A Micro-Mechanistic Study of Oxygen-Diffusion-Assisted Crack Growth in a Polycrystalline Nickel-Based Superalloy

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Nickel base superalloys are particularly applied for critical rotating turbine discs in the hot section of aero engines due to their exceptional high temperature mechanical properties. RR1000 is a new generation of nickel base superalloy developed at Rolls-Royce through powder metallurgy processes to meet the demand of increasing turbine entry temperatures and rotational speeds for modern design of aero engines. Very recently, RR1000 has gone into service as a turbine disc material in the latest Rolls-Royce Trent engines . From an academic perspective, RR1000 is representative of the best powder metallurgy, fine grained nickel based superalloys for use as aero-gas turbine discs.The demonstration of aero-engine structural integrity and safety must include an assessment of components' fatigue lives (repeating flights) in terms of crack initiation and propagation. Good crack propagation resistance is typically required for disc materials to give an acceptable level of damage tolerance life assessment for the critical turbine disc components. In addition to mechanical and thermal loading, high temperature gas environment makes considerable contributions to crack growth rates at a given stress intensity due to the attack of oxidation. For RR1000, fatigue and creep behaviour has been considerably studied and data and models are already available for life assessment of RR1000 turbine discs. However, oxidation effects on crack growth behaviour for RR1000, which is a crucial factor for life assessment of RR1000 turbine discs, have not been well studied yet. The proposed work is to investigate oxidation-assisted crack growh in RR1000 under high temperature fatigue. The outcome will provide an insight into the oxygen-embrittlement phenomenon at crack tip and the associated crack growth in such polycrystalline, fine grained nickel based superalloys.The major work is to study, both experimentally and analytically, the process of oxygen diffusion at a crack tip and the associated crack growth for RR1000. Oxygen diffusion at a crack tip is a dynamic process, a combined effect of time, temperature, local deformation and material microstructure. Knowledge of this dynamic process is vital to assess crack propagation behaviour under the attack of oxidation. In the proposed work, finite element analyses, complimented by experimental work, will be carried out to study such a process at a microscale (grain level) using a coupled mechanical-diffusion model. Effects of loading conditions and grain microstructures on oxygen diffusion process will be fully investigated. Connection between oxygen diffusion and crack growth will also be studied, and a macroscopic crack propagation model will be developed and validated for fatigue life prediction. The proposed work is novel in that it considers the contributions of oxygen diffusion ahead of the crack tip and local material deformation at grain levels to high temperature crack growth behaviour.The overall aim is to establish a micro-mechanics based connection between oxygen diffusion, grain-level cracking and macroscopic crack growth rate for safe life prediction of RR1000 turbine discs. The whole project will be in close collaboration with Rolls-Royce (Dr M Hardy) and Cranfield University (Professor JR Nicholls). Rolls-Royce shows a great interest in the proposed work and is willing to provide test pieces for the experimental work and technical advice for the whole project. Cranfield University will offer facilities and technical advice for oxidation kinetics studies, which will be carried out by the research associate through a three-month secondment to Cranfield.