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

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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.