Isothermal and thermo-mechanical fatigue behavior of the nickel base superalloy Waspaloy™ under uniaxial and biaxial-planar loading

•First thermo-mechanical biaxial-planar fatigue test rig which met the code of practice of TMF.•New fatigue life data under biaxial isothermal and thermo-mechanical fatigue.•Investigation of the failure mechanism including crack initiation and crack growth.•New lifetime model which based on a stress–strain approach is proposed.•Comparison with the common used lifetime models of the literature.

The thermo-mechanical uniaxial and biaxial-planar fatigue behavior of the forged nickel base superalloy Waspaloy™ was investigated for in-phase and out-of-phase loading between 673 K and 923 K. At these temperatures also the uniaxial and biaxial-planar isothermal material behavior was studied. In order to determine the influence of the multiaxial stress state on the fatigue life biaxial-planar isothermal tests were carried out at three different strain ratios. The uniaxial isothermal lifetimes coincide very well with literature data. A conservative lifetime description for thermo-mechanical in-phase and out-of-phase fatigue test was set based on the isothermal tests at the upper temperature. The comparison of lifetimes from uniaxial and biaxial-planar tests shows that the equivalent strain hypothesis according to von Mises correlates the fatigue lives of the different stress states within a scatter band of two. The failure mechanism including crack initiation, crack growth and the macroscopic crack path were studied by scanning electron microscopy. A change from a mainly transgranular crack growth at 673 K to a mostly intergranular fracture at 923 K as well as under thermo-mechanical fatigue loading was found. The fatigue lives of the isothermal and thermo-mechanical tests under both uniaxial and biaxial-planar loading were correlated by a new lifetime model which is based on a stress–strain approach.