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

金属材料和部件的疲劳是限制其寿命和冲击可持续性的主要原因之一。现代技术的许多领域正在进行小型化，例如微电子、医疗设备等，这就需要进一步了解小尺寸的机械性能，以确保其可靠性。控制损伤演化的典型疲劳位错结构尺寸在微米量级。因此，将零部件的尺寸降低到这种规模会引发这样的问题：这种微结构是否会发生，以及这会如何影响损伤演化。此外，晶界在多晶金属的疲劳过程中一直起着至关重要的作用，因为晶界通常是疲劳断裂的优先位置，并且影响着损伤的演化。在晶界的情况下，根据文献中不同的模型，局部应力和应变中的不相容与晶界处的损伤演化相关，因此具有特殊的意义。为此，本课题通过在扫描电子显微镜(SEM)上对单晶和双晶微观样品进行原位疲劳试验，研究了样品尺寸(0.5~15µm)、初始位错密度和晶体取向对疲劳组织发展和损伤演化的影响。在现场试验后，较厚的样品表面出现了类似于散体材料的挤压。利用扫描电子显微镜系统分析了材料的损伤演化和位错结构。随着加载幅值的增大，试件的损伤加剧。此外，较小样品的破坏和挤压的形成弱于较厚的样品。在小得多的样品(&lt；2微米)中，其他损伤机制似乎变得活跃。在双晶中，观察到各种位错机制(如位错堆积、形变影响区的形成)影响力学性能。使用微米尺寸试件的主要优点是了解局部应力和应变，这允许将应力-应变曲线的变化与微观结构事件联系起来。例如，可以将(局部)包辛格效应与晶界位错堆积的背应力联系起来。该项目的目的是了解疲劳微结构的发展与试件尺寸的关系，以预测小型化零部件的寿命。从原理上讲，所获得的知识也有助于在宏观尺度上更好地理解疲劳现象。

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.