Coarsening of liquid AlPb dispersions under reduced gravity conditions

In the multifurnace assembly (MFA) of the free-flying satellite EURECA, experiments were performed with molten AlPb alloys to investigate Ostwald ripening of liquid-liquid dispersions under diffusive conditions free from any sedimentation effect. AlPb alloys with very low volume fractions of dispersed phase (0.75 and 1.1 vol.%) were processed by annealing cylindrical samples within the liquid miscibility gap at 1073 K for 1, 5 and 20 h. The initial droplet size distribution was centered around 0.5 μm diameter. After the annealing treatments in the MFA a coarsening of the droplets to sizes around 5–10 μm was observed. The droplets were, however, not evenly distributed in the samples. There was some drift which can be attributed to Marangoni motion caused by small temperature gradients (0.5-3 K cm−1) during the annealing treatment and also the heating and cooling periods. The microstructural observations agree with similar results obtained in Spacelab 1 with ZnPb alloys. The coarsening is a superposition of diffusional Ostwald ripening and coagulation of droplets by Marangoni motion and discussed with a model based on population dynamics.