Phase separation and solidification of immiscible metallic alloys under low gravity

Abstract

Alloy melts being immiscible in the liquid state separate on earth rapidly by Stokes sedimentation of the droplets of the minority phase being nucleated and grown during cooling through the miscibility gap. Their Marangoni motion is, however, of similar importance to the evolving microstructure during a casting process, since ahead of the solidification front temperature gradients exist. The joint action of Stokes and Marangoni-motion determines the microstructure of castings on earth. In space using the EuroMIR'95 mission and parabolic flights in fall of 94 and 95 we investigated the effect of Marangoni motion alone on the microstructure during casting and directional solidification of AlPb, AlBi and AlPbBi alloys. The experimental results on earth and under low gravity are described with a population dynamical model analytically and with a numerical model. The experiments and the quantitative description of the microstructure give new insights into the role of Marangoni motion and coagulation processes between droplets.