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

• Applicant: Professor Dr. Christian Motz
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 270913401

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.

• DFG Programme: Research Grants