Samples of Pb-Sn eutectic alloy with a high density of $8.5\times {10}^3{\mathrm{k}\mathrm{g}/\mathrm{m}}^3$ are levitated with a single-axis acoustic levitator, and containerlessly melted and then solidified in argon atmosphere. High undercoolings up to 38 K are obtained, which results in a microstructural transition of “lamellas–broken lamellas–dendrites.” This transition is further investigated in the light of the coupled zone for eutectic growth and the effects of ultrasound. The breaking of regular eutectic lamellas and suppression of gravity-induced macrosegregation of (Pb) and (Sn) dendrites are explained by the complicated internal flow inside the levitated drop, which is jointly induced by the shape oscillation, bulk vibration and rotation of the levitated drop. The ultrasonic field is also found to drive forced surface vibration, which subsequently excites capillary ripples and catalyzes nucleation on the sample surface.

• Received 6 June 2002

DOI:https://doi.org/10.1103/PhysRevE.66.061601