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 ΔT, the microstructural evolution has shown a transition from lamellar eutectic to anomalous eutectic. A maximum cooling rate of 4.2 × 10⁴K/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 ΔT*₁ =101K and ΔT*₂ = 178K. When ΔT ≤ ΔT*₁, the microstructure for Al-51.6% Ge eutectic shows lamellar eutectic. If ΔT ≥ ΔT*₂, the microstructure shows anomalous eutectic. In the intermediate range of ΔT*₁ < ΔT < ΔT*₂ the microstructure is the mixture of the above two types of eutectics.