Scale-Bridging Microstructure-Sensitive Assessment of Intergranular Cracking during High-Temperature Dwell-Time Fatigue of Polycrystalline Superalloys

航空发动机和陆基燃气轮机的可靠性、安全性和燃油效率，以及为优化利用可再生能源而开发的新工艺，都要求材料兼具高疲劳和蠕变强度以及优异的耐腐蚀性。对于这种高温应用，现有的多晶变形镍基高温合金的块体性能已经得到了优化。然而，在提高使用性能时，晶界可能是最薄弱的环节。主导失效机制被称为“动态脆化(DE)”，它通过脆化元素向晶界的应力辅助扩散来降低界面粘聚力。多晶高温合金的De高度依赖于材料的微观组织，特别是决定扩散速率的晶界特征，从而决定脆性元素在晶界的局部浓度。然而，微结构和动能之间的相关性目前还不是很清楚，也不存在对微结构敏感的基本建模方法来评估相关性。因此，该研究项目的目标是开发一种微观结构敏感的模拟方法，用于研究扩散控制的界面疲劳断裂。该模型将基于微观结构的有限元模型与应力辅助界面扩散的有限差分方法和与晶界性质相关的从头计算相结合。基于微观结构的有限元模型考虑了穿晶和沿晶疲劳裂纹的扩展，并包含了关于多晶组织和晶界特征的统计信息。单晶的性质用循环单晶粘塑性来描述，而晶界的性质用粘聚区单元的牵引-分离定律来模拟。基于微观结构的有限元模型与有限差分法耦合计算应力辅助界面扩散，从而使脆化单元在裂纹前沿前方晶界的集中程度取决于外加应力水平、应力比和保持时间。根据晶界特征和脆化元素的浓度，从头计算得到了与扩散和牵引-分离行为相关的结构-性质关系。根据多尺度模拟方法，将基于X射线计算机层析成像、电子显微镜和原子探针层析成像对尺度桥接微结构进行量化。此外，还将对DE裂纹的扩展进行现场监测，从而确定材料的性能并验证建模方法。调查中考虑了718合金。

Reliability, safety and fuel efficiency of aero engines and land-based gas turbines that are nowadays used at strongly variable operating conditions as well as the exploitation of new processes to optimize the use of renewable energies require materials that combine high fatigue and creep strength with an excellent corrosion resistance. For such high-temperature applications, the bulk properties of existing polycrystalline wrought nickelbase superalloys have been optimized. However, when increasing the in-service performance, grain boundaries may act as the weakest links. The governing failure mechanism is known as "Dynamic Embrittlement (DE)" where interface cohesion is lowered by stress-assisted diffusion of an embrittling element into the grain boundary. DE of polycrystalline superalloys is highly depending on the material’s microstructure, especially on the grain boundary character that determines diffusion rates and, thus, the local concentration of the embrittling element in the grain boundary. However, the correlations of microstructure and DE are not well understood today and fundamental microstructure-sensitive modelling approaches to assess the correlations do not exist. Thus, it is the objective of the proposed research project to develop of a microstructure-sensitive modelling approach for diffusion-controlled interfacial fatigue fracture at elevated temperature due to DE. The proposed modelling approach combines microstructure-based finite-element models with a finite difference method solution for stress-assisted interface diffusion and ab-initio calculations related to the grain boundary properties. The microstructure-based finite-element models consider transgranular and intergranular fatigue crack growth and include statistical information on polycrystalline microstructure and grain boundary characteristics. The properties of the single crystals are described by cyclic single-crystal viscoplasticity, while the properties of the grain boundaries are modeled with traction-separation laws using cohesive zone elements. The microstructure-based finite-element models are coupled with the finite difference method for the calculation of stress assisted interface diffusion, so that the concentration of the embrittling element in the grain boundary ahead the crack front is known depending on applied stresses level and ratio as well as hold time. Structure-property relationships related to diffusion and traction-separation behavior are obtained from the ab-initio calculations depending on grain boundary characteristics and concentration of the embrittling element. According to the multi-scale modelling approach, scale-bridging microstructure quantification will be based on X-ray computer tomography, electron microscopy and atom-probe tomography. Moreover, DE crack propagation will be in-situ monitored, so that material properties can be determined and the modeling approach can be validated. Alloy 718 is considered in the investigations.