Refractory Metal - Steel Diffusion Bonds for Nuclear Fusion

The materials issues for nuclear fusion are a major barrier to building the first generation of demonstrator fusion power plants. In-vessel materials will be subject to harsh conditions, including high neutron fluxes, high temperatures, potential exposure to corrosive liquids, and possible plasma erosion. Refractory alloys are of immense importance to nuclear fusion due to their high-temperature and environmental compatibility properties. Refractory alloys – tungsten and vanadium-alloy – are candidate materials for use in STEP for essential in-vessel components including the divertor, first-wall, and blanket structure. Despite the consideration of refractory alloys for these key components of the STEP tokamak, these will unlikely comprise the only material in such components. Steels such as RAFM or austenitic stainless-steel variants are still likely to be required as structural material within the tokamak, and yet strategies to join steels to refractory alloys remain low TRL.

Diffusion bonding is an attractive method of making high quality joints between materials (similar or dissimilar) based on the solid-state diffusion of atomic species between the two parts being joined. As no filler material is required the quality of the joint can be very high and little residual stress is generated compared to traditional welding methods. However in the case of dissimilar metal joints interdiffusion will result in a compositional gradient. Depending on the phase diagrams and reaction kinetics this can result in the generation of a range of second phases, often intermetallic compounds which are usually brittle and deleterious to bulk mechanical behaviour.

This project will use a combination of high-resolution SEM based EDX and EBSD and TEM based diffraction and EDX microscopy to study the production of such phases in a range of diffusion couples between vanadium alloys and steels. These systems have been chosen as they represent likely joints in breeder blanket systems. The mechanical properties of the phases produced will be studied using ultra high resolution nanoindentation using newly installed equipment, funded through the Royce Institute. This will be conducted both at room and operational (650oC) temperatures. Data sets can consist of 10,000-100,000 indents allowing mechanical behaviour to be correlated to microstructure. Method development will focus on correlating local micromechanical measurements with high resolution microscopy data to develop a full understanding of the performance of the joints. Through this improvements to bonding methods (including the use of interlayers) will be developed. The project will be in collaboration with UKAEA. It would suit a student with strong background in materials science or engineering with an interest in developing high resolution micromechanical testing mthods and correlating these with high resolution microscopy data.