| Popis: |
Dendritic growth is the common mode of solidification encountered when metals and alloys freeze under low thermal gradients. The growth of dendrites in pure melts is generally acknowledged to be controlled by the transport of latent heat from the moving crystal-melt interface into its supercooled melt. The Ivantsov formulation solves the equation of heat flow from a paraboloidal dendrite tip for the case of diffusive heat transport. However, this formulation is incomplete, and the physics of an additional selection rule, coupled to the Ivantsov solution, is necessary to predict the dendrite tip velocity and radius of curvature as a unique function of the supercooling. Unfortunately, the experimental evidence is not definitive because dendritic growth can be complicated by buoyancy-induced convection, which is normally unavoidable under terrestrial conditions. Recent experiments performed in the microgravity environment of the space shuttle Columbia (STS-62) show quantitatively that convection alters the tip velocities and radii of curvature of dendrites in both terrestrial and microgravity conditions. In addition, these data can be used to evaluate both how well the Ivantsov diffusion solution and the selection rule (the product of the dendrite tip velocity and the tip radius of curvature squared is a constant) match the dendritic growth data under microgravity conditions. |
