Solidification of Al-Pb-base alloys in low gravity

Multicomponent hypermonotectic Al-Pb-base alloys were solidified on earth under normal gravity and under reduced gravity conditions of a parabolic flight. The cast microstructure obtained in a reduced gravity environment is dominated by Marangoni motion of the Pb droplets and their shape by the interaction of the droplets with the dendritic zone. The cast microstructures are analyzed theoretically with respect to the Marangoni motion as the main factor of spatial phase separation. Some simple theoretical considerations are presented treating the microstructure evolution as a concurrent action of nucleation, diffusional growth and Marangoni motion of Pb and Bi droplets. The interaction of the minority phase droplets with the solidification front is discussed on the basis of the microstructural features and the ternary and quarternary phase diagrams of the alloys.