Local investigation of the grain boundary resistance against 3D-stage I crack propagation: Combination of stress and geometry concept - extension of models and validation of results

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The interaction of dislocations with grain boundaries (GBs) determines the mechanical response of a material to plastic deformation essentially. Simultaneously, the slip transfer behavior of the GB affects the lifetime of a material under fatigue significantly but is only understood by basic approaches. Therefore, an improvement of lifetime prediction models requires a detailed understanding of the interaction of dislocations and cracks - especially of microstructurally short cracks - with the local microstructure which means an experimentally tested and quantified knowledge of the slip transfer resistance of GBs. This is due to GBs being the main obstacles in the process of crack initiation and the early but lifetime-determining short fatigue crack growth.During the recent years, much knowledge has been received in the field of simulation and modeling. However, due to the complexity there is a leakage of experiments to proof these results and to obtain input parameters for further simulations and calculations. This is the aim of this project.In the first period of the project, a measuring strategy for the local stress concentration at the GB was developed to measure the breakthrough stress for slip transfer. Short stage-I-fatigue cracks were initiated at FIB-notches in a polycrystalline modification of the nickel-based superalloy CMSX-4 near GBs and their propagation behavior, in particular the interaction of the cracks and their plastic zones with selected GBs, was studied by a combination of insitu experiments in the AFM, in the optical microscope and in the SEM. Common geometrical concepts based on the compatibility of the active slip systems in both grains were checked if and how far they describe the particular resistance of a GB. Based on this geometrical considerations, the STRoNG-concept was developed (Slip Resistance of Transfer Neighboring Grains).The aim of the second part of the project is to find a proofed functional relationship between validated and quantified GB resistance and the geometry of the slip systems and the GB which means a combination of geometry and stress concept into a consistent concept. To this the influence of strain-hardening and anisotropy have to be included. Therefore, the results from the first period are tested with a strain-hardening, binary aluminum-lithium alloy. In situ experiments with CMSX-4 micro-specimens will be used to integrate elastic anisotropy into the current measuring strategy. Both together expands the STRoNG-concept for technical materials and a more general material behavior to the (eXtended) X-STRoNG concept. This concept provides a quantified and verified prediction of the slip transfer resistance of a GB. Finally, the influence of GB precipitations on the slip transfer resistance becomes a subject of investigation as a possibility for grain boundary engineering.