INTERFACE DYNAMICS AND COUPLED GROWTH IN DIRECTIONAL SOLIDIFICATION IN PRESENCE OF BUBBLES

The formation and dynamics of gas bubbles in Bridgman growth of succin onitrile-acetone alloys is examined. The experimental results show for the first time the rich dynamics that are associated with the formati on and propagation of bubbles during directional solidification of all oys. The strong coupling of bubbles with the solid-liquid interface is found to result in the growth of elongated bubbles, either attached t o a flat solidification front or forming localized cellular as well as dendritic duplexes (bubbles wrapped by a solid envelope). The couplin g of the bubble with the solidification front is shown to cause oscill ations in the bubble, which are characterized by fast Fourier transfor ms. When several duplexes are formed, coupled growth and screening may occur. The basic factors that give rise to oscillations, namely compe tition between source and sink of acetone assisted by capillary convec tion at the bubble cap, are discussed qualitatively through the develo pment of an internal oscillator model. Coherent sidebranching observed on dendritic duplexes is shown to be due to resonant modes between th e bubble cap and the solid envelope.