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Open Access Publications from the University of California

Scholarly Works (3 results)

  • Thesis
  • Peer Reviewed
UC Santa Barbara Electronic Theses and Dissertations (2015)

A variety of processing approaches were employed to fabricate ductile-phase-toughened (DPT) tungsten (W) composites. Mechanical testing and analytical modeling were used to guide composite development. This work provides a basis for further development of W composites to be used in structural divertor components of future fusion reactors.

W wire was tested in tension, showing significant ductility and strength. Coatings of copper (Cu) or tungsten carbide (WC) were applied to the W wire via electrodeposition and carburization, respectively. Composites were fabricated using spark plasma sintering (SPS) to consolidate W powders together with each type of coated W wire. DPT behavior, e.g. crack arrest and crack bridging, was not observed in three-point bend testing of the sintered composites.

A laminate was fabricated by hot pressing W and Cu foils together with W wires, and subsequently tested in tension. This laminate was bonded via hot pressing to thick W plate as a reinforcing layer, and the composite was tested in three-point bending. Crack arrest was observed along with some fiber pullout, but significant transverse cracking in the W plate confounded further fracture toughness analysis.

The fracture toughness of thin W plate was measured in three-point bending. W plates were brazed with Cu foils to form a laminate. Crack arrest and crack bridging were observed in three-point bend tests of the laminate, and fracture resistance curves were successfully calculated for this DPT composite.

An analytical model of crack bridging was developed using the basis described by Chao in previous work by the group. The model uses the specimen geometry, matrix properties, and the stress-displacement function of a ductile reinforcement (“bridging law”) to calculate the fracture resistance curve (R-curve) and load-displacement curve (P-D curve) for any test specimen geometry. The code was also implemented to estimate the bridging law of an arbitrary composite using R-curve data.

Finally, a parametric study was performed to quantitatively determine the necessary mechanical properties of useful toughening reinforcements for a DPT W composite. The analytical model has a broad applicability for any DPT material.

    Cover page: Ductile-Phase-Toughened Tungsten for Plasma-Facing Materials
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2016)
    The capability of nanostructured ferritic alloys (NFAs) to manage high levels of transmutation product helium will help resolve one of the grand challenges to transforming the promise of C-free fusion energy into a reality. NFAs are dispersion strengthened by an ultrahigh density of Y-Ti-O nano-oxides (NOs), which result in both high strength and temperature limits, as well as unique irradiation tolerance. Here, aberration-corrected high-resolution transmission electron microscopy was used to characterize the NOs in four NFA conditions, including following severe deformation and extreme neutron radiation exposure. Fast Fourier Transform analysis of focal series images revealed the NO crystal structure, including the smallest at < 2 nm in diameter, to be Y2Ti2O7 pyrochlore in all cases, consistent with both exit wave analysis and scanning transmission Z-contrast imaging of the atomic columns in a larger feature. The faceted NOs exhibit a quasi-epitaxial orientation relationship with the ferrite matrix: [110]YTO||[100]Fe and [001]YTO||[010]Fe, forming a 5 × 7 near coincidence site interface. The NOs also exhibit size-dependent strains in both the oxide and matrix ferrite phases.
      Cover page: The crystal structure, orientation relationships and interfaces of the nanoscale oxides in nanostructured ferritic alloys
      • Article
      • Peer Reviewed
      UC Santa Barbara Previously Published Works (2020)
      Nanostructured ferritic alloys (NFAs) contain an ultra-high density of 2–4 nm fcc pyrochlore Y2Ti2O7 nano-oxides (NOs) embedded in a bcc Fe-14Cr ferritic matrix. Characterization of helium interactions with NOs and associated Fe-Y2Ti2O7 interfaces is important to the development of structural materials for nuclear fusion and fission applications. A benchmark 14YWT NFA was first annealed to coarsen the NOs, then insoluble helium was implanted at 700 °C to produce a high number density of bubbles. High-resolution scanning transmission electron microscopy characterization shows two dominant Fe-Y2Ti2O7 crystallographic orientation relationships (cube-on-edge and cube-on-cube). The smallest NOs (≈ 2 nm) are associated with the smaller bubbles (≈ 1.5 nm), while some of the largest NOs (> 6 nm) have larger, and sometimes multiple, bubbles. NO corner {111} facets are the preferred sites for He bubble nucleation. A refined sequence of events for He trapping and bubble formation is presented. These observations offer new insight on He management in NFAs, and provide a foundation for detailed modeling studies.
        Cover page: Characterization of polyhedral nano-oxides and helium bubbles in an annealed nanostructured ferritic alloy
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