Effective Structural Unit Size in Polycrystals: Formation, Quantification and Micromechanical Behaviour

晶粒尺寸的概念在多晶金属的工程应用中起着重要的作用。在铸造和随后的锻造加工过程中，使用经过试验和测试的方法来细化晶粒尺寸，以提高延展性并增加拉伸、屈服和疲劳强度。电子显微镜为基础的实验技术，如电子背散射衍射（EBSD）和聚焦离子束（FIB）加上纳米压痕的出现提供了新的，有趣的见解更深层次的结构演化和结构/性能的关系。这包括初步确定有效结构单元尺寸（而不是晶粒尺寸）在确定机械性能方面的关键作用。然而，对于大多数系统，对处理和有效结构单元大小之间的关系的理解仍然处于起步阶段。因此，现在可以在理解结构的演变，包括再结晶过程和相变过程中的变体选择方面取得重大进展。这提供了细化的结构范围广泛的工程材料的相变过程中起着重要的作用，如钢，钛，锆等的疲劳过程是非常复杂的，但可以在概念上简化成萌生和裂纹扩展的潜力。对于高周疲劳（HCF）制度，其中施加的应力循环次数很容易超过10，000，000，材料评估依赖于样本或组件测试。HCF的大部分寿命都是用来初始化一个缺陷，然后迅速增长到失效。对于经受这种HCF机制的材料，设计原则是保持在经验定义的耐久应力以下，从而防止引发。对于低周疲劳（LCF）的情况是不同的，因为初始寿命和增长寿命都可以用来预测一个安全的组件寿命。典型地，再次通过机械测试凭经验确定引发。目前无法预测疲劳萌生的基本原则源于这样一个事实，即裂纹萌生是由相互作用的晶粒，这是固有的难以量化和建模。因此，对于重要的最终用户应用，工程师只有很少的知识来定义材料的哪些方面或其处理，除了通过机械测试来影响其性能，这是非常耗时和昂贵的。直到最近，对疲劳的大量科学探索在很大程度上未能在其他重要方面帮助材料生产商和最终用户。在钛基合金的特定情况下，晶粒边界的定义和随后的晶粒尺寸的测量仅通过光学检查是非常困难的。类似取向的结晶单元的大群体的存在可以鼓励广泛的平面滑移结构的发展。反过来，通过相对较弱和较强单元之间的应力重新分布过程，这可能对部件性能产生潜在的灾难性影响。确定最终用户感兴趣的机械性能的关键问题包括：a）边界如何表现以及对于给定的载荷制度，什么构成边界。B）决定有效结构单元尺寸的加工和热处理因素。c）提供机械行为（包括HCF起始和短裂纹扩展速率）的定量预测的建模能力。所有这些问题构成了当前研究建议的基础。

The concept of grain size playing an important role in the engineering application of polycrystalline metals is well established. During casting and subsequent wrought processing, tried and tested methods are used to refine grain size in order to enhance ductility and increase tensile, yield and fatigue strengths. The advent of electron microscopy based experimental techniques such as electron back scatter diffraction (EBSD) and focussed ion beam (FIB) plus nano-indentation have provided novel, intriguing insights into the deeper aspects of both structural evolution and structure / property relationships. This has included preliminary identification of the critical role of effective structural unit size (rather than grain size) in determining mechanical behaviour. However, understanding of the the relationship between processing and effective structural unit size remains in its infancy for most systems. Consequently, significant progress can now be made in understanding the evolution of structures including recrystallisation processes and variant selection during phase transformation. This offers the potential of refining the structure of a wide range of engineering materials for which phase transformation plays an important role during processing such as steel, titanium, zirconium etc. The fatigue process is very complex but can be simplified conceptually into initiation and crack growth. For high cycle fatigue (HCF) regimes where the number of applied stress cycles can easily exceed 10,000,000 material evaluation relies on specimen or component testing. The majority of the HCF life is spent initiating a defect that then grows rapidly to failure. For materials subject to such HCF regimes, the design principle is to stay below an empirically defined endurance stress so that initiation is prevented. For low cycle fatigue (LCF) the situation is different in that initiation life and growth life can both be used to predict a safe component life. Typically, initiation is again determined empirically by mechanical testing. The current inability to predict fatigue initiation from basic principles stems from the fact that crack initiation is dominated by interactions from grain to grain which are inherently difficult to quantify and to model. Thus, for significant end user applications, the engineer has minimal knowledge defining what aspects of a material, or its processing, influence its performance other than by mechanical testing, which is very time consuming and expensive.Considerable scientific exploration of fatigue has until recently largely failed to assist the material producer and end user in other important ways. In the specific case of the titanium-based alloys, the definition of grain boundaries and subsequent measurement of grain size are notoriously difficult through optical inspection alone. The existence of large colonies of similarly orientated crystallographic units can encourage extensive planar slip structures to develop. In turn, through a process of stress redistribution between relatively weak and strong units , this can have a potentially disastrous effect on component performance. Key issues which determine mechanical properties of interest to the end user include:a) How boundaries behave and what constitutes a boundary for a given load regime.b) Factors in processing and heat treatment that dictate effective structural unit size.c) Modelling capability to provide quantitative predictions of mechanical behaviour including HCF initiation and short crack growth rates.All of these issues form the basis of the current proposal for research..