Abstract
A high cycle tension-tension fatigue test was conducted on specimens of different diameters of aged Inconel 718 superalloy after an ultrasonic surface rolling process (USRP). The results show that although all USRP treated specimens had similar compressive residual stress and hardening layer, the fatigue limit of small diameter specimens (3.4 mm) decreased considerably than that of non-USRP specimens with the same diameter, while that of larger diameter specimens (7.0 mm) increased. The larger the diameter was, the higher the fatigue limit (The diameter of specimens: φ3.4 mm ∼ φ7.0 mm). The fatigue life first exhibits an increasing tendency and then a decreasing tendency with increasing distance between the fatigue crack initiation position (FCIP) and the surface of specimen. Focused ion beam (FIB) analysis of the dislocation configuration in the wake regions of small fatigue cracks at different FCIPs showed that when the FCIP was close to the centre of the specimens where high stress triaxiality hindered emission and movement of the dislocation at the crack tip. The dislocation density and the size of the plastic zone were greatly reduced, and the stress concentration increased. These factors reduced the critical condition required for small crack propagation, and thus diminished the fatigue limit.
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