Grain interactions under thermo-mechanical loads investigated with coupled crystal plasticity simulations and high-energy X-ray diffraction microscopy

Nickel (Ni)-based superalloys are used for safety-critical components in the aerospace industry because of their high strength at elevated temperatures. These materials are subjected to complex thermal and mechanical cycling due to service conditions, which leads to thermo-mechanical fatigue (TMF) failure. The failure mechanisms associated with TMF are dependent on microstructural characteristics and loading parameters, such as temperature, strain range, and strain-temperature phasing, requiring a large set of experimental test matrices to study TMF failure. One means to reduce the necessity of extensive and exhaustive testing for material qualification is with model-based approaches. In this work, to study the complexities of micromechanical TMF damage, a temperature-dependent, dislocation density-based crystal plasticity model for a Ni-based superalloy is generated with uncertainty quantification. The capabilities of the model's temperature dependency are examined via direct instantiation and comparison to a high-energy X-ray diffraction microscopy (HEDM) experiment under coupled thermal and mechanical loads. Unique loading states throughout the experiment are investigated with both crystal plasticity predictions and HEDM results to study early indicators of TMF damage mechanisms at the grain scale. Via a crystal plasticity-based failure metric that can predict likely sites for crack initiation, regions of extreme values indicate the spatial location of microstructural damage, while showing correlation with high strain gradients. The results also indicate that the extreme value of thermo-mechanical damage accumulation is influenced by the surrounding grains characteristics including slip system activity, lattice curvature, and neighboring grain compatibility interactions, quantified via a grain interaction strength-to-stiffness ratio orthogonal to the grain boundary.