The formed long needle like β Fe phase in Al Si alloy is harmful to mechanical properties of the alloy. It was found during directional solidification that fracture phenomenon of needle like iron phase in transition zone occurs. It was considered the mechanism of the fracture phenomenon is the same as split off of iron phase in non equilibrium heat treatment. But in the given condition

the needle like iron phase is held and solidified in a wider range temperature interval during directional salidification so as to quickly split into short one. Iron phase has been split into granular structure with near length/width ratio in some local areas. This kind of granular structure is less harmful to the properties of the alloy than the needle like iron phase

and even can be functional to strengthen structure.