Slip transfer across grain/twin boundaries in polycrystalline Ni-based superalloys

This study focuses on the interactions of slip bands with grain boundaries which is one of the main sources of intergranular cracking in polycrystalline nickel-based superalloys. For specific configurations where the transmission of slip bands from a grain to its neighbor is difficult, a local but significant crystal elastic rotation may appear leading to highly localized stresses in front of the tip of the active slip band in the adjacent grain. In previous works, such regions were found to be at the origin of fatigue cracks. The objective of this work is to focus on slip band activity near grain boundaries and slip transfer processes through them in order to identify which crystallographic conditions (linked to geometrical relationships between slip systems activated on both sides of the grain boundary) are more prone to favor the development of the aforementioned regions. For this purpose, a statistical analysis was carried out over a large dataset of grain boundaries configurations obtained from SEM images and EBSD scans performed on deformed samples of a Udimet™ 720 Li. A special attention was paid to the Σ3 coherent twin boundaries. The twist angle between projections of slip bands on the boundary plane was found to govern slip transfer processes in this case. The influence of the twist angle is also discussed in the case of more complex general grain boundaries but no similar conclusion has been drawn.