Microstructure-sensitive fatigue crack nucleation and propagation in nickel-based superalloys using a novel multi-scale CPFEM-CDM model with stored energy density

High-temperature nickel-based superalloys underpin many aerospace components owing to their superior strength and durability at elevated temperatures, yet they remain vulnerable to fatigue-induced cracking. This study develops a multi-scale crystal plasticity finite element model coupled with continuum damage mechanics that, for the first time, employs non-local stored energy density as a unified metric for both crack nucleation and subsequent growth. The framework clarifies how grain orientation and misorientation dictate crack path selection and propagation rate. Simulations reveal that polycrystals with high misorientation nucleate cracks more readily than low-misorientation aggregates or single crystals because high-angle grain boundaries favour the formation of intense slip bands and associated localised energy accumulation. Grains possessing larger Schmid factors also show higher nucleation propensity. In single crystals, propagation paths are orientation-dependent: linear trajectories arise when one dominant slip system prevails, whereas competition among multiple systems produces tortuous paths and slower growth. Crossing a grain boundary, increasing misorientation progressively reduces the crack growth rate and deflects the path by generating elastic mismatch and non-uniform energy distributions that impede damage accumulation. Conventional misorientations exert a greater retarding effect than special misorientations, the former being governed jointly by orientation contrasts and grain-strength differences, while the latter are dominated by orientation alone. The insights obtained provide quantitative guidance for fatigue-life prediction and microstructural design of high-temperature superalloys.