Fatigue crack initiation in lead-bismuth eutectic and its effect on the cyclic stress behaviour of austenitic stainless steel 316L

Active media such as liquid metals have an effect on the fatigue resistance of steels when compared to vacuum or inert environments. This effect might impact the initiation of fatigue cracks, their propagation, or both. This paper focusses on the initiation of fatigue cracks in an austenitic stainless steel when in contact with three environments: vacuum, air, and lead-bismuth eutectic (LBE), and the impact of such cracks on cyclic loading behaviour. Solid cylindrical samples of a 316L-type steel were fatigued under strain control in vacuum, air, and LBE in order to induce the initiation of cracks on their surface. The characteristics of fatigue damage in the form of surface microcracks were analysed with microscopy techniques. It was found that the nucleation of fatigue microcracks is enhanced by LBE, but the majority of these cracks do not propagate beyond the grain size of the steel (50 μm). An analysis with finite element methods showed that the large number of small (<10 μm) microcracks nucleated in LBE environments has a negligible impact on the sample's stiffness, as opposed to the fewer but deeper (100–500 μm) microcracks that initiate in air, which produce a loss of stiffness, observable on the mechanical stress response of the fatigue sample. A model based on the adsorption of LBE atoms along surface slip bands is proposed to explain the phenomenon of enhanced fatigue crack nucleation in LBE.