The penetration of liquid Cu into the grain boundaries of Fe was experimentally analyzed. During penetration, the Cu diffused into the Fe grains. In this case, a Kirkendall effect occurred and led to vacancies which diffused into the grain boundaries and condensed. A grain-boundary crack formed, which filled with liquid Cu. The driving force for the penetration of the Cu-liquid into the grain boundary was the condensation of vacancies and the surface/interface free-energy change.

On the Mechanism of Liquid Copper Penetration into Iron Grain Boundaries. H.Fredriksson, K.Hansson, A.Olsson: Scandinavian Journal of Metallurgy, 2001, 30[1], 41-50