Effect of Stress Ratio on High Cycle Fatigue Behavior of a Single Crystal Superalloy

A series of axial force controlled fatigue tests at 1070 degrees C were performed to elucidate the influence of stress ratio (-1/-0.33/0.1/0.5/0.8) on high cycle fatigue behavior of DD6 single crystal superalloy. Fracture morphology and microstructure of these samples were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) in order to analyze the fracture micro-mechanism at different stress ratios. The results indicate that the high cycle fatigue life of DD6 alloy with different stress ratios decreases with the increase of stress amplitude and mean stress. When the stress amplitude is constant, the fatigue life of DD6 alloy decreases with the increase in stress ratio. The fatigue life of DD6 alloy increases with the increase in stress ratio when the mean stress is constant and the stress ratio is less than 0.5. However, when the mean stress increases to a certain extent, the stress ratio has no significant effect on the fatigue life of the alloy. Analysis on fracture surfaces of DD6 alloy at 1070 degrees C demonstrates that a quasi-cleavage mode is observed under the condition of low stress ratio, while a dimple mode is found at high stress ratio. The fatigue fracture mode under medium stress ratio has the mixing characteristics of the above mentioned modes. High temperature oxidation under low stress ratio is helpful to the initiation and propagation of cracks.