Direct measurement of anisotropy of interfacial free energy from grain boundary groove morphology in transparent organic metal analong systems

Both academia and industry alike have paid close attention to the mechanisms of microstructural selection during the solidification process. The forces that give rise to and the principles which rule the natural selection of particular morphologies are important to understanding and controlling new microstructures. Interfacial properties play a very crucial role to the selection of such microstructure formation. In the solidification of a metallic alloy, the solid-liquid interface is highly mobile and responds to very minute changes in the local conditions. At this interface, the driving force must be large enough to drive solute diffusion, maintain local curvature, and overcome the kinetic barrier to move the interface. Therefore, the anisotropy of interfacial free energy with respect to crystallographic orientation is has a significant influence on the solidification of metallic systems. Although it is generally accepted that the solid-liquid interfacial free energy and its associated anisotropy are highly important to the overall selection of morphology, the confident measurement of these particular quantities remains a challenge, and reported values are scarce. Methods for measurement of the interfacial free energy include nucleation experiments and grain boundary groove experiments. The predominant method used to determine anisotropy of interfacial energy has been equilibrium shape measurement. There have been numerous investigations involving grain boundaries at a solid-liquid interface. These studies indicated the GBG could be used to describe various interfacial energy values, which affect solidification. Early studies allowed for an estimate of interfacial energy with respect to the GBG energy, and finally absolute interfacial energy in a constant thermal gradient. These studies however, did not account for the anisotropic nature of the material at the GBG. Since interfacial energy is normally dependent on orientation of the crystallographic plane of the solid with respect to the liquid, a better calculation of interfacial energy was needed. Herring described this orientation dependence, which related the interfacial undercooling to the principle interfacial curvatures. The present study pertains to the measurement of the anisotropy of interfacial energy by comparison of experimental and theoretical GBG geometries in pure succinonitrile (SCN) and pivalic acid (PVA). A quantity of SCN and PVA was distilled and zone refined using a process that is defined in the experimental procedure portion of this paper. Very thin (100 {micro}m) slide assemblies were created and filled with these organic materials. For each system, several grooves were photographed and their shapes were compared with theoretical predictions. The correlation between experiment and theory was quantified and plotted as a function of the anisotropy for each of the GBG's examined, and a maximum correlation corresponded to the anisotropy of interfacial energy which describes that particular rotation of the GBG. The results from several rotations were statistically analyzed to ensure confidence in the measurement of the anisotropy of interfacial energy and, finally, compared to reported values obtained with other techniques