Modelling environmentally assisted cracking in Ni-based superalloys
Ni-based superalloys in aircraft engines are routinely subjected to temperatures in the range of 650-700oC during operating conditions. However, their application at these and higher temperatures is limited by a complex failure process called environmentally-assisted cracking (EAC). EAC involves many competing micromechanical and metallurgical mechanisms, and understanding it better is essential to predicting the life of the alloys currently used and the development of new compositions. Effective management is also of significant interest to the aero-engine manufacturing industry as it would increase engine lifetimes, higher operating temperatures and increased fuel efficiency.
To understand EAC we need to identify the kinetic bottleneck in the evolution of damage, so that we can exploit it. In microstructurally complex Ni-based super alloys, this kinetic process is influenced by rate-dependent mechanisms such as stress relaxation during creep deformation, crack tip oxidation and oxygen ingress along grain boundaries (GBs). Certain alloying elements have been found to alleviate this effect, presumably because the chemical reactions which are promoted are then altered in subtle ways.
This project aims to use computational modelling to understand the EAC growth behaviour by developing kinetic models for the underpinning mechanisms of stress-assisted GB oxygen diffusion and oxidation ahead of a crack tip. This will involve investigating the following strongly coupled processes:
1. GB ingress of elemental oxygen and its microstructural trapping at lattice defects, carbides and interfaces
2. Oxidation kinetics ahead of a crack tip, which is mediated by the competitive microstructural partitioning, diffusion and oxidation affinity of the alloying components
3. Micromechanical driving forces for crack growth, determined by crystal plasticity and local stress relaxation mechanisms
The detailed and systematic understanding of the GB oxidation process will then be used by Rolls-Royce, in collaboration with universities like Cambridge University, Imperial College London and Birmingham University to develop improved strategies for EAC resistance through controlled alloying and heat treatment.
Candidates for this position should have a degree in Materials Science, Physics or related discipline with strong computational skills. Experience is materials modelling is helpful but not essential, however candidates are should be enthusiastic about using mathematics and computers to model materials behaviour. This PhD position will be sponsored by Rolls-Royce. There will be opportunities to spend time at Rolls-Royce and contribute to neutron and synchrotron diffraction experiments led by other university partners. As well as developing excellent computational modelling skills, the PhD student will acquire expertise in advanced Ni-based superalloys and how they are used in aero-engines.
For successful applicants, tuition fees will be covered and a stipend of £15,000 per year will be awarded from Rolls Royce. The PhD duration is 3.5 to 4 years and the proposed start date is October 2019.
Applicants should have or expect to achieve at least a 2.1 honours degree in Mathematics or a physical science. The applicant should have demonstrated an aptitude for the more mathematical and theoretical aspects of their degree.