Effect of Microstructure on Fatigue Crack Growth Rate of Alloy 718 at 650C

This paper examines the influence of serrated grain boundary morphology and dislocation substructures on the fatigue crack growth resistance of Alloy 718 at 650"~ in laboratory air. The serrated grain boundary work was conducted on experimental plate stock whereas the influence of dislocation substructures was examined on specimens machined from service exposed discs that were retrieved from aero engines. Serrated grain boundaries retard the fatigue crack growth rates (FCGR'S) by suppressing grain boundary sliding and providing a tortuous path for crack growth. The influence of dislocation substructures on FCGR's can vary markedly with the type of substructure present. High dislocation densities in the form of planar arrays dramatically increase the FCGR's relative to cellular dislocation arrangements. The origin of dislocation substructures in relation to remnant forging strains or service induced LCF damage is considered in detail. The influence of stress cycling on substructure development in service exposed discs is also discussed.