Composition-dependence of core-shell microstructure formation in monotectic alloys under reduced gravity conditions

Solidification microstructures have been examined in Cu-Pb and Fe-Sn alloys over a wide range of hypermonotectic compositions in which the core-shell structure was produced under reduced gravity condition in a 3 m drop tube. The contributions of alloy composition to the formation potential of core-shell microstructure were determined. It was found that the core-shell structure was obtained in the alloys with compositions near and on the left side not far away from critical point and it could be double- and triple-layer. The low melting point phase, Pb-rich phase in Cu-Pb alloys and Sn-rich phase in Fe-Sn alloys, always forms the shell in two-layer structure and it also forms the core in the three-layer structure. The Marangoni velocity was calculated to show the migration characteristics. The alloys with the critical composition experiences the longest migration period partly due to the less time consuming of the spinodal decomposition than the nucleation and partly due to the deepest miscibility gap, so that the core-shell microstructure is produced easily. With the composition departs from the critical point to the two sides in the miscibility gap, the formation ability of the core-shell structure decreases. The small migration velocity caused by the different thermophysical parameters of the two liquid phases for the alloy with the composition larger than the critical value leads to the difficulty for forming the core-shell structure.