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

位错与晶界的相互作用从本质上决定了材料对塑性变形的力学响应。同时，GB的滑移传递行为会显著影响材料在疲劳下的寿命，但仅通过基本方法来理解。因此，寿命预测模型的改进需要详细了解位错和裂纹-特别是微观结构短裂纹-与局部微观结构的相互作用，这意味着GB的滑移传递阻力的实验测试和量化的知识。这是由于晶界是裂纹萌生和扩展过程中的主要障碍，是决定寿命的早期疲劳短裂纹扩展的主要障碍。然而，由于复杂性，有泄漏的实验来证明这些结果，并获得进一步的模拟和计算的输入参数。在第一期工程中，开发了一种用于测量滑移传递的突破应力的GB局部应力集中测量策略。短阶段-I-疲劳裂纹开始在FIB缺口的多晶改性的镍基高温合金CMSX-4附近的GB和他们的传播行为，特别是相互作用的裂纹和他们的塑性区与选定的GB，通过结合原位实验在AFM中，在光学显微镜和在SEM中进行了研究。共同的几何概念的基础上的兼容性的主动滑移系统在两个晶粒进行了检查，如果和多远，他们描述的特定阻力的GB。基于这种几何考虑，开发了STRoNG概念（相邻颗粒转移的滑动阻力）。该项目第二部分的目的是找到经过验证和量化的GB阻力与滑移系统的几何形状和GB之间的经过验证的函数关系，这意味着几何形状和应力概念的组合成为一个一致的概念。为此，应变硬化和各向异性的影响必须包括在内。因此，从第一阶段的结果进行了测试与应变硬化，二元铝锂合金。CMSX-4微试样的原位实验将用于将弹性各向异性集成到当前的测量策略中。两者一起将技术材料的STRoNG概念和更一般的材料行为扩展到（扩展）X-STRoNG概念。该概念提供了GB的滑移传递阻力的量化和验证的预测。最后，GB沉淀对滑移传递阻力的影响成为晶界工程的可能性调查的主题。

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.