High-temperature fatigue crack growth behaviour of GH4169 alloys with different heat treatment methods

The widely used GH4169 alloy can meet various application through different heat treatment methods. In order to investigate the effect of heat treatment method on crack resistance at elevated temperature, fatigue crack growth tests of direct aging heat-treated GH4169 alloy (DA-GH4169) and standard heat-treated GH4169 alloy (SHT-GH4169) were performed at 650 degrees C in air. Under 10 Hz sinusoidal waveform, the fatigue crack growth rate of DA-GH4169 is slightly higher than that of SHT-GH4169 at low-Delta K Paris regime. The rates become close as Delta K increased. Under 0.5 Hz triangular waveform, the fatigue crack growth rate of DA-GH4169 is always higher than that of SHT-GH4169 in the tested Delta K scope. The change of fracture morphology from intergranular to transgranular in the Paris regime was observed for two GH4169 alloys. It is attributed to the interaction of cyclic plastic damage and dynamic embrittlement at crack tip, which is influenced by load, microstructure and frequency. Completely different propagation modes were found in the near-threshold region. While intergranular delta phases mildly inhibit grain boundary oxidation in the Paris regime, the transgranular propagation induced by intergranular phases in the near-threshold region is responsible for lower threshold.