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

多晶的有效结构单元尺寸：形成、定量和微机械行为

基本信息

批准号：
EP/E044700/1
负责人：
David Dye
金额：
$ 17.82万
依托单位：
Imperial College London
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2007
资助国家：
英国
起止时间：
2007 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FE044700%2F1
关键词：
Effective Structural Unit Size Polycrystals

项目摘要

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.

晶粒尺寸的概念在多晶金属的工程应用中发挥着重要作用，这一概念已得到充分证实。在铸造和随后的锻造加工过程中，采用经过试验和测试的方法来细化晶粒尺寸，以增强延展性并提高拉伸强度、屈服强度和疲劳强度。基于电子显微镜的实验技术（例如电子背散射衍射（EBSD）和聚焦离子束（FIB）加上纳米压痕）的出现，为结构演化和结构/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）在确定机械行为方面的关键作用。然而，对于大多数系统来说，对处理和有效结构单元大小之间关系的理解仍处于起步阶段。因此，现在在理解结构演化（包括重结晶过程和相变过程中的变体选择）方面可以取得重大进展。这提供了细化各种工程材料结构的潜力，这些材料在加工过程中相变起着重要作用，例如钢、钛、锆等。疲劳过程非常复杂，但可以在概念上简化为引发和裂纹扩展。对于高周疲劳 (HCF) 状态，所施加的应力循环次数很容易超过 10,000,000 次，材料评估依赖于样本或部件测试。 HCF 寿命的大部分时间都花在引发缺陷上，然后迅速发展到故障。对于受此类 HCF 状态影响的材料，设计原则是保持低于经验定义的持久应力，以防止引发。对于低周疲劳（LCF），情况有所不同，起始寿命和生长寿命都可以用来预测安全部件寿命。通常，引发再次通过机械测试凭经验确定。目前无法根据基本原理预测疲劳萌生，这是因为裂纹萌生主要是晶粒间的相互作用，而这种相互作用本质上难以量化和建模。因此，对于重要的最终用户应用，工程师除了通过机械测试之外，对定义材料或其加工的哪些方面影响其性能的知识很少，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才在很大程度上未能以其他重要方式协助材料生产商和最终用户。在钛基合金的具体情况下，仅通过光学检查来定义晶界和随后的晶粒尺寸测量是非常困难的。类似取向晶体单元的大群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的单元之间的应力重新分配过程，这可能会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) 边界如何表现以及给定载荷状态的边界由什么构成。b) 决定有效结构单元尺寸的加工和热处理因素。c) 建模能力提供机械行为的定量预测，包括 HCF 引发和短裂纹扩展速率。所有这些问题构成了当前研究提案的基础。