Thermomechanical fatigue of a directionally-solidified Ni-base superalloy: Smooth and cylindrically-notched specimens

Directionally-solidified (DS) Ni-base superalloys are used in high temperature gas turbines because of their excellent properties in the most aggressive mechanical, thermal, and environmental operating conditions. Fatigue life prediction approaches need to account for the presence of cooling holes and other geometric stress concentrations, time-dependent dwells, thermomechanical cycling, and anisotropic material properties. In working towards such a life model, this paper describes the cyclic deformation response and damage mechanisms resulting from thermomechanical fatigue (TMF) of directionally-solidified CM247LC DS. Linear in-phase (IP) and out-of-phase (OP) TMF cycles were performed on both smooth and cylindrically-notched round-bar specimens in both longitudinal and transverse grain orientations. Results take into consideration anisotropy, time-dependent deformation, and the severity of the notch in addition to the waveform and temperature cycle characteristics. The active damage mechanisms are identified as a function of these parameters. Conclusions are drawn in light of fractography, microscopy, and finite element analysis conducted to evaluate geometric and microstructural influences on the time-dependent deformation and fatigue behavior.