Microstructural Investigation of LME Crack Initiated in Ferritic/Martensitic Steel T91 Loaded in Liquid Lead-Bismuth Eutectic at 300 °C.

Liquid lead-bismuth eutectic (LBE) is one of the candidate materials for advanced nuclear systems. The structural materials used in contact with LBE are selected according to the resistance to liquid metal corrosion, irradiation embrittlement, and compatibility with the coolant. However, simultaneous presence of mechanical strain and LBE environment can induce liquid metal embrittlement (LME) in these materials. In this study, a specimen from candidate ferritic-martensitic steel T91 was tested by Constant Extension Rate Tensile (CERT) test exposed to PbBi environment with oxygen concentration 6 × 10-6 wt % at 300 °C up to rupture. Post-test examination using scanning electron microscopy (SEM) showed a deep crack indicating features of LME in the plastic strained region of the tested specimen. Further investigations focused on characterization of the fracture path and microstructure determination using focused ion beam (FIB) and energy dispersive X-ray spectrometry/electron backscatter diffraction (EDX/EBSD). This observation revealed that the dominant LME failure mode of the observed crack is translath or transgranular and the crack stopped at the high-angle grain boundary. The role of oxides in the crack initiation is discussed.