Three-dimensional morphological characterization of coarsened microstructures

The three-dimensional morphological evolution of a directionally solidified Al-15 wt.% Cu alloy during isothermal coarsening was examined. The microstructure was analyzed using the Interfacial Shape Distribution (ISD), which gives the probability of locating an interfacial patch with a given pair of principle curvatures. By scaling the ISD's by a characteristic length, the inverse of the surface area per unit volume, it was possible to determine if the system reached a scaled time-invariant state. During coarsening, cylindrical interfaces appear in the second coarsened sample and remained for later coarsening times. The appearance of the corresponding peak in the ISD indicated that the microstructure was not evolving in a self-similar manner, despite the linear relationship between the inverse surface area per unit volume and the cube root of coarsening time. Stereographic projections of the interfacial normals indicated that the microstructure of the earliest coarsened sample had a four-fold symmetry that disappeared during coarsening and that the majority of interfacial area for all coarsened samples is parallel to the direction in which the pre-coarsened samples were solidified.