Effect of Local Recrystallized Grains on the Low Cycle Fatigue Behavior of a Nickel-Based Single Crystal Superalloy

This paper aims to understand the effect of local recrystallization (RX) on the low cycle fatigue fracture of a turbine-blade single crystal nickel-based superalloy. The fatigue life of the single crystal superalloy was evidently decreased by local recrystallization. In single crystal specimens, casting porosity is the preferential fatigue crack initiation site, which is followed by crystallographic crack propagation along one or several octahedral slip planes. For all RX specimens, fatigue cracks preferred to initiate from local recrystallized grains and propagated through the recrystallized grains in a transgranular manner, followed by crystallographic crack propagation in the substrate single crystal superalloy. Moreover, fatigue tests indicated that locally recrystallized specimens exhibited temperature dependent fracture modes, i.e., transgranular cracking dominated at 550 degrees C, whereas intergranular cracking was preferred at 850 degrees C. Evident oxidation of fracture surfaces and strength degradation of grain boundaries at 850 degrees C was evidenced by scanning electronic microscopic observations. The present study emphasized the need to evaluate the effect of recrystallization according to the working conditions of turbine components, i.e., the local temperature.