Oxidation Damage at a Crack Tip and Its Significance in Crack Growth under Fatigue-Oxidation Conditions

Nickel-based alloys are widely used in power generation, nuclear and aerospace industries due to their superior mechanical properties at high temperature. As structural materials, a strong resistance to crack initiation and propagation is particularly required for safe-life design and assessment of their components. At elevated temperature, crack growth rates in such alloys exposed to air can be drastically accelerated, by two and even three orders of magnitude, due to the attack of oxidation. Over time, significant effort has been made to investigate the crack tip oxidation mechanism in order to provide a basis for the development of quantitative models that predict crack growth under operational temperatures and loading conditions. However, this problem has been neither fundamentally nor fully understood, and current lifing practice in industries is still predominantly empirical and relies on expensive and extensive experimental data on crack growth.

This research aims to investigate the physical process of oxidation damage at a crack tip and the associated crack growth behaviour for nickel alloys, which will provide a direct insight, for the first time, into the oxidation-embrittlement phenomenon at crack tip. Oxidation damage at a crack tip is a combined effect of time, temperature, local deformation and material microstructure. Knowledge of this process is vital to assess crack propagation behaviour under the attack of oxidation. In the proposed work, single crystal, directionally solidified and polycrystal nickel alloys will be used for crack growth testing under fatigue-oxidation conditions in controlled environments (vacuum, air, oxygen-18). Advanced microscopy analyses will be carried out to characterise and measure the oxygen penetration and microstructural damage at a crack tip, and the results will be used to calibrate important diffusion and damage parameters during oxidation. Numerical analyses will be carried out to model such processes at a microscopic scale using a coupled mechanical-diffusion model. Effects of loading condition and grain boundary character on oxygen diffusion will be fully investigated, especially the connection between oxidation damage and crack growth. A crack propagation model will be ultimately developed and validated for accurate fatigue-oxidation life prediction.

The work draws together three established groups to tackle these fundamental problems in a collaborative, systematic and multi-scale manner. Interaction between oxidation damage and crack tip deformation requires carefully designed specialist testing on fatigue crack growth in a controlled environment, which is the expertise of UoS. The problem also requires advanced microscopy characterisation and physical measurements of the phenomena using the established techniques at IC. The new models will be developed, with validation against these experimental results, by UoP who has a strong background in material and crack growth modelling. Owing to our complementary skills, this joint project should establish a physically based connection between oxidation damage and crack growth for fatigue design and safe life prediction of nickel alloy components.

The research will generate unique and practically-useful data and models which can be quickly exploited through our committed industrial collaborators including E.On, Alstom, NASA and Dstl. The results will also be of generic use to other industries striving to achieve maximum service life and temperature capabilities of critical high-temperature components. Researchers and academics working on high-temperature materials and related areas will also directly benefit from our targeted dissemination activities including workshops, conferences and journal papers. A wider audience will be reached via specially designed public engagement programmes and continuously updated web sites.

The research has a direct impact on power generation and gas turbine propulsion systems. The fast growing energy demand and concerns about climate changes drive industries to implement higher operating temperature and longer maintenance intervals to achieve high-efficiency power and energy systems. Fatigue-oxidation behaviour of high temperature materials is a key issue in the performance of high temperature plant. This project aims to address this critical problem for nickel alloys, an important class of high temperature materials, through development of accurate models for the design, performance prediction and lifing calculations for successful commercial applications. Exploitation of the research outcomes will be carried out with our project partners in the energy and aerospace industries (E.ON, ALSTOM, NASA, DSTL) via the bi-annual review meetings and contacts over the project period. The research outcomes will provide scientific guidance and support for industries to gain a maximum service life through the optimisation of service conditions and material microstructures. The model developments will enable industries to conduct "numerical experiments", in place of expensive, risky and time-consuming experimental tests wherever possible, and save significant costs in product development. This will contribute to ensuring the structural integrity and safety in terms of fatigue design and life management of critical nickel alloy components under service conditions. Our research team will also take a lead in going out and promoting the impact of the work with the assistance of our existing collaborative network and the experienced Knowledge Transfer Network teams at three institutions. Support from regional organisations, such as Solent Local Enterprise Partnership (www.solentlep.org.uk), will also be sought to additionally exploit the research outcomes for local business growth and prosperity.

Key scientific findings generated from the research will have a direct impact on research communities working on high temperature materials, particularly the physical measurements of oxidation damage near a crack tip and the predictive modelling tools for crack growth under fatigue-oxidation conditions. The research also delivers an underlying generic contribution which should also benefit research communities in physics, mathematics, biomechanical and petrochemical engineering. A number of methods will be used to maximise the impact across communities, including publication in journals for different audiences and presentation at a variety of conferences, seminars and workshops. Additionally all research data produced from the project will be archived on the Materials Data Centre hosted at UoS, which will allow other researchers to access the relevant data and models linked to our published findings.

The proposed research will enable the three PDRAs to establish themselves with further significant skills and experience in the areas of fatigue, fracture, characterisation and modelling. The PDRAs, as well as the pledged PhD student at UoS, will supply a critical engineering skills shortage in the UK. This joint research grant will also be an important step for the investigators to develop and expand their leadership roles in respective fields.

Wider public audiences will be easily engaged due to the direct relevance of this research to power generation and air travel. We will provide a number of infrastructure opportunities for researchers to develop research-led engagement activities. Also key summaries about significant findings and developments will be published on our continuously updated websites. In addition, all three institutions have school visits where the issues regarding oxidation and mechanical behaviour will be directly linked to the content of current chemistry and physics GCSE and A-level syllabi.