Crack Initiation Mechanisms in Dwell‐Fatigue of Forged Alloy 718 Ni‐Based Superalloy

The dwell‐fatigue behavior of forged Alloy 718 Ni‐based superalloy was investigated at 610 °C, with tests performed in stress‐controlled mode at for different hold times ranging from 1 to 90 s. The cyclic life decreases with the addition of a hold time compared to pure fatigue conditions. The plastic strain rate increases with the hold time. Furthermore, a plastic strain transition corresponding to the onset of plastic strain acceleration and identified at the minimum plastic strain rate was evaluated for each maximum stress applied. Fractographic analyses reveal that all crack initiation sites were either surface carbides/nitrides or surface grain boundaries. Interrupted fatigue tests were performed to follow the evolution of damage mechanisms along the gage part. Different crack initiation scenarios were proposed depending on the hold time.