Effect of Nb on the Growth Behavior of Co sub(3)Sn sub(2) Phase in Undercooled Co-Sn Melts

The growth behavior of the primary beta -Co sub(3)Sn sub(2) phase in (Co sub(67)Sn sub(33)) sub(100-x)Nb sub(x) (x = 0, 0.5, 0.8, 1.0) hypereutectic alloys at different melt undercoolings was investigated systematically. The growth pattern of the beta -Co sub(3)Sn sub(2) phase at low undercooling changes with the Nb content from fractal seaweed (x = 0, 0.5) into dendrite (x = 0.8) and then returns to fractal seaweed (x = 1.0) as a response to the changes in interface energy anisotropy and interface kinetic anisotropy. As undercooling increases, the dendritic growth of the beta -Co sub(3)Sn sub(2) phase in (Co sub(67)Sn sub(33)) sub( 99.2)Nb sub(0.8) alloy gives way to fractal seaweed growth at an undercooling of 32 K (-241 degree C). At larger undercooling, the fractal seaweed growth is further replaced by compact seaweed growth, which occurred in the other three alloys investigated. The growth velocity of the beta -Co sub(3)Sn sub(2) phase slightly increases at low and intermediate undercooling but clearly decreases at larger undercooling due to the Nb addition. The growth velocity sharply increases as the growth pattern of the Co sub(3)Sn sub(2) phase transits from fractal seaweed into compact seaweed.