Rapid solidification of highly undercooled Si and Si-Co melts

We present direct experimental observations of growth velocity and morphology in undercooled melts of pure Si and dilute Si100-xCox alloys (x=0.1, 0.5, and 2.5 at. %). An anomalous behavior in the growth velocity-undercooling relation of the Si-Co alloys is found, exhibiting an abrupt break at a critical undercooling Delta T-* which depends on Co concentration. In contrast to pure Si, the anomalous growth behavior in Si-Co alloys cannot be described by current dendrite growth theory even if nonequilibrium effects at the interface as solute trapping are taken into account. The growth morphology changes essentially with increasing undercooling and Co concentration. (c) 2006 American Institute of Physics.