Abstract
The objective of this study was to clarify the effects of creep and oxygen-related damage induced by tensile dwell at high temperature on fatigue crack propagation in a polycrystalline Ni-base superalloy, Alloy 718. Crack propagation tests with single tensile dwell applied during fatigue loading were performed at 650 °C. Five tests were conducted, involving various stress intensity levels and different dwell times of the tensile dwell, to observe the creep and oxygen effects on the fatigue crack propagation behavior. When the stress intensity level is high, acceleration of the fatigue crack propagation occurred after the tensile dwell. On the other hand, when the stress intensity level is low, retardation of the fatigue crack propagation occurred following temporary acceleration after the tensile dwell. Parallelly, the same experiments, but interrupted right after the tensile dwell, were conducted to observe the damage zone ahead of crack tips using a scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). By comparing the damage zone observed by the SEM and EDS and the crack propagation rates affected by tensile dwell, it was revealed that the acceleration of the fatigue crack was caused by the oxygen-related damage along grain boundaries induced by the tensile dwell. Also, a transition from the retardation to the acceleration depending on the stress intensity levels was rationalized based on a size comparison between the damage zone and stress relaxation zone ahead of the crack tip caused by the tensile dwell.
