The ferritic martensitic steel T91 is among the candidate materials for internal structural components of future Heavy Liquid Metal cooled reactors. The position of T91 on the list of the candidate materials is now being questioned due to the evidence of its sensitivity to liquid metal embrittlement (LME), mainly in PbBi eutectic. Therefore, a renewed interest is currently focused on austenitic steels, in particular on 1.4970 steel, as there is a lack of evidence of LME for them.The main goal of this work was to provide an insight into crack initiation phenomena in Liquid Metals for the two different steels and the evaluation of possible mechanisms initiating and propagating the cracks.Slow Strain Rate Tensile (SSRT) tests were carried out with flat specimens in PbBi at 300 °C with low oxygen content (about 10-8 wt. %) and, for comparison in air, up to the Ultimate Tensile Stress (UTS) point. Tests were performed with flat tapered specimens, which were meant to create a uniform variation of stress along the gauge length, with the maximum stress concentrated in the smallest area. This specimen geometry can be used for determination of the threshold stress of the crack initiation.The behaviour of the two materials was affected by the environment, as their cracking mode changed in PbBi when compared to the testing in air. Post-tests examinations were carried out with a Scanning Electron Microscope (SEM) equipped with Focused Ion Beam (FIB). The role of oxides formation and microstructure is discussed and correlated to the initiation of cracks.Under these experimental conditions 1.4970 steel was not affected by the environment, as small plastic cracks were observed around the Ti-rich precipitates in both air and PbBi.By contrast T91 steel showed a market tendency to the crack initiation and rapid growth in PbBi.Keywords: ferritic-martensitic steel, austenitic steel, lead-bismuth eutectic, tapered, crack initiation
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