Rapid Solidification Processing of Ag-5%Bi Alloy under Microgravity Condition inside Drop Tube

Liquid Ag-5% Bi alloy has been rapidly solidified in the form of 84-1140 mu m droplets under free fall condition. The results show that there are dendritic (Ag)(1) phase, interdendritic (Ag)(2) phase and (Bi) solid solution phase in this alloy respectively. The microstructure of (Ag)(1) phase displays a conspicuous "dendritic-equiaxed" morphology transition with the decrease of droplet diameter, which is attributed to the fragmentation of dendrites induced by the recalescence of highly undercooled liquid alloy. The theoretical analysis with the LKT/BCT dendritic growth model predicts that the growth velocity of (Ag) dendrite increases with undercooling, and the kinetics transition from solute diffusion-controlled growth to thermal diffusion-controlled growth occurs at 98 K during solidification. Meanwhile, the remarkable solute trapping effect takes place according to theoretical calculated calculation.