Understanding phase decomposition in SMART-W alloys for fusion first wall applications
The first wall of a fusion reactor experiences an incredibly harsh environment, with high neutron fluxes, high temperatures and plasma erosion. Tungsten is one of the few materials are able to withstand these conditions due to it’s high melting temperature, resistance to plasma erosion and minimal long lived radioactive waste. Therefore, tungsten-based materials are being considered for the first wall of many of the next-generation fusion power plants. However, tungsten oxidises readily at elevated temperatures, forming volatile hazardous oxides and mobile radioactive dust presenting a significant safety hazard if it were to escape the vessel. To solve this problem, novel SMART W-based materials are being developed that combine the advantages of tungsten with drastically lower oxidation rates. As such, these SMART alloys have drawn a lot of interest within the fusion community and Eurofusion support to develop manufacturing processes at industrial scale.
These novel SMART-W alloys include Cr and Y which enabling the formation of a passivating oxide layer and can also include other elements such as Zr. Importantly, there is a miscibility gap between W and Cr and modelling has indicated that the composition of SMART alloys lie within the spinodal curve, with the temperature at which this decomposition is expected to occur varying with Y and Zr content. During operation, the SMART alloys will experience temperatures that cycle through this decomposition temperature, and experience extended periods within the miscibility gap during maintenance. Localised phase decomposition has been observed in following thermal exposure but the nature of this decomposition and its impact on the performance of these materials in a fusion environment is unknown. Understanding the mechanism by which these alloys decompose and the effect on their oxidation behaviour and radiation damage will be crucial to implementing these safer W alloys.
This project will take state-of-the-art SMART alloys, exposing them to thermal treatments within and transversing the miscibility gap to give information about the microstructures which may form during realistic in-reactor opperation. The decomposition mechanism and resulting composition and crystallography of microstructures will be characterised both in-situ and after exposure using a combination of high-resolution SEM, TEM, DSC and XRD. The microstructural evolution will be linked to its impact on key properties needed for their application in the first wall, including oxidation performance, ductile to brittle transition temperature and thermal conductivity and explore how the diffusion-based phase transitions could impact radiation damage. This project will be in collaboration with UKAEA. It would suit a student with a strong background in materials science, with an interest high resolution microstructural characterisation and correlating this with materials properties.
