Description
The aim of this work was to investigate the precipitation and stability of nm-sized Ti oxides in vanadium-based alloys, a prime candidate material for future nuclear fusion reactors based on the magnetic confinement of the plasma. Fusion energy reproduces the nuclear reactions occurring in stars. It can potentially produce more energy than current nuclear fission power plants, and it is meant to be a solution to the clash of today's increasing energy demand with the continuous decrease of fossil-based energy sources, whose use is harmful for the environment. The operating conditions in a fusion reactor will be unprecedented in terms of ultra-high temperatures, stresses, radiation fields and very corrosive media. Only a limited number of materials may be able to withstand such combination of harsh environmental conditions, and vanadium-based alloys are among them. Recent research efforts have identified V-4Cr-4Ti as the most promising vanadium based alloy for application in the first wall of future fusion nuclear reactors such as DEMO and beyond. The presence of TiO-type precipitates, containing relatively small amounts of C and N, strongly influences the final mechanical properties and radiation resistance of the alloy. Therefore, a thorough understanding of the precipitate structure and evolution at both relatively high temperatures and radiation dose levels is primordial to predict and optimise the final performance of the structural component in the fusion reactor. This thesis is written in alternative format and collects one article already published in Scripta Materialia, and two additional articles to be submitted to peer-review scientific journals. Atomic resolution imaging of the precipitates, coupled with chemical analysis, constitutes the main body of the first article: a novel intergrowth of the fcc Ti oxide in the bcc V matrix is revealed at the precipitate/matrix interface. The evolution of the vacancies present in the TiO precipitates above 400 C, together with the recovery of dislocations in the matrix and the formation of extra precipitates, is studied in the second article by positron annihilation spectroscopy and micro-hardness measurements. The formation of additional precipitates below 400 C induced by radiation is assessed in the third article using proton irradiation as a surrogate of neutron damage. The structure of those additional precipitates and of the dislocation loops induced by the proton bombardment is characterized by advanced analytical electron microscopy.| Period | 31 Dec 2016 |
|---|---|
| Examinee | Andrea Impagnatiello |
| Examination held at |
|
| Degree of Recognition | International |