Effect of microstructure on crack propagation in high-temperature fatigue of directionally solidified Ni-based superalloy

In order to clarify the crack propagation properties of an anisotropic material (Ni-based directionally solidified superalloy), longitudinally loaded specimens (L-specimens) and transversely loaded specimens (T-specimens) with a crack are subjected to high temperature fatigue. The crack propagation rate is reasonably well correlated with the effective stress intensity factor range regardless of the propagation direction (specimens L and T), the stress range and the stress ratio. However, the crack propagation rate shows a notable fluctuation particularly in the T-specimens. It is at most about five times faster than the average. The fracture surface features can be classified into four types with three transgranular and one intergranular types. In the former, though the crack is along the {100} or {110} planes on a macroscopic scale, it threads through the {111} or {100} planes on a microscopic scale. Crack propagation is notably accelerated in the intergranular region, while deceleration is caused by crack branching.