Microstructure evolution of Cu-Pb monotectic alloys processed in drop tube

The rapid solidification of Cu-Pb monotectic alloys has been accomplished during free fall in a 3 m drop tube. Both macrosegregated and uniformly dispersed structures are observed in droplets of Cu-40 wt pct Pb alloy, whereas droplets of Cu-64 wt pct Pb alloy exhibit only macrosegregation morphologies. The microstructure is strongly dependent on the droplet size. The higher undercooling tends to facilitate the liquid phase separation and results in more extensive macrosegregation in smaller droplets. There exists a pronounced tendency for the Pb-rich liquid to occupy the surface of the droplets of both compositions, resulting from the quite lower surface tension of the Pb-rich phase and causing a Pb-rich layer at the surface of the solidified droplet. The nucleation of monotectic cells in the Cu-40 wt pct droplets with dispersed structures preferentially occurs at the droplet surface. The single nucleation event takes place more frequently as the droplet size is reduced.