The effects of size and concentration of droplet on the microstructure of rapidly solidified Cu-Co alloy in drop-tube have been investigated.The experimental results demonstrate that Co-rich particle in the droplet becomes more dispersive with the decrease of the droplet size.With Co concentration approaching towards the critical composition of metastable miscibility region

the liquid-liquid phase transformation is easier to be created in the molten alloy

and the average radius of Co-rich particle becomes bigger and bigger in the solidified microstructure.Co-rich particle is nucleated in the droplet and taken Marangoni movement towards high temperature region in center of droplet with temperature gradient during liquid-liquid phase transformation

resulting in the formation of Co-poor on the surface of the powder.