EVOLUTION OF DENDRITIC PATTERNS DURING ALLOY SOLIDIFICATION - ONSET OF THE INITIAL INSTABILITY

The evolution of a dendritic pattern from a planar solid-liquid interf ace during directional solidification of a binary alloy was investigat ed experimentally, The model alloy used was the transparent organic cr ystal succinonitrile doped with the laser dye coumarin 152. The buildu p of solute ahead of the initially stable planar interface and the sub sequent instability of the planar front were measured in detail and co mpared with recent theoretical calculations by Warren and Langer [Warr en, J. A. & Langer, J. S. (1993) Phys. Rev. E 47, 2702-2712]. The fluo rescence of coumarin 152 was used for direct observations of the evolu tion of the solute concentration profile ahead of the initially planar solid-liquid interface, UV absorption was used to produce thermal per turbations of the sample that generated spatially periodic modulations of the planar interface. This technique allows for measurement of bot h positive and negative linear growth coefficients (determined from th e growth or decay rate of the modulation after the perturbation is swi tched off) for a large range of wave vectors, Measurements of the evol ution of the concentration profile and the linear growth coefficients, and the occurrence of the initial instability, were in good agreement with the Warren-Langer predictions.