Precipitate characterization and stability in V-based alloys for nuclear fusion reactors

Abstract

Andrea Impagnatiello submitted this thesis entitled "Precipitate characterization andstability in V-based alloys for nuclear fusion reactors" for the degree of Doctor of Philosophyat the University of Manchester, 2016. The aim of this work was to investigate theprecipitation and stability of nm-sized Ti oxides in vanadium-based alloys, a prime candidatematerial 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 producemore energy than current nuclear fission power plants, and it is meant to be a solution to theclash of today's increasing energy demand with the continuous decrease of fossil-basedenergy sources, whose use is harmful for the environment. The operating conditions in afusion reactor will be unprecedented in terms of ultra-high temperatures, stresses, radiationfields and very corrosive media. Only a limited number of materials may be able to withstandsuch combination of harsh environmental conditions, and vanadium-based alloys are amongthem. Recent research efforts have identified V-4Cr-4Ti as the most promising vanadiumbasedalloy for application in the first wall of future fusion nuclear reactors such as DEMOand beyond. The presence of TiO-type precipitates, containing relatively small amounts ofC and N, strongly influences the final mechanical properties and radiation resistance of thealloy. Therefore, a thorough understanding of the precipitate structure and evolution at bothrelatively high temperatures and radiation dose levels is primordial to predict and optimisethe final performance of the structural component in the fusion reactor.This thesis is written in alternative format and collects one article already published in ScriptaMaterialia, and two additional articles to be submitted to peer-review scientific journals.Atomic resolution imaging of the precipitates, coupled with chemical analysis, constitutes themain body of the first article: a novel intergrowth of the fcc Ti oxide in the bcc V matrix isrevealed at the precipitate/matrix interface. The evolution of the vacancies present in the TiOprecipitates above 400C, together with the recovery of dislocations in the matrix and theformation of extra precipitates, is studied in the second article by positron annihilationspectroscopy and micro-hardness measurements. The formation of additional precipitatesbelow 400C induced by radiation is assessed in the third article using proton irradiation as asurrogate of neutron damage. The structure of those additional precipitates and of thedislocation loops induced by the proton bombardment is characterized by advanced analyticalelectron microscopy.

Date of Award	31 Dec 2016
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Enrique Jimenez-Melero (Supervisor) & Paul Mummery (Supervisor)