Dendrite growth in eutectic alloys: the coupled zone

Abstract

Casting alloys of eutectic composition are of great practical importance. Good microstructural control of these alloys is essential in obtaining the required properties in the cast state. Primary phases present in a eutectic matrix may have either a beneficial or a detrimental effect on these properties. Therefore, their appearance must be carefully controlled. In this review the authors describe the fundamentals and recent advances in the understanding of the extent of eutectic growth – the coupled zone. Because of the inherent difficulties and the present inapplicability of stability analyses to predicting eutectic to eutectic plus dendrite transitions, the simpler ‘competitive growth’ approach is presented in this review. As this involves separate consideration of eutectic and dendrite growth, the theory of these growth forms for temperature gradients is first reviewed. The competitivegrowth principle is then applied, i.e. the principle that the morphologies appearing in the microstructure, under given conditions, will be those having the highest interface temperature or the highest growth rate (depending on whether growth rate or undercooling is the controlled variable). On the basis of this theory, a modified classification of eutectics is proposed. Mter discussion of the experimental techniques available for determining the form and extent of the coupled zone, the experimental results are compared with theory. Conclusions are drawn concerning the reasons for the various forms of coupled zone.