The formation of lamellar-eutectic grains in thin samples

Difficulties of investigation of formation of lamellar eutectics can be addressed by using the method of thin-sample directional soldification of a transparent analogue to metallic eutectics, allowing continuous follow-up of dynamical processes with adequate spatiotemporal resolution. We present an experimental study of the formation of lamellar-eutectic grains in directional solidification of thin hypereutectic samples of the transparent nonfaceted alloy CBr sub 4 -C sub 2 Cl sub 6 . We start solidification from a partly stabilized solid residue. This solid is in a single phase ( beta phase) along the solid-liquid interface. The successive stages of the transient leading to the final lamellar structure are (1) the solute redistribution transient of the beta -liquid front; (2) the appearance, without nucleation, of seeds of the other solid phase ( alpha phase) onto the front; (3) the growth of the alpha phase along the beta -liquid front (primary invasion); (4) the secondary invasion of the newly formed alpha -liquid front by the beta phase; and (5) the oscillatory instability, called periodic lamellar branching, occurring during the secondary invasion. We study stages (2) through (5) in detail. Stages (2) through (4) are similar to those leading to banded microstructures in peritectics. Stage (5) is specifically responsible for the onset of two-phase growth and the formation of eutectic grains.