Development of High-Resolution STEM Methods to Map Diffusional Transport of Point Defects Produced via Kirkendall Diffusion

In nuclear reactors, irradiation damage tolerance can be increased by utilizing interfaces as vacancy sinks. Degradation mechanisms at interfaces can involve complicated defect production and evolution. Specifically, Kirkendall void formation has been attributed to the imbalance diffusion between two species of atoms, and as a result, this is counteracted by a flux of vacancies across the metal-metal interface, accumulating locally at grain boundary or dislocations to form voids. Current methods for probing accumulation and mobility of these vacancy-type point defects are largely based on bulk measurements (X-ray diffraction (XRD) and positron annihilation spectroscopy (PAS)) that do not provide the spatially resolved information necessary to track the early stage of interdiffusion at a bi-metal interface. 4D-STEM and high-resolution STEM methods have the potential to provide details on the formation of point defects by identifying changes in lattice parameters at the nanometer scale. This spatially resolved method is especially important for determining the influence of pre-existing microstructural features, such as grain boundaries or dislocations, and their role on defect kinetics. In this study, we consider a thin film bimetallic diffusion couple to track changes in vacancy concentration and Kirkendall void formation <i>in situ</i> at elevated temperatures. A Cu-Ni bimetal is utilized as an ideal specimen (both phases are miscible and no intermediate phase is expected to form) to correlate vacancy concentration fluctuation with Kirkendall void formation under designed in situ heating program. With high-resolution EDS, we track real-time motion and diffusivity/concentration of the solute atoms. 4D-STEM measurements map the local change in lattice parameter across the bimetallic interface, with custom patterned “bullseye” condenser apertures that are used to improve the accuracy of the lattice parameter mapping. DFT modeling is used to connect the divergence between solute induced lattice strain and vacancy-defect concentration. This extensive study fundamentally improves the understanding of interfaces in engineering materials (eg. Coating, welding joints, etc.) designed in future nuclear energy systems.