The sharp drop in fatigue crack growth life at a critical elevated temperature for a PM Ni-based superalloy FGH97

The fatigue crack growth (FCG) experiments for Ni-based powder metallurgy (PM) FGH97 superalloy were performed from RT to 800 °C in air. It has been found that the FCG life decreases in an accelerated manner with an increase in temperature in the service temperature range, and a sharp drop in FCG life was noticed at the inflection point temperature (Tc) of about 800 °C. The fractographic and microstructural analyses were carried out to investigate the primary reason for the phenomenon of observed sharp drop. The obtained results indicate that the fracture transforms from transgranular to intergranular mode during the crack propagation under elevated temperature, and the transition point of fracture mode (ΔKT) also declines rapidly with an increase in temperature. At 800 °C, ΔKT appears even from the crack starts to propagate, which is closely related to the significant decline of FCG life. Combined with the calculated apparent activation energies of FCG, it can be inferred that the sharp drop in FCG life is primarily attributed to the dynamic embrittlement induced weakening of grain boundary, which dominates in the intergranular FCG stage at 800 °C. Further, the evolution of the fracture mechanism governing the FCG process under the conditions of different temperatures and stress intensity factor range (ΔK) has been discussed. With an increase in temperature in the service temperature range, the dominant mechanism of the intergranular fractures transforms from stress assisted grain boundary oxidation to dynamic embrittlement, which results in the sharp decline of FCG life at Tc.