Advanced X-Ray Characterization of Fusion Materials

In the present work, we describe our efforts employing advanced non-destructive X-ray-based characterization to support the fabrication and post-irradiation examination of materials for advanced fusion energy systems. Specific material systems include castable nanostructured alloys (CNAs) and neutron irradiated tungsten. We quantify the microstructural and atomic properties of advanced first-wall materials through two-dimensional mapping and high-throughput X-ray diffraction (XRD). X-ray-based characterization techniques provide complimentary quantitative insights across multiple length scales needed to fill critical knowledge gaps and predict long-term behavior and performance.<br/><br/>CNAs have been under development by the fusion program over the last decade and are meant to provide enhanced elevated temperature performance as compared to reduced activation ferritic martensitic steels through the internal formation of irradiation-stable and dislocation-pinning precipitates. In this way, CNAs are a potentially more practical high-temperature option for high-heat-flux or other challenging fusion applications. The uniformity of the microstructure and engineered precipitates through various cast plates are investigated here by constructing 2D microstructural maps from XRD analysis.<br/><br/>Tungsten is presently the leading plasma-facing material candidate due to its high melting point, resistance to sputtering, and chemical compatibility with tritium. However, extended exposure of W to fusion plasmas and intense 14 MeV neutrons, resilience against plasma-induced surface damage (cracking, erosion/exfoliation, and fuzz formation), and degradation of bulk mechanical properties due to neutron irradiation raise significant concerns about its stability. We performed XRD experiments of unirradiated and neutron-irradiated polycrystalline W alloys after low-dose irradiation at 800°C (0.1 dpa) and after high irradiation temperature (&gt;800°C).<br/><br/>XRD patterns were collected for fifty neutron irradiated W, and 2D XRD mapping data for six CNA alloy plates were performed at the PDF beamline at the National Synchrotron Light Source-II. Analysis of the XRD patterns allows the determination of the lattice strain, change in microstructure parameters, and formation of secondary phases. We developed a high-throughput quantitative XRD analysis routine to generate microstructural data of both sets of specimens. Using Perl and Python scripts, we ran batch analysis of our data through the Materials Analysis Using Diffraction software and then generated heatmaps that plot our collected data in 2D mappings.<br/><br/>The quantitative XRD analysis of the exposed CNA samples show a complex hydrogen uptake in the metal carbide crystallites, and the lattice expansion and crystallite size reduction will have non-negligible implications on the mechanical properties and retention of transmutation products within this important structural component. The heatmaps visualize microstructural features (carbides) that provide strength to the steel and are trapping sites for radiation-induced gaseous products. The steels are strangely affected by the exposure and expand (lattice expansion).<br/><br/>Our initial success in demonstrating the effective characterization of fabricated and as-irradiated materials supports our goal of fabricating functionally graded or composited first wall tile structures (i.e., W-CNA), which may be essential to mitigate the very high heat flux loading anticipated as we move beyond ITER towards DEMO-like fusion systems.<br/><br/>AZ, MZ, and RK acknowledge the Garcia Research Program and the Morin Charitable Trust for their contributions to making this work possible. These experiments and analysis were supported by the U.S. Department of Energy Office of Fusion Energy Sciences under contract DESC0018322 with the Research Foundation for the State University of New York at Stony Brook and DE-AC05–00OR22725 with UT-Battelle LLC.