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

晶粒度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度，以提高延展性和抗拉强度、屈服强度和疲劳强度。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)+纳米压痕的出现，为结构演变和结构/性质关系的更深层次提供了新的、有趣的见解。这包括初步确定有效结构单元尺寸(而不是颗粒尺寸)在确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大小之间的关系的理解仍处于初级阶段。因此，现在可以在理解结构的演变方面取得重大进展，包括重结晶过程和相变过程中的变体选择。这为细化各种工程材料的组织提供了可能性，对于这些材料，相变在加工过程中起着重要作用。疲劳过程非常复杂，但在概念上可以简化为萌生和裂纹扩展。对于高周疲劳(HCF)制度，施加应力循环的次数很容易超过10,000,000次，材料评估依赖于样品或部件测试。HCF寿命的大部分时间都花在启动缺陷上，然后该缺陷迅速增长到失效。对于受这种HCF制度影响的材料，设计原则是将其保持在经验定义的持久应力以下，以防止引发。对于低周疲劳，情况是不同的，因为起始寿命和增长寿命都可以用来预测部件的安全寿命。通常，启动也是由机械测试经验确定的。目前无法从基本原理预测疲劳萌生的原因是，裂纹萌生是由从颗粒到颗粒的相互作用主导的，这种相互作用本身就很难量化和建模。因此，对于重要的终端用户应用，工程师只有很少的知识来定义材料的哪些方面或其加工过程除了通过机械测试之外影响其性能，这非常耗时和昂贵。直到最近，对疲劳的可考虑的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。在钛基合金的具体情况下，仅通过光学检查定义晶界和随后测量晶粒度是出了名的困难。取向相似的晶体单元的大群体的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，这可能会对部件性能产生潜在的灾难性影响。决定最终用户感兴趣的力学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，边界是什么。b)决定有效结构单元尺寸的加工和热处理因素。c)建模能力，以提供力学行为的定量预测，包括HCF起始和短裂纹扩展速率。所有这些问题构成了当前研究提案的基础。

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.