Investigation of isothermal and thermo-mechanical fatigue behavior of the nickel-base superalloy IN738LC using standardized and advanced test methods

Abstract

The uniaxial and biaxial-planar isothermal fatigue behavior as well as the uniaxial thermo-mechanical fatigue behavior of the nickel-base superalloy IN738LC was investigated at high temperatures using standardized and advanced test technologies. Small scale specimens’ tests were carried out in order to study the specimen size effect and biaxial-planar tests under equibiaxial and shear loading to investigate the influence of the multiaxial stress state on the isothermal fatigue behavior both at 750°C. Standardized specimens were used to determine the uniaxial isothermal and thermo-mechanical fatigue behavior under in-phase and out-of-phase loading in the temperature range between 750°C and 950°C. The standard specimens’ results particularly served as reference data as well as for the verification of a recently developed lifetime prediction model for thermo-mechanical fatigue loading. The small scale specimens’ results were in good agreement with standardized specimens. The comparison of fatigue lifetimes of uniaxial and biaxial-planar tests showed that the equivalent strain hypothesis according to von Mises is applicable to correlate the fatigue life of the equibiaxial loading case within a scatter band of factor two, whereas the shear loading case was predicted conservatively. Furthermore, the recently developed lifetime prediction model correlated the fatigue lives of the isothermal and thermo-mechanical tests under uniaxial and biaxial-planar loading as well as literature data. Finally, the deformation and crack growth mechanism were studied.