Dendritic growth velocity and diffusive speed in solidification of undercooled dilute Ni-Zr melts

During rapid solidification of undercooled melts deviations from local equilibrium occur at the solid-liquid interface. With increasing interface velocity v, the interfacial undercooling ΔY_i increases and solute trapping becomes important. These phenomena are well characterized for planar interfaces, but for dendritic growth they must be incorporated into dendrite growth theory and the combination tested. The predictions of dendrite growth theory are very sensitive to the diffusive speed - the interface speed at which the solute trapping function is in mid transition between local equilibrium and complete trapping. Dendrite growth velocities have been measured as a function of undercooling in levitated droplets of Ni₉₉Zr₁ alloys. The results are described within current theory of dendrite growth taking into account deviations from local equilibrium. The diffusive speed is independently determined by preliminary pulsed laser melting experiments on thin film specimens for the same alloy system. Best fit values of the diffusive speed inferred from both sets of measurements are similar in magnitude. Given the preliminary nature of the pulsed laser melting measurements, this result is encouraging for the prospects of a parameter-free test of modern dendrite growth theory.