Unveiling liquid Pb-Bi embrittlement of 316LN stainless steel under fatigue crack propagation tests through multiscale advanced characterization

Abstract

316LN austenite stainless steel (AuSS) is a candidate structural material for the reactor vessel in lead-bismuth eutectic (LBE) cooled fast reactor. However, the potential risks of liquid metal embrittlement (LME) and the associated damage mechanisms for 316LN AuSS in LBE remain poorly understood. In this study, the micro-mechanisms of fatigue crack growth (FCG) of 316LN AuSS in oxygen-saturated liquid LBE at 200–400 °C were investigated by multiscale advanced characterization techniques. The FCG rate increased significantly when the LBE temperature exceeded 300 °C and the stress intensity range (ΔK) surpassed approximately 20 MPa·m^0.5. Initially, 316 LN AuSS exhibited ductile cracking, which subsequently transitioned to quasi-cleavage and ultimately to cleavage cracking. As the crack propagated, the interaction between plastic deformation and liquid LBE wetting induced the evolution of the microstructures ahead of the crack tip, accompanied by an increase in LME sensitivity. The crack preferentially propagated along the deformation-induced microstructural interfaces, including twin boundaries and planar dislocation bands at cleavage crack tip. Liquid Pb-Bi atoms preferentially segregated at these microstructural interfaces, promoting brittle cracking. Even trace amounts of Pb atoms doped on the high-density dislocation walls can promote the interfacial cleavage cracking under conditions of high stress concentration ahead of the sharpened crack tip.