R. Trivedi

Eutectic spacing selection in lead-based alloy systems

Directional solidification studies have been carried out in Pb-Au, Pb-Pd, Pd-Cd, and Pb-Sn systems to characterize the variation in eutectic spacing with velocity. In the Pb-Cd, Pb-Sn, and Pb-Pd systems, statistical distributions of spacings at each velocity were determined and a significant spread was observed. The smallest and the largest spacings, along with the average spacing, have been characterized. A broad distribution curve has been observed for low velocities which becomes sharper as the velocities increase. A comparison with the theoretical model shows that the average spacings are consistently larger than the spacings predicted by the minimum undercooling criterion, whereas the smallest observed spacing corresponds to the theoretical extremum value. Dynamical studies have been carried out to examine the spacing selection process at a given velocity by initially starting with a higher or a lower average spacing. The final spacing distribution at a given velocity was found to be the same irrespective of the initial starting condition which establishes that a definite distribution of spacings exists in eutectic systems without any hysteresis effect and that dynamical effects are not responsible for the existence of a band of stable spacing.

Orientation Dependence of Primary Dendrite Spacing

The orientation dependence of the primary dendrite spacing is examined through the solidification of a succinonitrile-3.61 wt pct acetone alloy with a Bridgman-type device. Primary dendrite spacing has been studied as a function of the primary dendrite trunk orientation with respect to the thermal gradient direction under different growth rate conditions. The observations show that the orientation dependence of the primary dendrite spacing can be related to the formation of new primary dendrites at divergent grain boundariesvia the branching mechanism. A branching-based model is developed to study the interaction between secondary and tertiary dendrite arms. Based on this model, a simple analytical relationship is proposed to account for the orientation dependence of the primary dendrite spacing.

Banded solidification microstructures

Banded microstructures are composed of alternate structures or phases which develop mostly parallel to the transformation front. At low growth velocities, bands of the same microstructure but with different scales form through periodic fluctuations of the solidification system. On the other hand, banding can occur as a transformation microstructure when the growth front becomes unstable to oscillations. This instability is either due, at low velocities, to nucleation of another phase (peritectics) or, at high velocities, to nonequilibrium effects at the interface which lead to periodic changes of the microstructure. In this article, the inherent banded patterns of low velocity peritectic solidification and high velocity nonequilibrium solidification will be presented and their origin will be discussed.

The Effects of Interface Kinetics Anisotropy on the Growth Direction of Cellular Microstructures

Directional solidification studies have been carried out in the pivalic acid-ethanol system in which significant anisotropies in interface kinetics and interfacial free energy are present. These anisotropic properties influence the microstructure formation and often lead to the formation of cells and dendrites which are tilted with respect to the heat flow direction. It is shown that dendrites always form in the preferred crystallographic direction, whereas the angle of tilt for cells is governed by the relative effects of heat flow and the anisotropic property of the crystal. This tilt angle for a given crystal orientation is found to increase as the velocity is increased. The angle of tilt reaches its largest value when the cell growth direction coincides with the preferred crystallographic direction,i.e., 〈001〈 direction for the cubic pivalic acid crystals. At this point, a transition from cellular to dendritic morphology occurs. The variation in the angle of tilt as a function of velocity is examined for the steady-state cellular structures. These results are then compared with the linear and the weakly nonlinear analyses of the planar interface stability to obtain the magnitude of the kinetic anisotropy effects. It is also shown that the cellular spacings as well as the amplitude of cells alter significantly with the angle of tilt under identical conditions of growth rate, temperature gradient, and composition.

Dendritic growth in the carbon tetrabromide and hexachlorethane system

Croissance dendritique par la methode de solidification dirigee dans le systeme CBr 4 -C 2 Cl 6 . Deux compositions differentes ont ete selectionnees afin detudier la croissance de dendrite CBr 4 ou de dendrite C 2 Cl 6 . Pour ces deux systemes VR 2 est constant et σ* vaut respectivement 0,022 et 0,019

The development of solidification microstructures in the presence of lateral constraints

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.

Nonequilibrium effects during the ledgewise growth of a solid-liquid interface

Directional solidification studies have been carried out in the napthalene-camphor system in which the interface advances through the formation and motion of ledges. Growth conditions have been varied so as to characterize both the planar interface growth and the condition for the instability of the planar interface. It is found that the mechanisms of planar interface instability and the subsequent morphological development of the interface depend significantly on the crystallographic orientation of the interface. Appreciable interface kinetics effects are present during the growth of a planar interface, and experimental studies have been designed to quantitatively evaluate the nonequilibrium conditions at the interface. These nonequilibrium effects for the napthalene-camphor system have been determined experimentally and have been characterized by two response functions that describe the interface temperature and the interface composition in the liquid under nonequilibrium conditions. The interface kinetic law is found to be exponential, indicating that the growth of the interface occurs through the process of nucleation of new layers.