The influence of microgravity on the solidification of Zn-Bi immiscible alloys

Abstract

Four experiments with alloys from the immiscible Zn-Bi system have been performed under microgravity conditions. Two alloys with a composition slightly within the miscibility gap (8 pct Bi) were cooled through the miscibility gap with two different cooling rates. It was found that Bi droplets were rather homogenously distributed in the samples, but the sizes of the droplets were somewhat larger than what would be expected from diffusion controlled growth. In samples with higher Bi contents (24 pct and 38 pct) larger areas of Bi-rich phase appeared in the center of the samples and as a layer around the samples. Calculations of the droplet size have been performed and compared with the experimental found droplet size. The calculated size was smaller and the difference was shown to depend on a collision coalescence. The layer around the samples was explained by a flotation of Bi-droplets to the surface.