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

镍基高温合金具有优异的高温力学性能，特别适用于航空发动机热段中的关键旋转涡轮机盘。RR 1000是罗尔斯·罗伊斯公司为满足现代航空发动机设计对涡轮机入口温度和转速的要求而通过粉末冶金工艺开发的新一代镍基高温合金。最近，RR 1000已作为涡轮机盘材料在最新的Rolls-Royce Trent发动机中投入使用。从学术的角度来看，RR 1000是最好的粉末冶金细晶粒镍基高温合金的代表，用于航空燃气涡轮机盘。航空发动机结构完整性和安全性的证明必须包括从裂纹萌生和扩展方面评估部件的疲劳寿命（重复飞行）。通常要求盘材料具有良好的抗裂纹扩展性，以使关键涡轮机盘部件的损伤容限寿命评估达到可接受的水平。除了机械载荷和热载荷外，高温气体环境由于氧化的侵蚀，在给定的应力强度下对裂纹扩展速率有相当大的贡献。对于RR 1000，已对疲劳和蠕变行为进行了大量研究，并且已获得用于RR 1000涡轮机盘寿命评估的数据和模型。然而，作为RR 1000涡轮机轮盘寿命评估的关键因素，氧化对RR 1000裂纹扩展行为的影响尚未得到很好的研究。本文研究了高温疲劳下RR 1000合金的氧化辅助裂纹扩展。本文的主要工作是从实验和分析两个方面研究RR 1000镍基多晶细晶高温合金裂纹尖端的氧扩散过程及其裂纹扩展规律。氧在裂纹尖端的扩散是一个动态过程，是时间、温度、局部变形和材料微观结构的综合作用。这个动态过程的知识是至关重要的，以评估裂纹扩展行为的攻击下的氧化。在拟议的工作中，有限元分析，补充实验工作，将进行研究这样的过程中，在微观尺度（晶粒级）使用耦合的机械扩散模型。研究了加载条件和晶粒组织对氧扩散过程的影响。氧扩散和裂纹扩展之间的联系也将进行研究，宏观裂纹扩展模型将开发和验证疲劳寿命预测。本文的工作是新颖的，因为它考虑了裂纹尖端前的氧扩散和晶粒级局部材料变形对高温裂纹扩展行为的贡献，总体目标是建立一个基于微观力学的RR 1000涡轮机盘的安全寿命预测的氧扩散，晶粒级裂纹和宏观裂纹扩展速率之间的联系。整个项目将与劳斯莱斯（M哈代博士）和克兰菲尔德大学（JR尼科尔斯教授）密切合作。罗尔斯·罗伊斯公司对拟议的工作表现出极大的兴趣，并愿意为整个项目的实验工作提供试验件和技术咨询。克兰菲尔德大学将为氧化动力学研究提供设施和技术咨询，研究助理将借调到克兰菲尔德三个月。

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.