Microstructure Evolution of Dendrite and Eutectic in Undercooled Al70ge30 Alloy

Al-Ge eutectic alloy, as a desirable bonding material, has been widely used in microelectromechanical systems (MEMS) packaging. The bonding property of Al-Ge eutectic alloy is closely related to their microstructures. However, the microstructure evolution and underlying mechanisms in undercooled eutectic Al-Ge alloy remain unclear. In this work, Al70Ge30 alloy was prepared and subjected to undercooling using a cycled superheating purification technology. The maximum undercooling can reach 90K (0.13TL). The solidified Al70Ge30 alloy exhibits concomitant dendrites and eutectic microstructures. The dendrites undergo rapid refinement at low undercooling and subsequently maintain nearly constant size as undercooling increases. Intriguingly, undercooling has a significant influence on eutectic microstructures. The eutectic microstructure transformed from feather structures to fishbone structures as undercooling increases then transformed into a line-like structure in eutectic cells. The evolution mechanisms of dendrites and eutectic structures are further elucidated based on several classical thermodynamic models. This study can provide theoretical and experimental guidance for the microstructure evolution of undercooled eutectic alloys with similar eutectic structures.