Dendrite growth during rapid solidification of Co-Sb alloys

Undercooling experiments were performed with Co-10%Sb and Co-60.5%Sb alloys in order to investigate the dendrite growth kinetics within the solutal and thermal diffusion-controlled growth regimes. These alloy compositions correspond to the two extremes of eutectic horizontal line and hence have the broadest equilibrium freezing temperature ranges. The maximum undercoolings obtained for these alloys are 274 K (0.16 T_L) and 168 K (0.11 T_L), which have either approached or exceeded their equilibrium freezing temperature ranges. The dendrite growth velocities were measured as a function of undercooling. The experimental results were analyzed within the current theories of dendritic growth. The results indicate that α-Co solid solution phase attains a growth velocity of 60 m s^-1 at 274 K undercooling, whereas β-CoSb intermetallic compound grows more slowly with a maximum velocity of 12 m s^-1. The investigations reveal a transition from solute diffusion-controlled growth to thermal diffusion-controlled growth by the onset of solute trapping at a critical undercooling. It is approximately 150 K in the case of Co-10%Sb alloy and 130 K for Co-60.5%Sb alloy. A 'dendrite-equiaxed' morphological transition is initiated once these alloys are undercooled by more than 100 K. This is interpreted within a recently developed model of dendrite fragmentation as a Rayleigh-like instability of rapidly growing dendrites.