Rapid eutectic growth in undercooled Al-Ge alloy under free fall condition

fEutectic growth in Al-51.6%wt Ge alloy has been investigated during free fall in a drop tube. With decreasing undercooling DeltaT, the microstructural evolution has shown a transition from lamellar eutectic to anomalous eutectic. A maximum cooling rate of 4.2x10(4)K/s and undercooling of up to 240K (0.35T(E)) are obtained in the experiment. The eutectic coupled zone is calculated on the basis of current eutectic and dendritic growth theories, which covers a composition range from 48%-59% Ge and leans towards the Ge-rich side. The two critical undercoolings for the eutectic transition are DeltaT(1)*=101K and DeltaT(2)*=178K. When DeltaT less than or equal to DeltaT(1)*, the microstructure for Al-51.6% Ge eutectic shows lamellar eutectic. If DeltaT greater than or equal to DeltaT(2)*, the microstructure shows anomalous eutectic. In the intermediate range of DeltaT(1)* < DeltaT < DeltaT(2)*, the microstructure is the mixture of the above two types of eutectics.