Space Flight Data from the Isothermal Dendritic Growth Experiment

The Isokrmal Dendritic Growth Experiment (IDGE) is a NASA space flight experiment which flew as part of the United States Microgravity Payload (USMP-2), in early 1994. The IDGE measured dendritic growth rates, tip radii, and crystal morphologies of ultra-pure succinonitrile [CN-(CH,),-CN] at supercoolings in the range tiom 0.05 2.0 K. Data taken in the form of slow-scan binary digitized images telemetered to the ground from orbit in near-real time, combined with the IDGE terrestrial data set, provide the first quantitative assessment of various theories on dendritic solidification and the effects of convection on dendritic growth. INTRODUCTION AND BACKGROUND Dendritic growth is the ubiquitous form of crystal growth encountered when metals, alloys, and many other materials solidify under low thermal gradients, a situation which typically occurs in most industrial solidification processes. A clear elucidation of dendritic growth kinetics under well-characterized diffusion-controlled conditions is crucial in order to achieve a rigorous test of dendritic growth theories, and, ultimately, to predict and achieve desired microstructures and hence, physical properties, in a variety of solidification processes. The growth of dendrites in pure liquids is generally acknowledged to be controlled by the diffusive transport of latent heat from the moving crystal-melt interface as it advances into its supercooled melt. Under terrestrial conditions this flow of heat, particulary at small supercoolings, and the resulting dendritic growth speed and size is greatly affected by natural convection. A number of theories of dendritic crystal growth, based on various tmmport mechanisms, physical assumptions, and mathematical approximations, have been developed over the last forty years (see the recent review by one the authors [ 11). A mathematical solution to the dendritic heat conduction problem was first analyzed by Ivantsov 121, who modeled the dendrite as a paraboloidal body of revolution, growing at a constant velocity, V. The resultant thermal diffusion field can be expressed mathematically and exactly in paraboloidal coordinates moving with the dendritic tip. This diffusion solution is, however, incomplete, insofar as it specifies the tip growth Pcclet number, VR/2a, (where R is the tip radius of curvature, V the tip velocity, and a is the thermal diffisivity) as a function of the supercooling AT. The explicit dynamic operating state, V, and R, is not explicitly specified. The Peclet number obtained from the Ivantsov sohrtion for each specified supercooling, yields instead an infinite manifold of V, and R ordered pairs (V,R), that satisfy the di&sion solution at that particular value of AT. Gne fmds experimentally that unique, steady, operating states (Vex,,, R,) are observed at any specified supercooling, AT. Considerable theoretical e&bits within the physics community have been directed to answering the question as to whether and under what conditions a second equation or length scale exists, which, when combined with the Ivantsov diffusion solutic~ selects the unique (i.e., observed) dynamic operating state (see Ref. [ 1,3-61). Although the underlying physics for these “theories of the second length scale” might in fact be quite different, their results are often encapsulated with a scaling constant that is defined as proportional to 1/VR2. A great deal of prior experimental work has been performed using succinonitrile (SCN: the chemical formula for which is NC-(CH,),-CN) as a model dendritic growth system because of its conveniently low melting