Probing the Ductile-to-Brittle Transition in BCC Blanket Alloys (Materials Strand project)

Supervisor/s: Dr Ed Pickering (University of Manchester), Professor João Fonseca (University of Manchester) and Huw Dawson (CCFE)

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. It 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 will probe the fundamentals of the ductile-to-brittle transition in BCC alloys using high-resolution digital image correlation (HRDIC), see right-hand image.  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.   Tests will be conducted on simple model systems before extending to candidate fusion alloys.  Later on, some of these materials will be irradiated before being tested, to provide insight into the effects of irradiation on fundamental material deformation behaviour and the DBT. If successful, the project will have established a new method for assessing the DBT, which would not only revolutionise our ability to assess candidate alloys for use in fusion reactors, but would also enhance current methodologies of determining the structural integrity of ageing fission reactors.

The successful candidate will have a good undergraduate degree in a relevant subject, e.g. materials science, physics or engineering. Previous specialisation in metallurgy or fusion energy is not required as students will gain a broad understanding through the taught component of the CDT programme and through their preliminary research. This project will provide the opportunity to develop transferable skills and knowledge of industrial processes, metallurgy, irradiation damage, electron microscopy, and other analysis techniques, which should ensure the candidate is prepared for a wide range of possible career paths after graduation.

The project will be based mainly in Manchester, but may involve short trips to international research institutions carrying out related work (such visits will be optional).

Generic transferable skills associated with programming, data manipulation and data interpretation will be gained.

This project is offered by University of Manchester. For further information please contact: Dr Ed Pickering (ed.pickering@manchester.ac.uk)