CuAl8 solder on S235JR, liquid metal induced stress corrosion cracking

The microstructural pictures impressively show the diffusion of Cu into Fe.
Iron has a decreasing solubility of maximum 2.2 wt.% Cu at 850 °C. The Cu diffuses along the grain boundaries of the iron and embrittles them. In the crack, the Cu concentrates to approx. 90 %.