Model predictions for the dendrite growth velocity at low undercoolings are deviating significantly from experimental data obtained in electromagnetic levitation with a capacitance proximity sensor (CPS) [K. Eckler, D.M. Herlach, Mater. Sci. Eng. A 178 (1994) 159]. In addition to that, previous data sets obtained by different techniques are not in good agreement with each other. For instance, growth velocity data for nickel melts obtained with a high-speed camera system [D.M. Matson, in: Solidification 1998, TMS, Warrendale PA, 1998 show higher values at low undercoolings than data obtained with the CPS. Within this work new measurements of dendritic growth velocity in levitated undercooled nickel samples were performed as a function of undercooling DT to investigate this discrepancy. Solidification of the undercooled melt was detected at undercooling levels within the range of 30 KoDTo300 K. The new data reveal high accuracy and low scattering. These data are compared with two independent growth velocity data sets and discrepancies are discussed. For verification of the new CPS data dendrite growth velocity was also measured by using a high-speed camera where the morphology of the intersection of the solidification front with the sample surface was investigated. The new experimental data are analyzed within the model of dendrite growth obtained on the basis of Brener's theory [E. Brener, J. Crystal Growth 99 (1990) 165] and the model of dendrite growth with melt convection in a solidifying levitated drop, presently being developed. Special attention is paid to the effects of convection and small amounts of impurities on the growth dynamics at small undercoolings. r

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The purpose of the present work was to measure the velocity of dendrite growth in undercooled Ni–Al alloy melts as a function of undercooling. The experiments were performed both by containerless electromagnetic levitation on Earth and under reduced gravity conditions during parabolic flight campaigns. While under terrestrial conditions, strong magnetic fields are required to compensate the gravitational force, the forces to compensate disturbing accelerations are decreased by orders of magnitude in reduced gravity. In turn, the alternating electromagnetic fields induce convection, which is strong under terrestrial conditions while much weaker in reduced gravity. The heat and mass transport in front of the solid-liquid interface during solidification controls the dynamics of dendrite growth. By comparing results obtained on Earth and in reduced gravity, it was demonstrated that the change of transport conditions by convection significantly alters the kinetics of solidification and the evolution of grain refined microstructures at undercoolings less than 100K.

Solidification from the undercooled melt is dominated by nucleation and subsequent crystal growth. For chemically ordered intermetallics, the kinetics of crystal growth can be markedly different from that of disordered solid solutions, manifested as substantial interfacial undercoolings or anomalous partitioning behaviour. In the present work the electromagnetic levitation technique is applied, as a containerless processing route, to melt and deeply undercool Ni-Al alloy melts of various compositions -mostly within the homogeneity range of the B2 (b) phase. The ensuing rapid solidification of the undercooled specimen is directly observed by time-resolved temperature measurements and monitored by a high-speed video camera. This allows for direct investigations of crystal growth velocities as a function of composition and undercooling. The experimental results show that the growth of stoichiometric B2 phase (NiAl) is -contrary to common expectations -more sluggish as compared to that of Ni-rich alloys, despite the hindering effect of solute rejection on the growth rate which seems to occur only in the latter. These findings are interpreted within current models of crystal growth taking into account non-equilibrium effects due to partial 'disorder trapping' at the solid-liquid interface.