Dresden 2003 – wissenschaftliches Programm

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M: Metallphysik

M 5: Symposium: Phasenumwandlungen in Schmelzen III

M 5.1: Vortrag

Montag, 24. März 2003, 14:45–15:00, IFW A

High accuracy measurements of dendrite growth velocities in undercooled melts of pure Ni — •Oliver Funke, Phanikumar Gandham, Peter Galenko, and Dieter M. Herlach — German Aerospace Center, DLR, Inst. of Space Simulation, Linder Höhe, 51147 Köln, Germany

Previously reported experimental data on dendrite growth velocities in undercooled Ni melts obtained by different techniques exhibit a large scatter at low undercoolings (30K < Δ T < 100 K). In order to measure dendrite growth velocities with high precision and small scatter a standard electromagnetic levitation facility was equipped with a Capacitance Proximity Sensor (CPS). The CPS measures the time needed by a central dendrite to propagate through the entire levitated drop. Nucleation of the dendrite is externally triggered at well defined surface position of the sample and at well defined time recorded by the CPS. We describe the experimental setup and report on measurements of growth velocities in pure Ni obtained at low, medium and high undercoolings by application of the CPS technique. The high accuracy data determined by the CPS reveal unambiguously large deviations between experimental results and predictions of theory in the small undercooling range while at medium undercoolings a reasonable good agreement is found. The deviations are observed in the velocity range of dendrites which becomes comparable with the speed of fluid flow in electromagnetically processed melts. The deviations are discussed in terms of changes of heat and mass transport due to fluid flow during the growth of dendrites in undercooled melts. Preliminary results for other systems are also reported briefly.

This work is supported within the DFG Schwerpunktprogramm SPP1120.