Dynamics of pattern formation during directional solidification of binary alloys

The dynamics of pattern formation during directional solidification of binary alloys was investigated experimentally on the transparent organic crystal Succinonitrile doped with the laser dye Coumarin 152 (solute). The UV fluorescence properties of Coumarin 152 were used to measure the solute concentration field in situ during solidification. Because UV absorption by the Coumarin 152 locally heats up the sample, UV illumination of the solid-liquid interface through a mask with rows of uniformly spaced holes was used to apply spatially periodic perturbations for a large range of perturbation wavelengths. Through the perturbations the interface was forced into stable and (temporarily) into unstable patterns of a desired wavelength. The decay mechanisms of unstable patterns were measured with this method and additional stable patterns, not accessible during the dynamics of pattern formation from an initially planar front, were found.;While only preliminary results were obtained on the complete dynamics of formation of a dendritic array as it evolves through a coarsening phase from the initial instability of a planar front, key aspects of the coarsening process (the initial instability, the cell to dendrite transition and dendritic array stability) were analyzed in detail.;Measurements of positive and also negative growth coefficients for perturbations of the planar interface were carried out and verified that the Warren-Langer time dependent linear stability analysis (J. A. Warren and J. S. Langer, Phys. Rev. E, 47, 2702 (1993)) provides a good description for the stability and initial instability of a planar interface.;At high crystal growth speeds, the stability of dendritic arrays was measured directly through perturbations and period doubling was found to be the mechanism limiting stability for small dendrite spacings, in agreement with the Warren-Langer dendritic stability analysis. (J. A. Warren and J. S. Langer, Phys. Rev. A, 42, 3518 (1990)).;In addition, at intermediate crystal growth speeds, the stability region and limiting instability mechanisms of asymmetric doublet cellular patterns were investigated and compared to phase-field model simulations by Kopczynski, Rappel and Karma.