Initiation and early-stage growth of internal fatigue cracking under very-high-cycle fatigue regime at high temperature

The initiation and early-stage crack growth under very-high-cycle fatigue (VHCF) at room temperature, 750 degC and 850 degC on directionally-solidified Ni-base superalloy have been investigated. There was little frequency effect of 20 kHz on fatigue lives when compared with 100 Hz, nor did the deformation and fracture mechanisms. Dislocation tangles re-arranged themselves to form well-defined networks at interface of gamma/gamma-prime, accounting for the enhanced fatigue strength at 850 degC in VHCF regime when compared to that at 750 degC. In most cases, internal casting pore was the crack initiation site. Crack initiation and early-stage growth occurred on one of the {111} planes or their intersecting planes, a characteristic of Stage I cracking. With the use of optimized intermittent loading conditions, both the initiation and early-stage crack growth processes were successfully tracked on the basis of fine but visible beach marks within the Stage I cracking region. The first registered fatigue beach mark can be as close as only 86 um to the crack initiation site and the crack length increased steadily over the whole early-stage crack growth stage. The enhanced fatigue strength at 850 degC can be rationalized with the higher threshold for propagating the early-stage crack. The fraction of fatigue life consumed for early-stage crack growth reduces with the decreasing stress, eventually leading to the initiation-controlling VHCF fatigue failure. The implications of these results are discussed with respect to the model prediction of fatigue life and fatigue strength.