Fatigue behavior in monocrystalline Ni-based superalloys for blade applications

Ni-based superalloys are used for turbine airfoil applications due to their excellent high temperature properties. Alloy design has historically focused on creep resistance as the critical design limiting parameter. Recently, the focus has shifted to include fatigue resistance, resulting in the need to better understand the effects of alloy microstructure on fatigue crack initiation and propagation. In this study, sustained peak low cycle fatigue tests of coated monocrystalline René N5 specimens oriented with the loading axis along [001] were conducted in strain control in the temperature range of 980-1100°C Tests were performed with either a tensile dwell (hold) time or a compressive dwell time. Fracture surfaces were characterized via scanning electron microscopy (SEM). Microstructural analysis was performed using a high resolution SEM. Far field deformation mechanisms were examined perpendicular to the loading axis and parallel to the loading axis using transmission electron microscopy. The occurrence of creep-fatigue interaction was characterized by the different types of rafting seen, and the deformation mechanisms varied between specimens with compressive hold time and tensile hold time. This project is a collaborative effort with GE Aviation and is funded by the AFOSR under the MEANS2 program.