Liquid metal embrittlement studies on model systems with respect to the spallation target technology: the importance of nano-metre-thick films

Liquid metal embrittlement (LME) is illustrated on the Cu-Bi and Cu-PbBi systems at 300 deg C using either constant strain-rate tests at 10-4 s-1 or constant load tests at 25% of yield stress. Intergranular penetration was studied in the Ni-Bi system at 700 deg C and was shown to result in the formation of slowly growing micro-metre-thick and rapidly growing nano-metre-thick films. Both induce very strong intergranular brittleness but only micro-metre-thick films are visible by SEM on polished cross-sections. Nano-metre-thick films were analysed by Auger electron spectroscopy after 'in situ' fractures within the spectrometer; in particular, a series of analyses on an Ni bicrystal proved the constant thickness of this film over several hundreds of microns. The severity of embrittlement due to bismuth penetration was confirmed in the analysis of the Ni-PbBi system. Based on these results, il is underlined that technological systems like T 91 steel / Pb or T91 / PbBi should work at temperatures below the wetting transition temperature (Tw), i. e. in the temperature range where intergranular films can't form. If it is not the case, nano-metre-thick films should be taken into account, both in the evaluation of the kinetics of embrittlement and in the modelling of intergranular penetration and LME. (authors)