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

目前，航空发动机和陆基燃气轮机在多变的运行条件下使用，其可靠性、安全性和燃油效率以及开发新工艺以优化可再生能源的使用，都需要联合收割机兼具高疲劳和蠕变强度以及优异的耐腐蚀性的材料。对于这样的高温应用，已经优化了现有的多晶锻造镍基高温合金的整体性能。然而，在提高服役性能时，晶界可能是最薄弱的环节。控制失效机制被称为“动态脆化（DE）”，其中界面内聚力通过脆化元素向晶界的应力辅助扩散而降低。多晶高温合金的DE在很大程度上取决于材料的微观结构，特别是决定扩散速率的晶界特征，以及因此决定晶界中脆化元素的局部浓度。然而，微观结构和DE的相关性还没有很好地理解今天和基本的微观结构敏感的建模方法来评估的相关性不存在。因此，它是拟议的研究项目的目标，发展的微观结构敏感的建模方法，扩散控制的界面疲劳断裂在高温下，由于DE。所提出的建模方法相结合的微观结构为基础的有限元模型与有限差分法解决方案的应力辅助界面扩散和从头计算相关的晶界性质。基于微观结构的有限元模型考虑穿晶和沿晶疲劳裂纹扩展，并包括多晶微观结构和晶界特征的统计信息。单晶体的性质由循环单晶体粘塑性描述，而晶界的性质用牵引分离法使用凝聚力区元素建模。基于微观结构的有限元模型与有限差分法相结合，用于计算应力辅助界面扩散，从而根据所施加的应力水平和比率以及保持时间，已知裂纹前缘前方晶界中脆化元素的浓度。根据晶界特征和脆化元素的浓度，从从头计算获得与扩散和牵引分离行为相关的结构-性能关系。根据多尺度建模方法，尺度桥接微观结构量化将基于X射线计算机断层扫描、电子显微镜和原子探针断层扫描。此外，DE裂纹扩展将在现场监测，以便可以确定材料性能和建模方法可以验证。研究中考虑了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.