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

Eutectic growth in Al-51.6%wt Ge alloy has been investigated during free fall in a drop tube. With decreasing undercooling \Delta T, the microstructural evolution has shown a transition from lamellar eutectic to anomalous eutectic. A maximum cooling rate of 4.2\times10^4K/s and undercooling of up to 240K (0.35T_\rm 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 \Delta T_1^*=101K and \Delta_2^*=178K. When \Delta T≤\Delta T^*_1, the microstructure for Al-51.6% Ge eutectic shows lamellar eutectic. If \Delta T≥\Delta T^*_2, the microstructure shows anomalous eutectic. In the intermediate range of \Delta T^*_1<\Delta T<\Delta T^*_2, the microstructure is the mixture of the above two types of eutectics.