X-ray Laue Microscopy to Understand Fatigue Damage

在疲劳过程中，内部和外部尺寸效应对特征位错结构形成的影响尚未得到详细的研究，更不用说理解了。特别是，由于缺乏合适的原位表征技术，目前还无法对单个晶界附近位错结构的形成及其在裂纹萌生中的作用进行详细的研究。在XMicroFatigue项目的第一个资助阶段，建立了一个“差孔径x射线显微镜”（DAXM），它提供了亚微米分辨率的位错密度、弹性应变场和取向的无损访问，同时允许微观物体的变形。双晶微弯曲悬臂梁-每一个在样品中心都有一个单一的晶界-受到循环加载，并研究了由此产生的位错结构。选用铜作为模型材料。我们的重点是对位错滑移转移不透明的晶界。第一个供资期关于就地DAXM的建造和验证的所有目标都已成功实现。显微镜以及评估例程是可用的劳用户社区。关于对位错运动不透明的晶界的疲劳，所有必要的实验都已成功完成，但对大数据集的评价仍在进行中，预计将在第二个资助期的第一年完成。为了克服评价大数据集的挑战，计划在第二个供资期开始时扩大劳厄评价程序。随后，对至今无法评价的非透明晶界的剩余数据进行分析和发表。第二个资助期的主要课题是研究不受位错运动影响的晶界的疲劳行为。将研究两种类型的晶界：没有预定义传输路径的透明晶界和具有预定义传输路径的透明晶界。第二个资助期的目标是全面了解循环加载过程中铜晶界附近的位错结构形成和疲劳裂纹萌生的初始阶段。

The influence of intrinsic and extrinsic size effects on the formation of characteristic dislocation structures during fatigue has not been studied in detail, let alone understood. In particular, the formation of dislocation structures in the vicinity of individual grain boundaries and their role in crack initiation could not be studied in detail until now, mainly due to the lack of suitable in situ characterization techniques.In the course of the first funding period of the XMicroFatigue project, a "Differential Aperture X-Ray Microscope" (DAXM) was built, which provides non-destructive access to dislocation densities, elastic strain fields and orientation with sub-micrometer resolution, while allowing the deformation of micro-objects. Bicrystalline micro bending cantilevers – each of which had a single grain boundary at the center of the sample – were subjected to cyclic loading and the resulting dislocation structures were studied. Copper was chosen as model material. Our focus was on a grain boundary that was intransparent to dislocation slip transfer.All objectives of the first funding period with respect to the construction and validation of the in situ DAXM have been successfully achieved. The microscope as well as evaluation routines are available to the Laue user community. With respect to fatigue of a grain boundary that is intransparent to dislocation motion, all necessary experiments have been successfully completed, but the evaluation of the large data sets is ongoing and is expected to be completed in the first year of the second funding period. To overcome the challenges of evaluating large data sets, an extension of the Laue evaluation routines is planned at the beginning of the second funding period. Subsequently, the remaining data of the non-transparent grain boundary that cannot be evaluated to date should be analyzed and published.The main topic of the second funding period deals with the fatigue behavior of grain boundaries transparent to dislocation motion. Two types of grain boundaries will be investigated: transparent grain boundaries without a predefined transmission path and transparent grain boundaries with a predefined transmission path. The goal of the second funding period is a comprehensive understanding of dislocation structure formation and the initial stages of fatigue crack initiation near copper grain boundaries during cyclic loading.