4D-STEM, so complimentary HR-STEM imaging was conducted via the 4D camera on TEAM 0.5. The microscope is capable of ultra-fast HR-STEM imaging <i>ex-situ</i> in and around the irradiated zones while exposing the sample to minimal dose. 0–90° scan rotated image pairs were corrected for scanning drift and atomic resolution strain maps are generated using a modified peak finding method that measures atomic positions in real space. This high precision (30 pm pixel resolution) measurement of strain associated with irradiation defects was directly compared with the 4D-STEM measurement. This combined approach to characterizing point defects through maximizing resolution (HR-STEM) and expanding field of view (4D-STEM) fundamentally improves the understanding of complex irradiation damage and provides a new pathway for engineering materials in future nuclear energy systems.