Abstract: 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 $4.2\times10^4$K/s and undercooling of up to 240K (0.35$T_{\rm E}$) 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.

Key words: growth, morphology, undercooling, coupled zone, free fall