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

镍基合金因其上级的高温力学性能而被广泛应用于发电、核能和航空航天等领域。作为结构材料，对裂纹萌生和扩展的强抵抗性对于其部件的安全寿命设计和评估是特别需要的。在升高的温度下，由于氧化的攻击，暴露于空气中的这种合金中的裂纹生长速率可以急剧加速两个甚至三个数量级。随着时间的推移，已经作出了重大努力，调查裂纹尖端氧化机制，以提供一个基础的定量模型，预测裂纹扩展的操作温度和负载条件下的发展。然而，这个问题既没有从根本上也没有得到充分的理解，目前工业上的寿命实践仍然主要是经验性的，并且依赖于昂贵的和广泛的裂纹扩展实验数据。本研究旨在研究镍合金裂纹尖端的氧化损伤的物理过程和相关的裂纹扩展行为，这将首次提供直接的见解，裂纹尖端的氧化脆化现象。裂纹尖端的氧化损伤是时间、温度、局部变形和材料微观结构的综合作用。了解该过程对于评估氧化攻击下的裂纹扩展行为至关重要。在拟议的工作中，单晶，定向凝固和多晶镍合金将用于在受控环境（真空，空气，氧气-18）疲劳氧化条件下的裂纹扩展测试。将进行先进的显微镜分析，以确定和测量裂纹尖端的氧渗透和微观结构损伤，其结果将用于校准氧化过程中的重要扩散和损伤参数。将进行数值分析，以模拟这种过程在微观尺度上使用耦合的机械扩散模型。研究了加载条件和晶界性质对氧扩散的影响，特别是氧化损伤与裂纹扩展之间的关系。最终将开发并验证裂纹扩展模型，以准确预测疲劳-氧化寿命。这项工作将三个已建立的小组聚集在一起，以协作、系统和多尺度的方式解决这些基本问题。氧化损伤和裂纹尖端变形之间的相互作用需要在受控环境中对疲劳裂纹扩展进行精心设计的专业测试，这是UoS的专业知识。这个问题还需要先进的显微镜表征和物理测量的现象，使用既定的技术在IC。新的模型将由UOP开发，并对这些实验结果进行验证，UOP在材料和裂纹扩展建模方面具有强大的背景。由于我们的技术互补，该联合项目应建立氧化损伤和裂纹扩展之间的物理联系，用于镍合金部件的疲劳设计和安全寿命预测。这项研究将产生独特的和实际有用的数据和模型，可以通过我们的坚定的工业合作伙伴，包括E.ON，阿尔斯通，NASA和DSTL快速利用。研究结果也将对其他致力于实现关键高温部件最大使用寿命和温度能力的行业具有通用性。从事高温材料及相关领域的研究人员和学者也将直接受益于我们有针对性的传播活动，包括研讨会，会议和期刊论文。我们会透过特别设计的公众参与计划和不断更新的网站，让更多市民参与。

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.