High temperature fatigue of Ni-base superalloys: Microstructural and environmental effects

The effects of environment and metallurgical factors on the fatigue crack growth rate (FCGR) behaviour of Ni-base superalloys are analysed in two alloys, Inconel 718 and Alloy N18, used for the fabrication of turbine discs. Inconel 718 is a widely used wrought alloy, while Alloy N18 is representative of the most advanced PM superalloys. It is shown that the FCGR behaviour of these materials is largely dependent on microstructural parameters, including gamma' precipitate size, grain size, and grain morphology. It is also confirmed that the FCGRs of these alloys tested at elevated temperature (approximate to 650 degrees C) and low frequency, in particular in fatigue with a tensile hold time, are largely dependent on oxygen partial pressure, PO2. In both materials a transition in FCGRs is observed when PO2 is reduced below 10(-3) mbar. This transition is related to the types of oxides formed ahead of the crack tip: spinel type above the transition and Cr2O3 type below the transition. In both alloys two extreme regimes for the FCGR behaviour are distinguished: (i) a pure cycle-dependent regime, and (ii) a pure time-dependent regime. These regimes are associated with widely different fracture modes: transgranular in the former case, intergranular in the latter. An attempt is made to give guidelines for modelling the FCGR behaviour in both regimes.