Liquid Metal-assisted Fracture of the T91 Martensitic Steel

摘要

Use of liquid lead-bismuth eutectic (LBE) as spallation target or as cooling fluid in accelerated-driven systems rises the question of the reliability of structural materials in terms of liquid metal embrittlement, corrosion and corrosion-fatigue.rnThe paper analyses the risks of corrosion damage and of accelerated fatigue damage by liquid lead-bismuth eutectic (LBE) of the T91 steel and consists of three parts. Firstly, the corrosion of the T91 in LBE is studied at CEA paying attention on the oxygen concentration. This results in dissolution process when the oxygen concentration is low while a protective oxide film forms under high oxygen concentration. The second part is devoted to the experiments carried out at Lille University on the low cycle fatigue behaviour of the T91 steel. It is shown that the stress response to strain cycling is not influenced by LBE. However, the fatigue resistance is reduced by a factor at least of 2 when cycling at 300℃ in LBE instead of air. Finally, the third part takes into account at once the effects of corrosion damage and the effects of fatigue damage. A pre-immersion of T91 fatigue specimens in a LBE bath is carried out at CEA. The immersion temperature was 600℃ for about 600 h and the dissolved oxygen concentration less than 10~10 wt.%. The average relative loss of mass estimated from reference plates was less than 0.3% as a result of dissolution process. The specimens are then fatigued at Lille University in the as-received conditions (no modification of the specimen surface before fatigue testing). Fatigue tests are carried out with the same experimental procedure in LBE at 300℃. The results obtained on the pre-corroded specimens show that the fatigue life is reduced by a factor of about 10. The role of LBE is then discussed on the bases of SEM metallographic observations. LBE appears to be not only a source of microcracks in T91 when dissolution process occurs but promotes as well the growth of short cracks nucleated by cyclic plasticity or of the sharp defects induced by corrosion. Finally, an oxide layer formed in oxygen saturated LBE behaves like a protective coating against fatigue damage in LBE.rnThe present methodology and the obtained results are therefore of interest for innovative nuclear systems, such as Generation IV reactors, critical and subcritical transmutation systems and fusion devices.