Change in Failure Behavior and Fatigue Life of Single-Crystal Ni-Based Superalloys Under Thermomechanical Fatigue Loading

This study discusses the thermomechanical fatigue (TMF) behavior of a first-generation Ni-based single-crystal superalloy. Tests were conducted for in-phase (IP) and out-of-phase (OP) TMF under identical strain conditions within 450-950 degrees C. The failure characteristics were investigated by studying the microstructural changes that occur because of TMF damage. During IP TMF, microscopic cracks were initiated at sites concentrated by localized stress within the material, where the gamma prime (gamma ' )-strength phase was dissolved, reducing the overall strength of the alloy. By contrast, the oxide layer formed on the surface makes it more susceptible to crack formation under OP TMF. The presence of deformation bands along the (111) crystallographic planes was attributed to the promotion of crack growth. The fatigue life of IP TMF was significantly longer than that of OP TMF. However, this difference decreased as the mechanical strain amplitude increased until a change in the trend occurred for the highest amplitude. The microstructures were analyzed in detail to discuss the differences in the changes of gamma' morphology during IP and OP TMF.