Rapid solidification of Al-18%Si hypereutectic alloy in drop tube

The drop tube technique was performed to achieve rapid solidification of undercooled Al-18% Si hypereutectic alloy. The droplets ranging from 60 similar to1 000 mum in diameter were obtained. The regular polygonal primary Si and lamellar eutectic homogeneously distribute on alpha (Al) matrix in the droplets larger than 500 mum. While in the droplets smaller than 500 mum the five-star primary Si was found, which is often accompanied by some spherical eutectic grains. The different morphologies of primary Si are due to varied undercoolings. Scanning electron microscopy suggests that the spherical eutectic grain is composed of anomalous eutectic in its core and lamellar eutectic radiating outside from its periphery. Such eutectic microstructure is presumed to be the result of combining large undercooling, microgravity with containerless processing during free fall.