In situ study of high frequency fatigue crack propagation in a single crystal superalloy

High frequency fatigue crack propagation tests were conducted in situ, in a scanning electron microscope, on single crystals of the nickel-base superalloy Rene N5. The threshold was found to be around 2 MPa√m, and the FCP curve appeared to be consistent with that of a similar polycrystalline alloy but with a lower threshold. Crack propagation was observed to occur on {111} as well as {001} planes at low δK. Strain maps were measured ahead of the crack tip with the DISMAP stereo-imaging technique. SEM images of plasticity associated with the crack tip indicated plastic zone sizes mainly consistent with isotropic linear elastic fracture mechanics estimates, but the plasticity was concentrated in highly planar slip bands parallel to the crack, and the shape of the plastic zone and strain distribution were not isotropic. It is suggested that the threshold behavior may be related to the fracture surface roughness, since the threshold increased in from single crystal to fine-grain polycrystal to coarse-grain polycrystal, in the same order as the fracture surface roughness.