Fatigue crack growth in poycrystalline IN 718 superalloy

The fatigue crack growth rate (FCGR) of a polycrystalline IN 718 superalloy was investigated at 650°C for two different grain sizes using single notch tension (SENT) specimens. In air the FCGR is insensitive to the grain size in the mixed time-cycle dependent regime and the oxidation-induced intergranular fracture controls the crack growth rate. When a hold time is applied the coarse microstructure shows a lower resistance to crack propagation both in air and in vacuum. A two parameter Paris equation was found to describe the relationship between the loading frequency and the crack growth rate, which provides valuable insight into the oxygen-induced intergranular fracture mechanisms operating at elevated temperature.