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，例如航空航天和发电工业中关键的燃气轮机叶片和盘。镍高温合金中短裂纹的行为对部件设计和寿命预测具有重要意义，因为在最终失效之前，小裂纹的扩展占据了使用寿命的很大一部分。由于局部微观组织和非均质应力场的强烈影响，短裂纹在疲劳作用下会出现异常生长，并呈现出高、不规则和分散的生长速率。尽管进行了大量的研究，但由于对裂纹尖端行为的理解有限，短裂纹扩展的物理驱动力仍未得到很好的理解。本研究旨在研究镍基高温合金在一定温度下疲劳作用下短裂纹尖端的基本变形机制。研究重点是裂纹尖端位错动态引起的局部塑性演化对短裂纹扩展的影响。位错与材料微观组织的相互作用是材料塑性不均匀和内应力集中的主要来源，导致短裂纹的萌生和扩展。在受控环境下进行短裂纹扩展试验，以研究这些合金在疲劳下短裂纹扩展的异常行为，这是UoS的专长。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，将对裂纹尖端区域进行死后透射电镜分析，以揭示位错的成核和增殖、位错在相/晶界的堆积和渗透以及晶粒取向偏差对位错行为的影响的详细机制。特别是，火柴棒样品将从疲劳测试试样的裂纹尖端断裂过程区提取，以便在疲劳下对裂纹尖端变形进行原位测量，这是UoM建立的技术。在这种情况下，高分辨率数字图像相关，借助于晶粒取向映射和扫描电子显微镜对金重塑表面的成像，将用于量化疲劳加载期间裂纹尖端附近形成的滑移痕迹中的剪切应变。此外，将进行高能同步x射线衍射研究，以测量裂纹尖端周围不同相之间的弹性应变响应和载荷传递，这将为位错在γ -素数析出物中的渗透提供见解。为了物理模拟材料的塑性行为，基于实验结果，将开发三维离散位错动力学（DDD）方法来模拟位错与材料微观结构之间的相互作用，即LU的强度。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。建立裂纹尖端变形分析的多尺度有限元方法，以确定短裂纹扩展的细观动力。计算模拟将针对局部应变测量（中尺度和微观尺度）、现场和死后测量以及提取的火柴棒样品的x射线断层扫描进行彻底验证。最终目标是为最终用户提供一个有效的有限元程序来预测短裂纹扩展，并根据实验数据进行充分验证。

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.