Low‐Angle Grain Boundaries and Re‐Segregation in Single‐Crystalline Ni‐Base Superalloys

The objective of the present work is to demonstrate that Re‐segregation to low‐angle grain boundaries (LAGBs) is affected by the misorientation angle associated with these boundaries. For this purpose, Re‐segregation at two LAGBs with misorientation angles of 0.5° and 5° in a negative misfit Ni‐base single‐crystal superalloy is characterized. The LAGBs form during directional Bridgman solidification and consist of dislocation networks, which separate small groups of parallel dendrites. Site‐specific specimen preparations for complementary investigations using high‐resolution electron backscatter diffraction, scanning electron microscopy, scanning transmission electron microscopy, and atom probe tomography (APT) were performed. APT data suggest that a higher level of misorientation is associated with a higher level of Re‐segregation. The results obtained in the present work also allow to conclude that grown in LAGBs act as dislocation sources throughout creep. In the early stages of creep, they provide the dislocations which fill the γ channels and form the dislocation networks at γ / γ ′ interfaces. During secondary creep, they are subjected to knitting‐in and knitting‐out reactions, which allow to establish a stress‐dependent average dislocation density. It is proposed that in both stages, Re‐segregation at LAGBs slows dislocation processes and thus contributes to creep strength.