PhD Studentship: Structural Integrity of Refractory High Entropy Alloys in Plasma-facing Fusion Tokamak Environments

Tungsten is currently the material chosen by the fusion community to manufacture plasma-facing components of the first wall and divertor in nuclear fusion tokamaks. Tungsten offers engineers a very high melting point, together with good mechanical strength and thermal conductivity at elevated temperatures, and a relatively low sputtering yield and fuel retention. However, tungsten components are predicted to experience high heat and particle fluxes in service, causing local temperatures potentially exceeding 1000 C and damage levels in the structure that can reach several dpa’s. In particular, the plasma-facing surface will experience helium fluxes with low-to-medium helium energies, as a by-product of the D-T reaction in the fuel plasma. As a consequence, the tungsten’s surface and its integrity will degrade over time, due to the generation of nano-bubbles, cracks and fuzzy structures, and causing also plasma contamination. Additionally, the interaction of plasma neutrons with tungsten will generate transmutant elements such as heavy elements (e.g. Os, Re), hydrogen or helium inside the tungsten’s structure, inducing further embrittlement in tungsten. All these degradation effects can compromise the integrity and expected lifetime of tungsten-base fusion components.

In recent years, Refractory High Entropy Alloys (RHEAs) containing tungsten and other high-melting point elements in (close to) equiatomic concentrations, have been designed and developed as promising alternatives as plasma facing materials in fusion technologies. RHEAs have been recently reported to outperform both coarse- and nano-grained tungsten in microstructural stability and resistance against radiation. The outstanding performance of RHEAs is mainly ascribed to their chemical complexity and nano-crystallinity. However, the mechanistic understanding of the surface and bulk microstructural evolution in RHEAs when exposed to fusion-relevant helium and neutron fluxes, and how the material evolution affects its structural integrity, is still largely missing and ultimately limits alloy design and the adoption of RHEAs in nuclear fusion technologies.

The aim of the project is to assess the influence of the alloy chemistry and microstructure on the high-temperature material’s response to fusion-relevant helium and neutron fluxes, and how the evolving surface and structure of RHEAs affects, or otherwise, the structural integrity of the engineered material. For this purpose you will expose at elevated temperatures, selected RHEAs to low-energy helium plasmas to induce (near-) surface modifications in the materials, and also to medium-to-high energy helium ion beams from accelerator-based sources, to implant helium atoms at different depths in the material and simultaneously trigger displacement cascades in the bulk structure. You will use advanced electron microscopy to characterise the helium-induced structures in RHEAs, and also test the integrity of those materials post-helium exposures, so as to be able to correlate surface and structural changes to potential degradations in the integrity of the material.

During this project, you will acquire a unique set of transferrable skills ranging from designing complex sample environments and experimental protocols, to programming and data mining, and effective communication skills. You will gain in-depth knowledge about physical metallurgy, mechanics of polycrystalline materials, and advanced material characterization tools.

Funding notes:

A 3.5-year PhD studentship is available in the fusion materials group of Prof. Jimenez-Melero within the UoB School of Metallurgy and Materials, with a stipend of at least £18,622 per year. This project is in collaboration with the UK Atomic Energy Authority, and it is also embedded within a multi-partner consortium aiming to develop and test advanced high entropy alloys for fusion applications.

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