Intergranular penetration of liquid bismuth was analysed in two pure metals, Cu at 500C and Ni at 700C; prepared either as polycrystals or as oriented bicrystals. At the liquid/solid interface, large grooves developed in Cu–Bi, while micrometer-thick films were observed in Ni–Bi. The bismuth concentration measurements obtained by Auger electron spectroscopy indicated a zone of monolayer Bi segregation followed by a diffusion-type profile over a distance of the order of 100μm for Cu–Bi and a nanometer-thick film followed by similar diffusion-type profile for Ni–Bi. In both cases, the kinetics of intergranular penetration and embrittlement was shown to be parabolic. It was concluded that no wetting occurred in the Cu–Bi system at 500C, while Bi wetted Ni at 700C. It was postulated that the mechanism of intergranular penetration operated at the very tip of the penetration front; as opposed to the tip of the liquid Bi film observed by scanning electron microscopy, and must be based upon diffusion rather than wetting phenomena. It was concluded that research in the area of intergranular penetration could be divided into two aspects: grain-boundary wetting above the wetting transition temperature and grain-boundary diffusion below it.

Grain Boundary Diffusion and Wetting in the Analysis of Intergranular Penetration. K.Wolski, V.Laporte: Materials Science and Engineering A, 2008, 495[1-2], 138-46