Effect of crystal grain arrangement on fatigue crack initiation for Ni-based directionally solidified superalloy

Low cycle fatigue tests at elevated temperature were conducted on a Ni-based directionally solidified superalloy subjected to transverse loading. To investigate the effect of the arrangement and the orientation of crystal grains on the crack initiation, the electron backscatter diffraction (EBSD) method was applied on the surfaces of the tested specimens. In addition, finite element analysis that considered the plastic behaviour of crystal grains was performed to evaluate the relationship between the crack initiation and the local stress. The results are summarised below. Regarding the effect of crystallographic properties on the surface of specimens without holes, cracks generally initiated near the grain boundaries. In addition, most cracks initiated near the grain boundaries that neighboured the grain whose secondary axis was more inclined to the normal direction of specimen surface than the grain located in the opposite surface. Similarly, the number of cycles to crack initiation tended to be small along the hole edges located on the grain boundaries for the specimens with holes. The results of finite element analysis showed non-uniform stress distribution due to the grain orientation. They revealed that high longitudinal stress developed near the grain boundaries with cracks for the specimens without holes. Stress correlated with the number of cycles to crack initiation along hole edges for specimens with holes.