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A microstructure-based capability for forecasting microcrack nucleation in the nickel-based superalloy LSHR is proposed, implemented, and partially verified. Specifically, gradient crystal plasticity is applied to finite-element models of the experimentally measured, 3D microstructure wherein a microcrack is known to have nucleated along a coherent Σ3 boundary. The framework is used to analyze this particular nucleation event and conduct an extensive grain boundary analysis study, the results of which underpin the importance that elastic anisotropy and coherency have in the localization of plastic slip.

A preliminary investigation into the microstructural factors potentially associated with fatigue crack initiation along coherent twin boundaries in the nickel-based superalloy LSHR is presented. Cracks appear to initiate predominantly along twin boundaries. Electron Back Scatter Diffraction (EBSD) maps, coupled with calculations of the local elastic stress point to an influence of the Schmid factor. The length of the twin appears, however, to be more strongly correlated with the tendency to crack.

A fast Fourier transform (FFT) based spectral algorithm is used to compute the full field mechanical response of polycrystalline microstructures. The field distributions in a specific region are used to determine the sensitivity of the method to the number of surrounding grains through quantification of the divergence of the field values from the largest simulation domain, as successively smaller surrounding volumes are included in the simulation. The analysis considers a mapped 3D structure where the location of interest is taken to be a particular pair of surface grains that enclose a small fatigue crack, and synthetically created statistically representative microstructures to further investigate the effect of anisotropy, loading condition, loading direction, and texture. The synthetic structures are generated via DREAM3D and the measured material is a cyclically loaded, Ni-based, low solvus high refractory (LSHR) superalloy that was characterized via 3D high energy x-ray diffraction microscopy (HEDM). Point-wise comparison of distributions in the grain pairs shows that, in order to obtain a Pearson correlation coefficient larger than 99%, the domain must extend to at least the third nearest neighbor. For an elastic FFT calculation, the stress–strain distributions are not sensitive to the shape of the domain. The main result is that convergence can be specified in terms of the number of grains surrounding a region of interest.