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Cracking jumps were observed in notched T91 (9Cr-1Mo ferritic-martensitic steel) specimens tested in liquid lead-bismuth eutectic (LBE) at 350 °C under tensile loading. The cracking jumps manifested themselves as being regular steps at tensile curves, while as being fatigue striation-like markings at fracture surfaces. The markings were consisted of liquid metal embrittlement (LME) zones and ductile zones. The liquid LBE was adsorbed at crack tip and likely reduced the interatomic bonding strength, leading to cracking in a brittle way and release of elastic energy in the load line. This unique phenomenon was found to be associated with coupled effects of LME and compliance/stiffness of tensile machines.

The influence of displacement rate and temperature on the severity of liquid metal embrittlement (LME) was investigated by performing fracture toughness tests on the ferritic-martensitic T91 steel in liquid lead-bismuth eutectic (LBE) at three different displacement rates at 200 °C and 350 °C. Compact tension specimens were used for testing and all specimens were pre-cracked in LBE environment at 350 °C prior to testing. The energy normalization (EN) method was used to construct the J-R curves. J Q of each specimen was calculated and the fracture surfaces were investigated by scanning electron microscope (SEM). The results were compared to the tensile test results of the same material where reduction in total elongation was used to determine the susceptibility to LME. Comparison of tests at 200 °C and 350 °C showed that the LME effect was less prominent at 200 °C which was evidenced by the higher J -values obtained at this temperature and the fracture surfaces which overall had a significant distribution of dimples and larger secondary cracks. The observation of reduced LME effect at lower temperature was found to be in accordance with the tensile data within the strain rates discussed in this paper. Although the tensile test results show reduced LME effect as the strain rate is sufficiently reduced at 350 °C, the fracture toughness tests point to a more severe LME effect with decreasing displacement rate. An LME mechanism of T91 in LBE was discussed based on the observations given in this work. The fracture toughness values in LBE were found to be very low to support the selection of T91 material for the LBE cooled nuclear reactors. Moreover, as the obtained toughness values do not reflect the conservative estimation of the LME effect, incorporation of LME in the design of such reactors is not feasible in the current state.