ARTICLE:

• EFFECTS OF BUOYANCY ON THE GROWTH OF DENDRITIC CRYSTALS  download article

Youn-Woo Lee
CFC Alternatives Technology Center, Korea Institute of Science and Technology, Seoul, 136-791 Korea

Richard N. Smith
Department of Mechanical Engineering, Aeronautical Engineering & Mechanics, Rensselaer Polytechnic Institute, Troy, NY 12180

Martin E. Glicksman
Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180

Matthew B. Koss
Department of Materials Engineering, Rensselaer Polytechnic Institute, Troy NY 12180

ABSTRACT

Recent experimental and theoretical considerations concerning dendritic solidification phenomena are discussed, with a concentration on the thermal dendrite as a phenomenological model for dendritic growth in general. Following a historical account of the fundamental physics of diffusion controlled dendritic growth, the considerable experimental evidence that natural convection effects play a significant role in determining the tip growth kinetics and morphology is presented. Paradoxically, this effect is more pronounced at lower values of the solid-liquid temperature difference. Theoretical considerations of the convective transport field around a dendrite tip, for forced or free convection, seem to establish that the natural convection effects are governed by a Rayleigh number based on the dendrite tip radius and the melt supercooling. In addition, an analogy between forced and free convection is quite useful in understanding the phenomena and in interpreting data for dendrites growing under the effects of buoyancy-induced convection or forced convection. An important conclusion is that convection becomes important when the fluid flow velocity is on the order of the dendritic growth velocity. The motivation for conducting fundamental experiments under microgravity conditions is followed by preliminary results that have been obtained from the Isothermal Dendritic Growth Experiment (IDGE) conducted in March 1994 aboard space shuttle Columbia. These data show a substantial reduction in natural convection effects and may provide the basis for a new assessment of the fundamental physics of dendrite growth. The results suggest that there is a considerable convective effect at 1 g at higher supercoolings than was previously suspected, that there are measurable convective effects even in microgravity, and that the Ivantsov solution for diffusion-controlled growth, combined with a selection rule of VR² = const, is consistent with the observed dependence on supercooling of growth velocities in microgravity in a diffusion-only regime.
Concluding remarks address some very recent considerations of the IDGE data in terms of additional analyses and experiments which have appeared in the literature [79,80]. Several very recent citations by the authors [81−84] are also noted for completeness of the presentation.

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