The low cycle fatigue behavior of the controlled transformation stainless steel alloy AM355 at 121, 204 and 315 °C

The axial, strain controlled, low cycle fatigue (LCF) behavior of the controlled transformation stainless steel alloy AM355 is investigated. The fatigue damage rate is determined as a function of temperature by postulating that the mechanism(s) responsible for the observed fatigue damage rates are thermally activated. The model yields an activation energy for fatigue damage that is significantly lower than the activation energies associated with either volumetric or grain boundary diffusion mechanisms in Fe-based alloys. The magnitude of the activation energy suggests that electric field assisted, low temperature oxidation controls the fatigue damage rate. The magnitude of the activation volume supports the idea that vacancy injection into the metal substrate, due to field assisted migration of metal ions to the oxide/gas interface, enhances crack nucleation as the test temperature increases. Fractographic examination of test samples indicates that increasing test temperature increases the number of surface crack initiation sites observed. The proposed model predicts the experimentally measured fatigue damage rates with reasonable accuracy. The mechanistic significance of the terms derived from the model are also discussed.