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Fatigue damage evolution in micro samples: the influence of specimen size and grain boundaries

微样品中的疲劳损伤演化：样品尺寸和晶界的影响

基本信息

批准号：
270913401
负责人：
Professor Dr. Christian Motz
金额：
--
依托单位：
Lehrstuhl für Experimentelle Methodik der Werkstoffwissenschaften
依托单位国家：
德国
项目类别：
Research Grants
财政年份：
2015
资助国家：
德国
起止时间：
2014-12-31 至 2018-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/270913401?language=en
关键词：
Fatigue damage evolution micro samples

项目摘要

Fatigue of metallic materials and components is one of the main reasons that limit their lifetime and impact sustainability. The ongoing miniaturization in many areas of modern technology, e.g. microelectronics, medical devices, etc. requires further knowledge of mechanical properties in small dimensions to guarantee their reliability. The size of typical fatigue dislocation structures which control the damage evolution is in the order of micrometers. Hence, reducing the size of parts and components down to this scale raises the question whether such microstructures can occur or not and how this affects the damage evolution. In addition, grain boundaries always play a crucial role in fatigue of polycrystalline metals because it is found that grain boundaries are usually a preferential fatigue cracking site and affect the damage evolution. In the case of grain boundaries, incompatibilities in local stresses and strains are of special interest as they correlate with the damage evolution at the grain boundary according to different models in the literature. For this reason, in the present project the development of fatigue microstructures and the damage evolution were studied by in-situ fatigue tests in the scanning electron microscope (SEM) on single and bi-crystalline micro samples depending on specimen size (0.5 to 15 µm), initial dislocation density and crystal orientation. After the in-situ tests, extrusions arise at the surface of the thicker samples that are similar to those in bulk materials. The damage evolution and dislocation structures were analyzed systematically using the SEM. The samples damage intensified by increasing the load amplitude. Furthermore, the damage and the formation of extrusions in smaller samples were weaker than in thicker samples. In much smaller samples (<2 µm), other damage mechanisms seem to become active. In bicrystals, various dislocation mechanisms were observed (e.g. pile-up of dislocation, formation of a deformation affected zone) affecting the mechanical properties. The main advantage of using micron-sized specimen is the knowledge of the local stresses and strains which allows to associate changes in the stress vs. strain curves with microstructural events. For example, it is possible to correlate the (local) Bauschinger-effect with the back stress of dislocation pile-ups at grain boundaries. The aim of the proposed project is to understand the development of fatigue microstructures in dependence of the specimen size to predict the lifetime of miniaturized parts and components. In principle, the achieved knowledge can also help to better understand fatigue phenomena at the macroscale.

金属材料和部件的疲劳是限制其寿命和冲击承受能力的主要原因之一。在现代技术的许多领域中，例如微电子、医疗设备等，正在进行的小型化需要进一步了解小尺寸的机械性能，以保证其可靠性。控制损伤演化的典型疲劳位错结构尺寸在微米量级。因此，将零件和组件的尺寸减小到这种规模提出了这样的微结构是否会发生以及这如何影响损伤演变的问题。此外，晶界在多晶金属的疲劳过程中起着至关重要的作用，因为它通常是一个优先疲劳裂纹的位置，并影响损伤的演变。在晶界的情况下，在局部应力和应变的不相容性是特别感兴趣的，因为它们相关的损伤演化在晶界根据不同的模型在文献中。因此，在本项目中，疲劳微观结构的发展和损伤演变是通过在扫描电子显微镜（SEM）中对单晶和双晶微观样品进行原位疲劳测试来研究的，这取决于样品尺寸（0.5至15 µm）、初始位错密度和晶体取向。在原位测试之后，在较厚的样品的表面处出现类似于块体材料中的挤出。利用扫描电镜对损伤演化和位错结构进行了系统的分析。随着载荷幅值的增大，试样的损伤程度加剧。此外，在较小的样品中的损伤和挤压的形成比在较厚的样品中弱。在更小的样品（<2 µm）中，其他损伤机制似乎变得活跃。在双晶体中，观察到影响机械性能的各种位错机制（例如位错堆积、变形影响区的形成）。使用微米尺寸的试样的主要优点是局部应力和应变的知识，其允许将应力-应变曲线中的变化与微观结构事件相关联。例如，可以将（局部）包辛格效应与晶界处位错堆积的背应力相关联。拟议项目的目的是了解疲劳微观结构的发展依赖于试样尺寸，以预测小型化零件和组件的寿命。原则上，所获得的知识也可以帮助更好地理解宏观尺度上的疲劳现象。