Crack Growth Behavior in Alloy 718 at 425°C

Subcritical crack growth behavior in Alloy 718 was studied at 425°C under static, cyclic, and combined loads. The results are analyzed using linear elastic fracture mechanics. Crack growth was shown to be cycle-dependent at all stress intensities at this temperature and hold times up to 10 min have no effect on the crack growth rates. On the other hand, crack growth is shown to occur readily under static loads at stress intensities less than half of the fracture toughness value. Comparison of the present results with those published in the literature showed that while crack growth is cycle-dependent at 425°C and time-dependent at 650°C, at an intermediate temperature, 540°C, it is time-dependent at low stress intensities and cycle-dependent at high stress intensities. Extrapolation of the present data shows that crack growth under static load could occur at temperatures above approximately 350°C and this may be a limiting factor for Alloy 718 for some of its high temperature applications.