The morphological evolution of dendritic microstructures during coarsening

The coarsening process in systems consisting of spherical particles in a matrix has been studied extensively. In contrast, coarsening in systems that possess both positive and negative curvature, such as those present following dendritic solidification, have received less study. Recent advances in experimental technology now allow for the routine analysis of metallic microstructures in three dimensions. A method has also been developed to determine the mean and Gaussian interfacial curvature, the analogs of the particle size distribution for spherical particles. The evolution of dendritic microstructures during coarsening is analyzed for a directionally solidified Al-15 wt pct Cu alloy. Samples were taken from this ingot and isothermally coarsened for 10 and 964 minutes. Probability density plots of the mean and Gaussian curvature as well as probability density plots of the principal curvatures show that extreme positive and negative mean curvatures decrease, that most of the interfaces are saddle-shaped, and that solid spherical shapes disappear to a greater degree than liquid spherical shapes. Probability density plots of the orientation of the surface normals within the microstructure show that the majority of the interfaces are parallel to the growth direction and that there is a fourfold symmetry in the 10-minute sample and a twofold symmetry in the 964-minute sample.