The development of solidification microstructures in the presence of lateral constraints

Abstract

The effect of a sudden change in cross section on the microstructural development has been investigated by the directional solidification technique. Experiments have been carried out in a transparent model system of succinonitrile-acetone so that the dynamical changes in the interface shapes can be monitoredin situ as the interface travels from a region of uniform cross section to a region of sharply reduced cross section. Different experimental conditions which give rise to initial steady-state planar, cellular, and dendritic interfaces have been investigated. Significant changes in microstructures have been observed as the interface approaches a sharply reduced cross section. The planar interface undergoes transitions to cellular and dendritic morphologies as the cross section is reduced, and reverse transitions are observed as the cross section is then increased gradually to its original value. An initial cellular interface is found to become dendritic as the cross section is reduced, and again it becomes cellular as the cross section is increased. When the experimental conditions are designed to give initial dendritic structures, the change in microstructure is found to occur only when the reduced cross section is of the order of primary dendrite spacing. When the reduced cross section is more than about 5 times the primary spacing, no appreciable change is observed in the dendritic array which travels across the cross-sectional change. The dynamical changes in the interface shape and the microstructural transitions that occur with the change in cross section have been examined quantitatively and discussed.