Containerless solidification of highly undercooled Al 2O 3–ZrO 2 eutectic melts on an aero-acoustic levitator

Abstract

The Al 2O 3–ZrO 2 eutectic oxide was levitated in an aero-acoustic levitator and melted by a continuous wave CO 2 laser beam and then solidified under a containerless condition. A high-speed video was operated to image the in situ recalescence behavior at different undercoolings. Based on the recalescence frames and ambient microstructures, copious nucleation was proposed to take place when the sample was solidified either spontaneously at the maximum undercooling or externally seeded at lower undercoolings. The recalescence front was not the reflection of the solid/liquid interface but induced by the simultaneous thermal release of concentrated crystallizing eutectic colonies. The occurrence of copious nucleation should be mainly ascribed to a chain-like successive nucleation reaction in undercooled melts. The temperature distribution inside a spherical sample was considered when applying dimensionless variables of the Biot number and Fourier number so as to elucidate the abnormal eutectic formation near the sample center. Further analysis from the viewpoint of thermal boundary layer indicated that there was a strong thermal interaction among crystallizing eutectic colonies in the flat thermal gradient region near the central part of a sample, which made it impossible to establish a steady thermal flux and thus generating abnormal eutectics after the coarsening process was taken into account.