Grain boundary diffusion, segregation and wetting in the analysis of intergranular penetration

Intergranular penetration of liquid bismuth has been analysed in two pure metals, Cu at 500°C and Ni at 700°C, used either as polycrystals or as oriented bicrystals. At the liquid/solid interface, large grooves have developed in Cu-Bi, while micrometer-thick films were observed in Ni-Bi. The bismuth concentration measurements obtained by Auger electron spectroscopy indicate a zone of monolayer Bi segregation followed by a diffusion type profile over a distance of the order of 100 µm 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 has been shown to be parabolic. It is concluded that no wetting occurs in Cu-Bi system at 500°C while Bi wets Ni at 700°C. It is postulated that the mechanism of intergranular penetration operates at a very tip of the penetration front, as opposed to the tip of liquid Bi film observed by scanning electron microscopy, and must be based on diffusion rather than wetting phenomena. Some suggestions are formulated for the future research in the area of intergranular penetration that can be split in two phenomena : grain boundary wetting above the wetting transition temperature and grain boundary diffusion below.