The formation of lamellar-eutectic grains in thin samples

We present an experimental study of the formation of lamellar-eutectic grains in directional solidification of thin hypereutectic samples of the transparent nonfaceted alloy CBr4-C2Cl6. 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 a 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 stages (2) through (4) are similar to secondary invasion. We study stages (2) through (5) in detail. Stages 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.