Fatigue crack growth mechanisms in superalloys: overview

Fatigue studies on disc and blade nickel based superalloys by the author and co-workers are reviewed. Crack initiation in single crystal turbine blade alloys is dominated by interdendritic porosity with oxidation processes affecting initiation position. Lifetime trends can be modelled using a multipart Paris type lifing approach. Orientation, loading state, temperature and environment determine stage I/II crack growth mechanisms and the resulting crack path and should be considered in lifing. Mechanistic insights on how complex stress states, subsurface failures and different temperatures/environments affect fatigue processes can thus improve turbine blade lifing, and direct alloy development programmes. In polycrystalline disc alloys cracks at high temperature may initiate at oxidised subsurface carbides or porosity. Grain size controls cycle and time dependent crack growth: the benefits of increased grain size in resisting grain boundary attack mechanisms predominate over those of gamma' distribution variation. Optimising grain boundary character and distribution should yield the best alloy design strategy for high temperature fatigue performance in turbine discs.