Free Solidification of Undercooled Eutectics

Abstract

The traditional eutectic solidification theories always stress the role of eutectic growth and morphology selection. However, when a eutectic alloy solidifies freely from an undercooled state, the first step commences from nucleation and then follows eutectic growth. This paper reviews the free solidification behavior of various undercooled eutectic alloys from the viewpoints of nucleation and growth. It is realized that an independent eutectic colony should be the basic unit when discussing eutectic solidification rather than a bulk sample, which urges us to re-examine the previous discussions and conclusions that were based on bulk solidification in which the role of nucleation was frequently ignored. Regarding to the eutectic growth after nucleation, particular emphasis is devoted to shedding light upon the physical mechanisms of the anomalous eutectic formation when the eutectic is undercooled beyond a certain critical undercooling. The interface attachment kinetics of terminal phases in a eutectic reaction is of great importance in yielding decoupled growth between a mobile interface and a sluggish interface. Further growth behavior after decoupled growth occurs is investigated when the effect of crystallization heat on solidification is considered. Future work is briefly directed to improve current understanding on the free solidification from undercooled states.