Growth of lamellar eutectic dendrites in undercooled melts

Abstract

Electromagnetic levitation is used to undercool bulk samples of eutectic Ni 21.4 at % Si alloys. Large undercoolings ΔT up to ΔT=220 K are achieved by containerless processing of the melts. Crystal growth velocities are measured as a function of undercooling. The growth kinetics during solidification of the eutectic alloys is controlled by atomic diffusion. A maximum in the relation of the growth velocity on undercooling is observed which is due to the progressively decreasing diffusion coefficient counteracting the enhancement of the driving force of crystallization with increasing undercooling. Microstructure analysis of as-solidified samples reveals colonies of eutectic lamellae enveloped by a nonplanar growth front of a dendritelike morphology. The experimental data are analyzed within current models of crystal growth taking into account a negative temperature gradient in front of the solidification front. While the thermal undercooling causes a dendriticlike morphology of the solidification front, constitutional and curvature undercooling control the formation of the eutectic colonies behind it.