Copper (Cu) has high thermal conductivity and is thus ideal for high heat flux, thermal heat sink applications in fusion power applications. Divertor designs for future fusion power plants will expose Cu alloys to extreme thermal (>10 MW m<sup>-2</sup>) and neutron fluxes (200 dpa) that destabilise the microstructure of Cu. To improve stability, strength and creep resistance, alloying additions are used commercially, but these compromise thermal conductivity. Dispersed oxide particles may offer the opportunity for improved mechanical stability and creep resistance even at very low volume fractions (<1%) while avoiding large reductions in thermal conductivity. However there are few studies on the processing-performance of oxide dispersion strengthened Cu alloys. In this thesis, novel oxide dispersion strengthened Cu alloys were prepared by room temperature mechanical alloying of Y<sub>2</sub>O<sub>3</sub> and the mechanism of dispersion investigated. A small fraction of Y<sub>2</sub>O<sub>3</sub>, up to 1%, was shown to disperse effectively during mechanical alloying at room temperature in Cu. Both severe fragmentation and some local disassociation of the Y and O occurred, allowing for re-precipitation of fine nanoparticles <10 nm during consolidation and exposure to elevated temperatures. A model alloy of Cu-2 wt.% Y<sub>2</sub>O<sub>3</sub> alloy had a mean oxide particle diameter of 7.1 ± 6.0 nm and a mean particle number density of 8.24 x 10<sup>22</sup> m<sup>-3</sup> following consolidation. Ternary Ti additions were investigated for nanoparticle refinement and best design alloy with a composition of Cu-0.25Y<sub>2</sub>O<sub>3</sub>-0.25Ti was produced that had a mean nanoparticle diameter of 3.2 ± 1.5 nm and a mean particle number density of 2.36 x 10<sup>23</sup> m<sup>-3</sup> , which after thermal ageing for 545 h at 350 °C, was largely unchanged at 3.8 ± 1.7 nm, and 1.74 x 10<sup>23</sup> m<sup>-3</sup> respectively. Comparing favourably with commercial Al<sub>2</sub>O<sub>3</sub> dispersion strengthened Cu, the alloy had a narrower particle size distribution and a higher particle number density. The fine dispersed oxide nanoparticles gave good grain boundary pinning, retaining an ultrafine mean grain size of 220 nm after thermal ageing. Thermal conductivity of the Ti-containing alloy was 332 ± 16W m<sub>-1</sub> K<sub>-1</sub> , and the addition of Ti increased the thermal conductivity with increasing temperature. The creep resistance was evaluated by small punch testing and in-house produced alloys outperformed commercial alloys at 350 °C. The work in this thesis indicates that mechanically alloyed Cu-Y<sub>2</sub>O<sub>3</sub> or Cu-Y<sub>2</sub>O<sub>3</sub>-Ti alloys, with further development and evaluation, have potential as thermal heat-sink materials for fusion divertor application.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:748807 |
| Date | January 2017 |
| Creators | Morrison, Alasdair |
| Contributors | Lozano-Perez, Sergio ; Grant, Patrick S. |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://ora.ox.ac.uk/objects/uuid:7a9f6c82-4fd1-4836-995d-eadf5dd74306 |
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