Abstract
Microstructural effects on near-threshold fatigue crack growth behavior in a Ni-base superalloy have been studied. TEM and SEM observations have been performed on corresponding microstructures near crack tip and fracture surfaces. The fatigue crack growth resistance increases with increasing grain size, i.e. the threshold value increases and the fatigue crack growth rate (FCGR) decreases; Correspondingly the fracture surface changes from "fatigue striations" into remarkable "facets". However, increasing γ' particle size at aging-peak condition leads to decrease of the fatigue crack growth resistance, i.e., the threshold value decreases and the FCGR increases; In this case the fracture surface shows "fatigue striations" feature. The high resistance to fatigue crack growth is closely related to the planar slip mode of dislocations in the plastic zone near crack tip, which gives rise to lower accumulation of damage and enhances crack closure resulted from facet-feature cracking. The optimum microstructure for best fatigue crack resistance is suggested as coarse grains with fine γ' particles just before aging-peak is reached.
