Monotectic growth morphologies and their transitions

Alloys of exact monotectic composition can decompose at the monotectic temperature into three different two phase microstructures. The minority phase can be arranged as fibers similar to eutectics, as string of pearls or as irregularly distributed droplets. The solidification of three Al-base monotectic alloys was investigated, Al-Pb, Al-In and Al-Bi. The solidified microstructures were characterized as a function of temperature gradient ahead of the solidification front and the solidification velocity. The experimental results are presented in form of stability diagrams for the three microstructures. The inter fiber spacing is analyzed and presented in from of Jackson and Hunt type relations. The transitions between the microstructures are discussed in view of new hypotheses for the transition from fibers to string of pearls and the origin of irregular droplets