Probing The Ductile-To-Brittle Transition in BCC Blanket Alloys

Conditions inside fusion reactors are exceptionally hostile, involving both high temperatures and severe levels of neutron irradiation. Directly adjacent to the fusion plasma in a tokomak reactor is the blanket, a construction of critical importance. The blanket not only protects the tokomak's superconducting magnets and supporting structures from high temperatures and energetic neutrons, but also converts the kinetic energy of the neutrons bombarding it to heat for power generation.

All the candidate materials for the blanket are metallic alloys that possess the body-centred cubic (BCC) crystal structure. Alloys with this structure undergo a change from ductile mechanical behaviour (high toughness) at high temperatures to brittle mechanical behaviour (low toughness) at low temperatures, see left-hand image. The temperature of this ductile-to-brittle transition (DBT) increases with irradiation, in effect decreasing the fracture toughness of the material and limiting the useful life of components. This increase in DBT temperature is an issue shared by nuclear fission reactors, where it is also life limiting and is the key material parameter in reactor life extension.

Typically, the DBT and the transition temperature are measured using macroscopic fracture tests, such as the Charpy impact test, the results of which are then used to predict the life of plant components. However, such methods require large volumes of material, and it is usually very difficult to obtain such quantities of material that have undergone irradiation damage. This is an even bigger issue in fusion reactors, where representative irradiated material simply does not exist, and where new alloys will be used, for which no historical test data exists.

This project aims to investigate the fundamentals of the ductile-to-brittle transition in BCC alloys using high-resolution digital image correlation (HRDIC). Pioneered at the University of Manchester , this technique not only requires very little material, but is able to provide detailed quantitative information about the pattern of deformation inside a material which is, in principle, related to the DBT. The project will test the thesis that HRDIC can be used to measure the DBT in alloys, since the transition involves a change in the deformation behaviour. Objectives include:

- Developing experimental aspects of the HRDIC technique for application to BCC alloys (the technique has only been used extensively on alloys with different crystal structures so far).
- Conducting room-temperature HRDIC investigations on simple model systems (e.g., pure Fe) before extending to candidate fusion alloys (e.g., EUROFER 97).
- Extending the HRDIC studies to include investigations of deformation (i) at cryogenic temperatures and (ii) following irradiation.

The project will use advanced characterisation facilities at the Univeristy of Manchester and the Henry Royce Institute. Any irradiations will be carried out at the Dalton Cumbria Facility.

If successful, the project will establish a new method for assessing the DBT, which could not only revolutionise our ability to assess candidate alloys for use in fusion reactors, but may also enhance current methodologies of determining the structural integrity of ageing fission reactors.