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。在铸造和随后的变形加工过程中，采用经过试验和测试的方法来细化晶粒尺寸，以提高延展性，提高拉伸、屈服和疲劳强度。基于电子显微镜的实验技术的出现，如电子背散射衍射（EBSD）和聚焦离子束（FIB）加纳米压痕，为结构演变和结构/性质关系的更深层次提供了新颖、有趣的见解。这包括初步确定有效结构单元尺寸（而不是晶粒尺寸）在决定力学行为中的关键作用。然而，对于大多数系统来说，对加工和有效结构单元尺寸之间关系的理解仍处于起步阶段。因此，现在可以在理解结构的演变方面取得重大进展，包括相变过程中的再结晶过程和变体选择。这就提供了改进各种工程材料结构的潜力，其中相变在加工过程中起着重要作用，如钢、钛、锆等。疲劳过程非常复杂，但在概念上可以简化为起裂和裂纹扩展。对于高周疲劳（HCF）状态，施加应力循环的次数很容易超过10,000,000次，材料评估依赖于试样或部件测试。HCF寿命的大部分时间都用于启动缺陷，然后迅速增长到失效。对于受此类HCF制度约束的材料，设计原则是保持在经验定义的耐久应力以下，以防止起裂。对于低周疲劳（LCF），情况有所不同，启动寿命和生长寿命都可以用来预测部件的安全寿命。通常，起裂又是由力学试验经验决定的。目前无法从基本原理预测疲劳起裂，这是因为裂纹起裂主要是由晶粒与晶粒之间的相互作用决定的，而这种相互作用本身就难以量化和建模。因此，对于重要的终端用户应用，除了机械测试之外，工程师对材料的哪些方面或其加工影响其性能的知识知之甚少，而机械测试非常耗时且昂贵。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者和最终用户。在钛基合金的特殊情况下，仅通过光学检测来定义晶界和随后的晶粒尺寸测量是出了名的困难。类似取向晶体单元的大量存在可以促进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配过程，这可能对组件性能产生潜在的灾难性影响。决定最终用户感兴趣的机械性能的关键问题包括：a)边界如何表现以及在给定负载状态下什么构成边界。b)加工和热处理中决定有效结构单元尺寸的因素。c)建模能力，提供包括HCF起始和短裂纹扩展速率在内的力学行为的定量预测。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.