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.