MEASUREMENTS OF DENDRITE GROWTH VELOCITIES IN UNDERCOOLED PURE NI-MELTS - SOME NEW RESULTS

We used a recently developed technique to measure dendrite growth velocities as a function of undercooling in levitated metallic droplets. The data obtained exhibit a low degree of scatter and allow for more accurate comparison with theoretical predictions compared with other measurements. The results for pure Ni-melts and the predictions of the widely used dendrite growth model by Lipton et al. (J. Lipton, W. Kurz and R. Trivedi, Acta Metall., 35 (1987) 957) diverge significantly at low undercoolings in the range DELTAT < 80 K, and at high undercoolings DELTAT > 200 K. Taking into account laminar fluid flow induced either by forced convection in the bulk liquid or by the volume shrinkage on solidification, reduces the discrepancy at low undercoolings but does not lead to an agreement between experiment and theory. In contrast, the recently developed theoretical approach to the dendrite growth problem by Ben Amar (M. Ben Amar, Phys. Rev. A, 41 (1990) 2080) seems to be able to describe the experimental data in almost the entire undercooling regime accessible to the measurements. However, on the basis of the models and the data available, it is not possible, as yet, to favour one theory over the other.