A Micro-Mechanistic Study of Oxygen-Diffusion-Assisted Crack Growth in a Polycrystalline Nickel-Based Superalloy
Lead Research Organisation:
University of Portsmouth
Department Name: Faculty of Technology
Abstract
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.
People |
ORCID iD |
Liguo Zhao (Principal Investigator) |
Publications
Karabela A
(2013)
Oxygen diffusion and crack growth for a nickel-based superalloy under fatigue-oxidation conditions
in Materials Science and Engineering: A
Karabela A
(2011)
Effects of cyclic stress and temperature on oxidation damage of a nickel-based superalloy
in Materials Science and Engineering: A
Karabela A
(2010)
Fatigue-oxidation interaction for a nickel-based superalloy
Lin B
(2011)
A crystal plasticity study of cyclic constitutive behaviour, crack-tip deformation and crack-growth path for a polycrystalline nickel-based superalloy
in Engineering Fracture Mechanics
Lin B
(2010)
Crystal plasticity modeling of cyclic deformation for a polycrystalline nickel-based superalloy at high temperature
in Materials Science and Engineering: A
Tong J
(2013)
Ratchetting strain as a driving force for fatigue crack growth
in International Journal of Fatigue
Zhao L
(2010)
Prediction of crack growth in a nickel-based superalloy under fatigue-oxidation conditions
in Engineering Fracture Mechanics
Zhao L
(2011)
Modeling of Oxygen Diffusion Along Grain Boundaries in a Nickel-Based Superalloy
in Journal of Engineering Materials and Technology
Zhao L.
(2015)
Cyclic deformation, crack growth and oxidation damage for nickel-based superalloys
in METAL 2015 - 24th International Conference on Metallurgy and Materials, Conference Proceedings
Description | The aim of this programme was to investigate oxidation assisted fatigue crack growth for nickel-based superalloy RR1000 utilised as disc rotors in the latest Rolls-Royce aero engines. Significant achievements have been made over the grant period, by a dedicated research team consisting of the PI, a postdoctoral researcher (Dr B Lin) and a PhD student (Mrs A Karabela), as evidenced by the significant published work. (1) Focused Ion Beam (FIB) examination of both smooth and crack specimens, fatigue tested at high temperature, revealed surface oxide scales, together with micro-voids and modified microstructure beneath the oxide scales (Karabela et al., 2011). The measured oxidation damage has been utilised to estimate the oxygen diffusivity and pressure factor, which are essential for modelling of oxidation damage and prediction of crack growth under fatigue-oxidation conditions. (2) Three dimensional crystal plasticity model has been established, for the first time, to simulate the cyclic deformation for RR1000 using the finite element method and the Representative Volume Element approach (Lin et al., 2010). Simulated results are in excellent agreement with the experimental results for stress-strain loops, cyclic hardening and stress relaxation behaviour. The model was further applied to study the effects of grain microstructure on crack-tip deformation in a compact tension specimen, from which a crystallographic slip-based criterion was proposed to predict the crack growth path (Lin et al., 2011). (3) Modelling of oxygen diffusion along grain boundaries, coupled with crystal plasticity, has been developed for the first time to quantify the fatigue-assisted oxidation damage for RR1000 (Zhao, 2011). Heterogeneous deformation presented at grain level imposes a great influence on oxygen diffusion at 750 Celsius degree and above, promoting further penetration of oxygen into the bulk material. In the case of an existing surface crack, oxygen tends to accumulate around the crack tip due to the high stress level presented near the crack tip, leading to localized material embrittlement and promotion of rapid crack propagation. (4) An engineering approach for prediction of oxidation-assisted crack growth has been developed for RR1000 based on finite element analysis of oxygen diffusion, coupled with viscoplastic deformation, near a fatigue crack tip (Zhao et al., 2010). A failure envelope, expressed in terms of oxygen concentration and accumulated inelastic strain, has been constructed and utilised to predict crack growth rates in a CT specimen under fatigue-oxidation conditions. The predictions were evaluated against the experimental results for triangular and dwell loading waveforms, with marked improvements achieved over those predicted from the mechanical deformation alone, i.e., neglecting the influence of oxidation. |
Exploitation Route | These achievements have significance in understanding and modelling of oxidation-assisted fatigue crack growth for RR1000, which are disseminated by six high-quality journal papers and numerous presentations at national and international conferences and seminars. The major application of this work has been in providing essential parameters and developing modelling methods for safe life prediction of RR1000 turbine discs in the presence of oxidation attack, which has great significance in air travel safety. |
Sectors | Aerospace, Defence and Marine,Energy,Transport |
Description | Exploitation of the results will be in gas turbine industry and will be facilitated through our continuing collaboration with Rolls-Royce. The project outcomes will provide scientific support for ensuring the structural integrity of gas turbine engines. The vital information on oxygen-assisted crack growth behaviour will be exploited for fatigue design and life management of future turbine engines under typical service conditions involving fatigue-creep-oxidation interaction. |
Sector | Aerospace, Defence and Marine |
Impact Types | Economic |
Description | International Travel Grant |
Amount | £400 (GBP) |
Organisation | Royal Academy of Engineering |
Sector | Charity/Non Profit |
Country | United Kingdom |
Start | 06/2009 |
End | 06/2009 |
Description | International Travel Grant |
Amount | £600 (GBP) |
Organisation | Royal Academy of Engineering |
Sector | Charity/Non Profit |
Country | United Kingdom |
Start | 06/2008 |
End | 06/2008 |
Description | Royal Society Conference Grant |
Amount | £1,300 (GBP) |
Organisation | The Royal Society |
Sector | Charity/Non Profit |
Country | United Kingdom |
Start | 09/2009 |
End | 09/2009 |
Description | Royal Society-Leverhulme Trust Senior Research Fellowship |
Amount | £40,000 (GBP) |
Organisation | The Royal Society |
Department | Royal Society Leverhulme Trust Senior Research Fellowship |
Sector | Charity/Non Profit |
Country | United Kingdom |
Start | 10/2008 |
End | 09/2009 |
Description | Rolls-Royce plc |
Organisation | Rolls Royce Group Plc |
Country | United Kingdom |
Sector | Private |
PI Contribution | Outcome data submitted to EPSRC on a Final Report |
Collaborator Contribution | Supply of materials, specimens and technical advice; Attendance of project review meetings. |
Impact | Joint journal publications and conference presentations; Share of test data, results and models. |
Start Year | 2007 |