Refractory body-centred cubic high-entropy alloys for nuclear fusion

Thesis

In this work refractory High-Entropy Alloys (HEAs) have been investigated for application as Plasma Facing Components in nuclear fusion reactors. Sample fabrication was guided by optimising composition in order to maximise the Ω parameter, which has been previously identified as correlating with the stability of solidsolution microstructures. Arc melting was used to fabricate billets of TiVZrHfTa, TiVZrTa, and TiVTa, each of equiatomic and optimised compositions. These samples were analysed by XRD and SEM in the as-cast condition and after being annealed at 1400 ◦C for twenty four hours. The matrix of each sample was found to be BCC, and each contained secondary phases with either BCC, HCP, or rocksalt crystal structures. The latter of these phases was found to be a carbonitride, formed from an air leak that occurred during melting. Finite-Element Analysis of thermal transport in arc-melted billets has shown that cooling rates generated are rapid, and even more so in smaller billets. Smaller 10 g billets of equiatomic alloys were found to have much lower volume fractions of secondary phases than as-cast 50 g counterparts, and this has been attributed to the differences in solidification and cooling conditions. The modelling also demonstrated large variations in cooling rate throughout larger 50 g billets, and so the effect of this on as-cast microstructures was also investigated. It was found that the differences in cooling rate was sufficient to create variations in microstructures, with an increase of 10% volume fraction of the carbonitride phase between the bottom and top of the billet, along with a large increase in porosity. The main conclusion of this portion of the thesis is that arc melting plays a larger role in determining as-cast microstructures than previously thought. Two ion-implantation experiments were run into four different alloys of TiVZrHfTa up to damages of 0.6 and 3.5dpa. Nanoindentation has then been used to show there is no measurable change in hardness, despite a strong reaction being found in a vanadium control sample. Separately, these alloys were used to demonstrate the efficacy and limitation of using a new nanoindentation technique called Express indentation, which rapidly makes thousands of indents in order to map the hardness and modulus of materials.

Authors: Waite, JC

• DOI: 10.5287/ora-5jv0bo6op
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Nuclear Fusion; High-entropy alloys
• Subjects: Materials