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本质上决定了材料对塑性变形的力学响应。同时，GB的滑移转移行为对材料在疲劳下的寿命有很大影响，但只能通过基本方法来理解。因此，改进寿命预测模型需要详细了解位错和裂纹--特别是微结构短裂纹--与局部微观结构的相互作用，这意味着对GBs的滑移转移阻力有一个经过实验检验和量化的知识。这是由于边界层是裂纹萌生和早期但决定寿命的短疲劳裂纹扩展的主要障碍。近年来，在模拟和建模领域获得了许多知识。然而，由于复杂性，为了验证这些结果并为进一步的模拟和计算获得输入参数，存在实验泄漏。这就是本项目的目的。在项目的第一阶段，开发了一种GB局部应力集中的测量策略，以测量滑移传递的突破应力。采用原子力显微镜、光学显微镜和扫描电子显微镜相结合的原位实验，研究了CMSX-4镍基高温合金近GBs多晶变质过程中FIB缺口处的短阶段I-疲劳裂纹的扩展行为，特别是裂纹及其塑性区与选定GBs的相互作用。检查了基于两个颗粒中活动滑移系统的兼容性的常见几何概念是否描述了GB的特定阻力以及它们在多大程度上描述了GB的特定阻力。基于这种几何考虑，提出了强概念(转移相邻颗粒的滑移阻力)。项目第二部分的目的是找到验证和量化的国标阻力与滑移系的几何形状和国标之间的函数关系，这意味着将几何和应力概念结合到一个一致的概念中。为此，必须考虑应变硬化和各向异性的影响。因此，第一阶段的结果是用应变硬化的二元铝锂合金进行检验的。利用CMSX-4微型试件的现场实验将弹性各向异性整合到当前的测量策略中。两者共同将技术材料的强概念和更一般的材料行为扩展到(扩展的)X-强概念。这一概念为国标的滑移传递阻力提供了一个量化和验证的预测。最后，作为晶界工程的一种可能性，GB析出物对滑移转移阻力的影响成为一个研究课题。

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.