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The Role of Oxides in Nanostructured Ferritic Alloys and Bilayers: Interfaces, Helium Partitioning and Bubble Formation


Despite the successful development of Tokamak nuclear fusion plasma physics devices, commercial power production remains elusive partly due to the severe environments produced during the deuterium-tritium fusion reaction. Nanostructured Ferritic Alloys (NFAs) are candidate structural materials for first-wall/blanket applications. The stainless steels are thermally stable up to 900 °C and remarkably irradiation tolerant. NFAs typically contain a high number density (5x1023/m2) of Y-Ti-O nano-oxides (NOs) with average diameters ≈ 2.5 nm. Most of the smallest NOs are Y2Ti2O7 (YTO) fcc pyrochlore. The NOs impede dislocation climb and glide, stabilize dislocation and grain structures, and trap He in fine-scale bubbles at matrix-NO interfaces. Detailed characterization and analysis of the NO-matrix interfaces is needed to develop first principles and atomic-scale models that are part of multi-scale efforts to predict the behavior of NFAs during processing and in irradiation service environments. YTO-matrix orientation relationships (ORs) are of particular interest because they impact selection of compositions and processing paths, service stability, mechanical properties and irradiation tolerance of NFAs.

X-ray absorption spectroscopy (XAS) measurements on embedded NOs are most consistent with Y2Ti2O7, while the slightly larger extracted oxides are primarily consistent with Y2TiO5. A bulk extraction and selective filtration technique was developed to dissolve the ferritic matrix, trap the larger Y2TiO5 oxides, and yield samples well suited for XAS measurements. Further, a 14YWT alloy was annealed to coarsen the NOs, and He implanted to produce bubbles. High resolution transmission electron microscopy shows two dominant ORs (cube-on-edge and cube-on-cube). The smaller NOs are associated with smaller bubbles, while some of the largest NOs (>6 nm) often have two bubbles. Most bubbles nucleate near dislocation cores at {111} NO facets.

The second research approach is to study a model bilayer system. For the first time, the dominant deposited Fe-YTO interface ORs are reported. Most Fe grains deposited on {111}YTO have the Nishiyama-Wasserman OR: {110}Fe//{111}YTO and <100>Fe//<110>YTO. The dominant OR for depositions on {100}YTO is: {110}Fe\{100}YTO and <111>Fe\<110>YTO. Finally, most Fe grains deposited on {110}YTO show axiotaxial texturing with off-normal {110}Fe planes parallel to off-normal {100}YTO planes. Room temperature He implantation of a Fe-{110}YTO bilayer shows a range of bubble sizes in the Fe film, and larger ~2 nm bubbles at the Fe-YTO interface. In this experiment, He did not diffuse into the YTO. In a second, high temperature implantation, 99.3% of the He remained in the Fe film and interfacial pores, but 0.7% was found in the YTO substrate. The studies performed in this dissertation provide crucial experimental inputs for the development of computational models that accurately predict NFA in-service behavior. The results provide an important step into turning the promise of fusion energy into a reality.

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