Interface Anisotropy and its Effect on Microstructural Evolution During Coarsening

The driving force for coarsening is provided by the excess interfacial free energy. In a system with isotropic interface energies, classical coarsening theories can predict the evolution of the sizes of spherical crystals. However, if the interfaces are anisotropic, crystal growth and shrinkage may be limited by the surface attachment/detachment rate, the motion of ledges, or the nucleation of new layers. The nucleation limited coarsening theory predicts the development of a transient bimodal grain size distribution consisting of large, growing grains with step producing defects and smaller, perfect grains that act as a source of material for the growing grains. To test the predictions of this theory, a comprehensive study of interfacial structure was conducted on the SrTiO3 system. The study consisted of evaluating the surface energy anisotropy of single phase SrTiO3, determining the grain boundary plane distribution, and characterizing the morphological evolution of the SrTiO3 crystals coarsening in titania rich liquid. The characterization of the evolving microstructure included determining the shape and the grain size distribution. However, the microstructures of the experimental systems only approximate the conditions of the theory and, in reality, because of the relatively high solid fraction, coarsening and grain growth occur simultaneously. To differentiate the mechanisms, the coarsening kinetics of SrTiO3 in 15 volume% titania rich liquid at 1500°C were compared with the grain growth kinetics of SrTiO3 at the same temperature with no intentionally added liquid phase. The results show that while large grains in the coarsening system grew at a much greater rate than any grain in the single-phase system, the small grains in the coarsening system grew more slowly. It appears that the increase in the average size of the small grains can be attributed to grain growth rather than coarsening. The simultaneous existence of a constant number of crystals that coarsen rapidly and a decreasing number of small grains that grow only by grain boundary migration is consistent with the nucleation limited coarsening theory.