Solidification of alloys with a liquid miscibility gap

Abstract

The solidification of alloys with a liquid miscibility gap, called monotectic alloys, is puzzling since decades especially compared to eutectic solidification to which they seem very similar. Alloys with exact monotectic composition solidify at the monotectic temperature by decomposition into a solid and a liquid phase and thus differ from eutectic ones only by the occurrence of a liquid phase at the nonvariant point. Alloys with hypermonotectic composition have to pass the liquid miscibility gap and thus a large region ahead of the monotectic reaction front is thermodynamically in a two-phase liquid state. Four different microstructures are observed in solidification of such alloys: fibrous arrangement of the liquid phase, string of pearls, irregularly dispersed droplet and fully segregated two layer arrangement. The paper reviews the monotectic solidification and the occurrence of fibres and string-of-pearls in light of new experimental results and describes the importance of nucleation in the case of hypermonotectic alloys when inoculants trigger the two-phase liquid microstructure.