Intergranular penetration and embrittlement of solid nickel through bismuth vapour condensation at 700°C

Grain boundary penetration of liquid bismuth in polycrystalline nickel is investigated at 700°C. The contact between the two metals is ensured by bismuth transport through vapour phase. The formation of a nanometre-thick Bi-rich layer on external surfaces of solid nickel is revealed by glow discharge optical spectroscopy. This layer is the consequence of Bi vapour condensation on nickel substrate at 700°C. The liquid Bi/solid Ni contact leads to grain boundary penetration of liquid bismuth in the form of a film of nanometric thickness as revealed by Auger electron spectroscopy. The presence of these intergranular films causes strong room temperature brittleness of nickel, as shown by tensile and bending tests. The comparison with results due to the direct contact between solid nickel and bulk liquid bismuth-rich alloy indicates that in both cases intergranular penetration rate and embrittlement are of the same magnitude. Based on these results, a concept of a new device for liquid metal embrittlement (LME) tests is outlined.