Dislocation-Microstructure Interaction at a Crack Tip - In Search of a Driving Force for Short Crack Growth

镍基高温合金特别用于涉及高温和应力的应用中，例如航空航天和发电工业中的关键燃气涡轮机叶片和盘。镍高温合金中短裂纹的行为对于部件设计和寿命预测特别重要，因为在最终失效之前，大部分使用寿命都花费在小裂纹的生长上。由于局部微观结构和非均匀应力/应变场的强烈影响，短裂纹在疲劳下会快速生长，并且往往表现出高的、不规则的和分散的生长速率。短裂纹扩展的物理驱动力仍然没有得到很好的理解，但尽管密集的研究工作，主要是由于有限的理解裂纹尖端behavior.This建议的目的是调查的基本变形机制在短裂纹尖端的镍基高温合金疲劳下在一个温度范围内。研究将集中在不断发展的局部塑性的影响，诱导在裂纹尖端的位错动力学，对短裂纹扩展。位错与材料微观结构的相互作用是材料非均匀塑性和内应力集中的主要来源，是短裂纹萌生和扩展的主要原因。将在受控环境中进行短裂纹扩展测试，以研究这些合金在疲劳条件下短裂纹扩展的异常行为，这是UoS的专业知识。为了观察温度变化对裂纹尖端附近滑移行为的临界影响，将改变温度。在裂纹扩展试验之后，将对裂纹尖端区进行事后透射电子显微镜分析，以揭示位错的成核和增殖、位错在相/晶界处的堆积和渗透以及晶粒取向差对位错行为的影响的详细机制。特别是，将从疲劳试验样本的裂纹尖端断裂过程区提取火柴棒样本，以允许在疲劳下原位测量裂纹尖端变形，这是UoM的既定技术。在这种情况下，高分辨率数字图像相关性，在金重塑表面的晶粒取向映射和扫描电子显微镜成像的帮助下，将用于量化疲劳加载期间裂纹尖端附近形成的滑动痕迹中的剪切应变。此外，亦会进行高能同步辐射X射线衍射研究，以量度裂纹尖端不同相之间的弹性应变反应及荷载转移，从而了解位错渗入γ ′沉淀物的情况。将开发三维离散位错动力学（DDD）方法来模拟位错和材料微结构之间的相互作用，这是LU的强度，根据实验结果。DDD模型将与粘塑性和晶体塑性模型相结合，并进一步应用于研究位错动力学在描述短裂纹扩展中的作用。将建立一个多尺度有限元方法的裂纹尖端变形分析，其目的是确定一个基于细观力学的驱动力的短裂纹扩展。将根据局部应变测量（中尺度和微尺度）、现场和死后测量以及提取的火柴棒样品的X射线断层扫描对计算模拟进行彻底验证。最终的目标是提供一个有效的有限元程序来预测短裂纹的增长，与实验数据的充分验证，为最终用户。

Nickel-based superalloys are particularly used in applications involving high temperatures and stresses, such as the critical gas-turbine blades and discs in aerospace and power-generation industries. The behaviour of short cracks in nickel superalloys is of particular importance for component design and life prediction, as a large proportion of service life is spent in the growth of small cracks before final failure. Due to the strong influence of local microstructure and heterogeneous stress/strain fields, short cracks are known to grow anomalously under fatigue and tend to exhibit high, irregular and scattered growth rates. The physical driving force for short crack growth is still not well understood yet despite intensive research effort, mainly due to the limited understanding of crack-tip behaviour.This proposal aims to investigate the fundamental deformation mechanism at the tip of a short crack for nickel-based superalloys under fatigue at a range of temperatures. The research will focus on the influence of evolving local plasticity, induced by dislocation dynamics at the crack tip, on short crack growth. The interaction between dislocation and material microstructure is the major source for heterogeneous plasticity and internal stress concentration, leading to initiation and growth of short cracks. Short crack growth testing in a controlled environment will be carried out to study the anomalous behaviour of short crack growth in these alloys under fatigue, which is the expertise of UoS. Temperature will be varied in order to observe the critical effect of temperature change on the slip behaviour near the crack tip. Following crack growth tests, post-mortem transmission-electron-microscopy analyses of crack-tip zone will be performed to reveal the detailed mechanisms for nucleation and multiplication of dislocations, pile-up and penetration of dislocations at phase/grain boundaries and the influence of grain misorientations on dislocation behaviour. In particular, match-stick samples will be extracted from the crack-tip fracture process zone of fatigue-tested specimens to allow in-situ measurements of crack tip deformation under fatigue, which are the established techniques at UoM. In this case, high resolution digital image correlation, with the assistance of grain orientation mapping and scanning-electron-microscopy imaging of gold remodelled surfaces, will be used to quantify shear strain in slip traces formed near the crack tip during fatigue loading. In addition, high energy synchrotron X-ray diffraction studies will be carried out to measure the elastic strain response and load transfer between different phases around the crack tip, which will provide insight regarding the penetration of dislocations into the gamma-prime precipitates.To physically simulate the material plasticity behaviour, a three-dimensional discrete-dislocation-dynamics (DDD) approach will be developed to model the interaction between dislocations and material microstructures, which is the strength of LU, based on experimental results. The DDD model will be interfaced with viscoplasticity and crystal plasticity models, and further applied to investigate the role of dislocation dynamics in depicting short crack growth. A multi-scale finite element method will be established for the crack-tip deformation analyses, which aims to identify a micromechanics-based driving force for short crack growth. Computational simulations will be thoroughly validated against local strain measurements (at both mesoscale and microscale), in-situ and post-mortem measurements as well as X-ray tomography of extracted match-stick samples. The ultimate goal is to deliver an efficient finite element procedure to predict short crack growth, with full validation against the experimental data, for end users.