X-ray Laue Microscopy to Understand Fatigue Damage

• Applicant: Professor Dr. Christoph Kirchlechner
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 316662945

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.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partner: Dr. Jean-Sébastien Micha
• Co-Investigator: Dr. Patric Alfons Gruber