Experimental study of the planar-to-cellular transition during thin-film directional solidification: Observations of the long-time-scale dynamics of microstructure formation

The morphological evolution of the crystal-melt interface formed during the thin-film directional solidification of the succinonitrile-acetone alloy is studied using an experimental system that permits extremely-long-time-scale experiments under conditions close to those for the onset of cellular growth from the planar interface. Long exposure times of the alloy to high temperatures result in thermal decomposition of the succinonitrile, which increases the total concentration of solute or impurity in the sample. This effect alone may be responsible for the apparent hysteresis observed by others in the measurement of the critical solidification rate ${\mathit{V}}_{\mathit{c}}$ for the onset of cellular growth and may explain the differences between those experiments and numerical calculations. We show that the evolution of the planar interface into cellular structures occurs first in packets of more rapidly growing undulations separated by regions of slower growing cells. These cellular structures propagate over the entire interface and at very long times the interface exhibits shallow cells without a selected wavelength. The interface dynamics appears to be spatiotemporally chaotic. The wavelength distribution is dispersed about a mean that is almost a factor of 4 below the critical value expected from linear stability theory. Deeper cells exhibit stronger wavelength selection behavior and are observed at solidification rates only slightly above ${\mathit{V}}_{\mathit{c}}$; the mean wavelength increases with increasing V for deep cells.