Liquid lead-bismuth eutectic enhanced intrusion/extrusion formation and early crack initiation in T91 steel under low-cycle fatigue

Abstract

The fatigue life of T91 steel in liquid lead-bismuth eutectic (LBE) environment is significantly reduced by the liquid metal embrittlement (LME) effect. While most studies focus on the fatigue crack propagation behavior, the fatigue initiation mechanisms influenced by the LME effect is less explored and unclear, especially at the early stage where localized plasticity predominates. In this study, full-life low-cycle fatigue and interrupted fatigue experiments were carried out on T91 steel at 350 °C and the stress amplitude of 360 MPa, in both air and oxygen-saturated (OS) LBE. The density, length, and severity of extrusions/intrusions in liquid LBE is found to be significantly higher than those in air, suggesting that LME affects fatigue initiation by promoting localized plasticity (e.g. extrusions/intrusions formation). Intrusions and extrusions in OS LBE predominantly occur within grain interiors characterized by higher dislocation density beneath the surface and preferential Pb-Bi infiltration. Enhanced dislocation mobility accelerates LME through two synergistic pathways: vacancy generation via dislocation interactions enables Pb-Bi penetration, and dislocation-mediated plasticity facilitates surface adsorption. Therefore, the enhanced formation of intrusions/extrusions in liquid LBE can be primarily attributed to a synergistic mechanism involving adsorption-induced reduction in cohesion, adsorption-enhanced dislocation emission, and dislocation-facilitated Pb atom infiltration. In addition, while the oxide film preserves the structural integrity, oxidation simultaneously degrades both the substrate material and intrusion tips, accelerating the evolution of the intrusion towards initial crack.