Effect of long-term aging on the fatigue crack growth rate of a nickel-based superalloy

Fatigue crack growth rate test was conducted for a nickel-based superalloy at room temperature to investigate the effect of long-term aging at 700 °C and 750 °C. It is found that the precipitation of topologically close packed (TCP) phases as well as the coarsening of γ′ phase and carbides at grain boundaries is induced by long-term aging. Fatigue crack growth rate of the superalloy is increased by long-term aging except that the alloy under aging at 700 °C for 500 h exhibits better crack growth resistance in the near-threshold region than the alloy under standard heat treatment. The difference in fatigue crack growth rate at the transition point from Paris regime to near-threshold regime can be rationalized by considering crack closure effect, crack tip stress shielding effect and crack tip effective stress intensity factor range. The precipitation of TCP phases and coarsening of γ′ phase cause changes in the three aspects. The crack closure effect and crack tip stress shielding effect are mainly induced by roughness and crack branching, respectively.