Modelling environmentally assisted cracking in Ni-based superalloys

Ni-based superalloys are extensively used in aero-engine applications, such as gas-turbine discs or blades, due to their high strength and creep resistance. However, oxygen diffusion into the material at elevated temperatures can lead to in a complex failure process known as environmentally-assisted cracking (EAC). With aircraft engine manufacturers driven to increase operating temperatures in excess of 700 oC due to fuel efficiency demands and the need to reduce emissions, EAC in these alloys is becoming increasingly prevalent and is now the major limitation to their use in aero-engine applications. EAC involves many competing mechanical and chemical processes, and a better understanding of these processes is essential to predicting the life of currently used alloys as well as the development of new EAC-resistant materials.

The aim of this PhD project will be to investigate how changes in composition and microstructure affect the susceptibility of Ni-based superalloys to EAC. EAC susceptibility is strongly influenced by the rate of uptake of elemental oxygen, the affinity of the material to grow oxides and relaxation mechanisms to accommodate stresses that build up in the material. The student will develop computational models for these highly interrelated processes, and will apply these models to investigate EAC behaviour in newly developed alloys and how they compare with more standard alloys and microstructures currently used.

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.

The EPSRC Centre for Doctoral Training in Advanced Metallic Systems was established to address the metallurgical skills
gap, highlighted in several reports [1-3] as a threat to the competitiveness of UK industry, by training non-materials
graduates from chemistry, physics and engineering in a multidisciplinary environment. Although we will have supplied ~140
highly capable metallurgical scientists and engineers into industry and academia by the end of our existing programme,
there remains a demonstrable need for doctoral-level training to continue and evolve to meet future industry needs. We
therefore propose to train a further 14 UK based PhD and EngD students per cohort as well as 5 Irish students per
cohort through I-Form.

Manufacturing contributes over 10% of UK GVA with the metals sector contributing 12% of this (£10.7BN [4,5]) and
employing ~230,000 people directly and 750,000 indirectly. It is estimated that ~2300 graduates are required annually to
meet present and future growth [5]. A sizeable portion of these graduates will require metallurgical expertise and current
numbers fall far short. From UK-wide HESA data, we estimate there are ~330 home UG/PGT qualifiers in materials and
~35 home doctoral graduates in metallurgy annually, including existing AMSCDT graduates, so it is unsurprising that
industry continues to report difficulties in recruiting staff with the required specialist metallurgical knowledge and
professional competencies.

As well as addressing this shortfall, the CDT will also impact directly on the companies with which it collaborates, on the
wider high value manufacturing sector and on the UK economy as a whole, as follows:

1. Collaborating companies, across a wide range of businesses in the supply chain including SMEs and research
organisations will benefit directly from the CDT through:

- Targeted projects in direct support of their business and its future development and competitiveness.
- Access to the expertise and facilities of the host institutions.
- Involvement in the training of the next generation of potential employees with advanced technical and leadership skills
who can add value to their organisations.

2. The UK High-Value Manufacturing Community will benefit as the CDT will:

- Develop the underpinning science and advanced-level knowledge base required by future high technology areas, where
there is high expectation of gross added value.
- Provide an enhanced route to exploitation, by covering the full spectrum of technology readiness levels.
- Ensure dissemination of knowledge to the sector, through student-led SME consultancy projects, the National Student
Conference in Metallic Materials and industry events.

3. The wider UK economy will benefit as the CDT will:

- Promote materials science and engineering and encourage future generations to enter the field, through outreach
activities developed by the students that will increase public awareness of the discipline and its contribution to modern
life, and highlight its importance to future innovation and technologies.
- Develop and exploit new technologies and products which will help to maintain a competitive UK advanced
manufacturing sector, ensure an internationally competitive and balanced UK economy for future generations and
contribute to technical challenges in key societal issues such as energy and sustainability.

References:
1. Materials UK Structural Materials Report 2009
2. EPSRC Materials International Review 2008
3. EPSRC Materially Better Call 2013
4. The state of engineering, Engineering UK 2017
5. Vision 2030: The UK Metals Industry's New Strategic Approach, Metals Forum