A Thermomechanical Fatigue Crack Initiation Model for Directionally-solidified Ni-base Superalloys

Nickel-base superalloys have been used extensively in power generation applications due to their high strength at temperatures up to 1093°C. When the material is subjected to complex mechanical loading superimposed with thermal loading, fatigue interacts with either creep and/or oxidation. Directionallysolidified (DS) materials are often used in place of polycrystalline (PC) materials since they derive their strength from the large directionally-oriented crystallographic grains. As a consequence, the propagation of microstructurally small cracks is controlled by certain microstructural features such as dendrite size, orientation and morphology, carbides in the interdendritic regions, oxide and gamma prime depleted layers. In this study a cumulative damage model capturing the key microstructural features influencing crack initiation is used to characterize a DS Ni-base superalloy with application to both longitudinal (L) and transverse (T) orientation. Both L and T specimens were used to carry out uniaxial ε-controlled isothermal and thermomechanical fatigue (TMF) tests. The mechanistically-based model presented captures the combined effects of fatigue, creep, and oxidation on the formation of low cycle fatigue (LCF) cracks and the microstructurally small crack growth phase, both of which comprise the crack "initiation" stage. Key parameters that relate to the microstructure and affect the crack initiation life are included in the model.