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

  • Author(s): Stan, Tiberiu
  • Advisor(s): Odette, George R
  • et al.

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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