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Dendrite growth velocities have been measured as a function of undercooling in droplets of dilute alloys of B in Ni undercooled by up to 306 K using an electromagnetic levitation technique. At a critical undercooling, depending strongly on the B concentration, a steep rise of the dendrite growth velocity is observed. The analysis of the results within current theories of dendrite growth gives evidence for a transition from diffusion-limited to purely thermally controlled growth with partitionless solidification as a consequence.Rapid solidification processing is a well-established method for the preparation of metastable materials, giving access to a range of materials properties, in particular supersaturated solid solutions. Here the crystal-growth velocity is the decisive parameter governing the trapping of solute in the solvent material. ' Rapid solidification is usually achieved by rapidly quenching the melt in techniques such as melt spinning or atomization. However, such experiments do not allow direct investigations of the growth process. They are restricted to analyses of the already solidified products, which are subject to considerable modification of the material through, e.g. , aging processes.Alternatively, rapid solidification conditions prevail even at slow cooling rates of bulk melts, provided that a substantial amount of undercooling is achieved prior to solidification. The high driving force for crystallization accumulated in this way leads to rapid crystallization, which can be observed quantitatively. In the present case, an electromagnetic levitation technique has been used to avoid heterogeneous nucleation on container walls, with the extra benefit of a suspended drop accessible to direct observation and even external stimulation of solidification.The dominant crystal-growth mechanism in undercooled melts is dendritic growth, which has attracted recent attention, both theoretical ' and experimental. "' In particular, measurements of the dendrite growth velocity as a function of undercooling on levitation undercooled Cu-Ni alloys' have revealed that nonequilibrium effects at the crystallization front play a decisive role in rapid solidification. The measurements indicated a velocity dependent partition coe%cient as predicted by theory at high solidification velocities, '" in agreement with laser heating investigations. ' In this work, we report measurements of dendrite growth velocities as a function of undercooling on levitation undercooled dilute alloys of Ni-8 which, in contrast to previously investigated alloys, are characterized by a small equilibrium partition coeScient kE «1. Such a system exhibits an extremely high sensitivity of the partition coeScient on the growth velocity, therefore allowing a direct observation of a theoretically predicted transition' ' from diffusion-controlled dendrite growth to the thermally controlled regime. This is evidenced by the observation of a sudden rise in the growth velocity at a critical undercooling temperature 5 T'. Undercoolings above the crit...

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Co–Pd alloy melts are characterized by a low heat of fusion and, as a consequence, by a comparably small critical undercooling for the hypercooling limit ΔThyp of the order of 300 K. It is shown that containerless processing of bulk melts by electromagnetic levitation offers undercooling levels of ΔT≈350 K thus exceeding the hypercooling limit considerably. Solidification of undercooled melts from the hypercooling regime leads to rapid crystallization of the entire sample under nonequilibrium conditions. The electromagnetic levitation technique in combination with time-resolved recalescence detection was used to measure the growth velocity of Co–Pd alloy melts as a function of undercooling prior to solidification. The growth velocities of undercooled metallic melts were measured in an undercooling range exceeding the hypercooling limit. The experimental results are discussed within current theory of dendritic growth.