Abnormal growth relationship between Si and other phases in undercooled Al–Cu–Si alloy

The dependence of the growth relationship between Si and other two phases on undercooling was investigated in terms of microstructural characteristics in Al77.2Cu13.6Si9.2 ternary alloy. As undercooling increases, the independent growth of faceted Si phase and the cooperative growth of Al and θ phases are replaced by the attaching growth of Al phase on faceted Si phase in the ternary eutectic. At the same time, θ phase and a small amount of non-faceted Si phase tend to grow cooperatively in both pseudobinary and ternary eutectics. Once undercooling exceeds a critical value, the independent nucleation and growth of the three non-faceted phases result in anomalous ternary eutectic.

► Growth relationship among phases is influenced by undercooling ΔT in Al–Cu–Si alloy.
► Faceted Si phase grows alone whereas Al and θ phases grow cooperatively.
► As ΔT increases, the attaching growth of Al phase on faceted Si phase was found.
► The three non-faceted phases nucleate and grow independently if ΔT is high enough.