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

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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.