Crystallography and elastic anisotropy in fatigue crack nucleation at nickel alloy twin boundaries

Fatigue crack nucleation at annealing twin boundaries (TBs) within polycrystal nickel-based superalloy Rene 88DT is investigated with a microstructure-sensitive crystal plasticity (CP) model, digital image correlation strain measurements and experimental SEM crack nucleation observations. Strong slip localizations at TBs were experimentally observed and predicted by the CP model, which also showed high predicted geometrically necessary dislocation and corresponding stored energy densities, capturing experimental observations of crack nucleation. In a systematic study, elastic anisotropy was found to drive local elastic constraint and hence resolved shear stress, slip activation, GND density and stored energy density, demonstrating for this reason that TBs are preferential sites for crack nucleation in this alloy. The parent grain / twin pair crystallographic orientation with respect to remote loading was also demonstrated to be key to slip activation parallel to TBs and hence to stored energy density and fatigue crack nucleation, and the range of most damaging parent grain orientations has been identified.

Automated summary

Based on investigation of fatigue crack nucleation at annealing twin boundaries (TBs) in polycrystal nickel-based superalloy Rene 88DT, it was found that elastic anisotropy plays a key role in driving local elastic constraint and slip activation, leading to TBs being preferential sites for crack nucleation. The crystallographic orientation of parent grain/twin pair also plays a crucial role in slip activation and fatigue crack nucleation, with the most damaging parent grain orientations identified.