Containerless rapid solidification of undercooled Cu-Co peritectic alloys

Droplets of Co-16%Cu and Co-71.6% Cu peritectic alloys were solidified during containerless processing in a 3-m drop tube. The microstructures of Co-16%Cu alloy droplets were characterized by dendritic or equiaxed α-Co phase with a small amount of Cu-rich solid solution distributed on α-Co phase boundaries. Two thresholds of droplet diameter were observed for Co-16%Cu alloy at which "equiaxed-dendritic-equiaxed" morphological transitions occur to primary α-Co phase. This conspicuous refinement of primary α-Co grains results from the fragmentation of α-Co dendrites caused by recalescence effect. For Co-71.6% Cu alloy, the primary α-Co phase forms as coarse columnar dendrites in large droplets and equiaxed dendrites in small droplets. Theoretical calculations indicate that Marangoni migration contributes more to the growth of disperse Co-rich spheres by stimulating collision and coalescence than Stokes motion caused by the residual gravity in the falling Co-71.6% Cu alloy droplets.