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 inte rface velocity v, the interfacial undercooling Delta T-i increases and solute trapping becomes important. These phenomena are well character ized for planar interfaces, but for dendritic growth they must be inco rporated into dendrite growth theory and the combination tested. The p redictions of dendrite growth theory are very sensitive to the diffusi ve speed-the interface speed at which the solute trapping function is in mid transition between local equilibrium and complete trapping. Den drite growth velocities have been measured as a function of undercooli ng in levitated droplets of Ni99Zr1 alloys. The results are described within current theory of dendrite growth taking into account deviation s from local equilibrium. The diffusive speed is independently determi ned by preliminary pulsed laser melting experiments on thin film speci mens for the same alloy system. Best fit values of the diffusive speed inferred from both sets of measurements are similar in magnitude. Giv en the preliminary nature of the pulsed laser melting measurements, th is result is encouraging for the prospects of a parameter-free test of modern dendrite growth theory. (C) 1997 Elsevier Science S.A.