Compressive Strain Hold Effect on High Temperature Low-Cycle Fatigue Crack Growth in Superalloys

Abstract

Crack growth mechanisms taking into account the oxidation and mean stress effect in nickel- and cobaltbased superalloys were determined by crack growth tests using a strain waveform including compressive strain hold. Cracks were found to propagate on the dendrite boundary in both superalloys. Crack growth rates were increased by increasing the compressive hold time. Oxidation around the crack tip was detected by electron probe micro analysis (EPMA) and was found to accelerate the fatigue crack growth. The oxidation component in the normalized crack growth rate by oxidation is proportional to about the 1/3 power of loading frequency for Ni-based superalloy, whereas the power for Co-based superalloy is about 1/2. Furthermore, mean stresses in the nickel-based superalloy, which has a high proof strength even at high temperature, were increased by the compressive strain hold. It was also found that increasing the mean stress accelerates the crack growth. A crack growth model that considers effects of both oxidation at crack the tip and mean stress was discussed from these test results.