DENDRITIC GROWTH OF SUCCINONITRILE IN TERRESTRIAL AND MICROGRAVITY CONDITIONS AS A TEST OF THEORY

Dendritic growth is the common mode of solidification encountered when metals and alloys freeze under low thermal gradients. The growth of d endrites in pure melts is generally acknowledged to be controlled by t he transport of latent heat from the moving crystal-melt interface int o its supercooled melt. The Ivantsov formulation solves the equation o f heat flow from a paraboloidal dendrite tip for the case of diffusive heat transport. However, this formulation is incomplete, and the phys ics of an additional selection rule, coupled to the Ivantsov solution, is necessary to predict the dendrite tip velocity and radius of curva ture as a unique function of the supercooling. Unfortunately, the expe rimental evidence is not definitive because dendritic growth can be co mplicated by buoyancy-induced convection, which is normally unavoidabl e under terrestrial conditions. Recent experiments performed in the mi crogravity environment of the space shuttle Columbia (STS-62) show qua ntitatively that convection alters the tip velocities and radii of cur vature of dendrites in both terrestrial and microgravity conditions. I n addition, these data can be used to evaluate both how well the Ivant sov diffusion solution and the selection rule (the product of the dend rite tip velocity and the tip radius of curvature squared is a constan t) match the dendritic growth data under microgravity conditions.