THE EFFECTS OF INTERFACE ATTACHMENT KINETICS ON SOLIDIFICATION INTERFACE MORPHOLOGIES

The presence of an interface kinetic effect significantly influences microstructures that form during the solidification of alloys. In order to quantitatively evaluate the effect of interface kinetics on microstructure formation, critical directional solidification studies have been designed in the pivalic acid-ethanol (PVA-Eth) system, in which significant anisotropies in interface properties are present. The interface kinetic effect is studied in high-purity PVA by measuring the interface temperature of a planar interface which is growing under steady-state conditions. In a binary system of PVA-Eth, the interface kinetic effect is characterized by examining the variations in dendritic microstructural scales with velocity and composition and by examining the planar interface instability condition. The variations in the dendrite tip radius, R, the primary spacing, and the secondary arm spacing near the dendrite tip with velocity, V, as well as with composition, have been characterized. Experimental results at a given composition showed VR2 to be constant, and those at constant velocity showed DELTA-T(S)R2 to be constant, where DELTA-T(S) is the product of the liquidus slope and the concentration difference at the dendrite tip. In order to characterize the system properly, additional experiments were carried out to measure the liquidus temperatures of the system. These experimental results are then compared with the theoretical models of planar interface instability and of dendritic growth to evaluate the role of interface kinetics on microstructure formation. Based on the theoretical models for planar and dendritic growth in an anisotropic system, the results on the interface kinetic effects are analyzed to give an insight into the possible phenomena which contribute to the complex kinetic behavior that is observed experimentally in the PVA system.