A model for slip irreversibility, and its effect on the fatigue crack propagation threshold in a nickel-base superalloy

A theoretical model for slip irreversibility in a polycrystalline nickel-base superalloy has been developed. This alloy deforms in a planar manner, and the slip irreversibility parameter was based on the fraction of dislocations which exit the free surface as a result of the loading. Defined in this manner, the slip irreversibility parameter can take values between zero, which corresponds to fully reversible slip and one, which denotes fully irreversible slip. It has been demonstrated that existing irreversibility models, which take into account the irreversibility of surface strains, are actually special cases of our general model. The theoretical calculations were compared to experimental observations of slip offsets resulting from a single stroke compression test in this superalloy. It was found that slip irreversibility increased as temperature increased and it has been proposed that this variation causes the observed decrease in fatigue crack propagation threshold values with increasing temperature.