Residual-Impurity Effects in Directional Solidification: Long-Lasting Recoil of the Front and Nucleation-Growth of Gas Bubbles

Abstract
Directional-solidification experiments are often perturbed by the nucleation of gas bubbles and other residual-impurity effects. We present a detailed experimental study of these phenomena in the system CBr ₄-C ₂Cl ₆ directionally solidified in thin films. As is usual in this type of experiments, we use zone-refined and outgased products, but do not fill and seal the samples under vacuum. We study the solute-redistribution transient (initial recoil of the growth front) in samples of as-refined CBr ₄, in the absence of bubbles. This in situ method allows us to detect all the impurities present in our samples and to estimate their concentrations and partition coefficients. We point out several artefacts that may be caused by the dissolved residual impurities (shift of the cellular threshold velocity, modifications of the morphology of the cellular front). We then describe in detail the nucleation-growth process of gas bubbles during the course of solidification. There exists a steady growth pattern consisting of a periodic array of tubular bubbles similar to the rod-like pattern of eutectics and monotectics. We study the morphology of this state as a function of the growth velocity. We show that, at high velocity, the pattern decomposes in a series of independent local two-phased structures, the solid-vapour (SV) fingers, of width a few diffusion lengths. Between the SV fingers, the front is unperturbed.