THERMOMECHANICAL FATIGUE OF A DIRECTIONALLY-SOLIDIFIED NI-BASE SUPERALLOY IN THE PRESENCE OF STRESS CONCENTRATIONS

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. Complex thermomechanical loading of turbine blades is induced by repeated engine start-up, firing, and shut-down transients. These histories make life prediction for such components difficult and subjective. In addition, accurate techniques need to account for the presence of cooling hole stress concentrations, time-dependent dwells, thermal gradients, and anisotropic material properties. In working towards such an analytical life model, this paper describes the cyclic deformation response and damage mechanisms resulting from thermomechanical fatigue (TMF) of directionally-solidified CM247LC DS. Experimental LCF tests consisted of linear in-phase (IP) and out-of-phase (OP) TMF cycles performed on smooth and notched round-bar specimens in both longitudinal and transverse grain orientations. Results take into consideration anisotropy, time-dependent deformation, and notch effects 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 material behavior.