The evolution of dendrites during coarsening: Fragmentation and morphology

The process of fragmentation of dendrite arms during coarsening remains poorly understood. We perform isothermal coarsening experiments of dendritic solid-liquid mixtures using PbSn alloys aboard the International Space Station (ISS), since arms that fission from the stem do not sediment and thus can be detected. The morphology of the structure and the number of fragments (fissioned arms) were determined using three-dimensional reconstructions. The evolution of the microstructure, change in length scale, interfacial shape distributions, number and distribution of fragments as well as the connectivity of the structures (handles) across coarsening time are discussed. We find that: the inverse of surface area per unit volume S_V⁻¹ increases with time as t^1/3 in a manner that is almost identical to a sample coarsened on earth; the number of fragments per unit volume scaled by S_V⁻³ is independent of time. Thus, it is possible to predict the number of fragments during coarsening by a measurement of S_V; the connectivity of the structures as measured by the number of handles in the structure scaled by S_V⁻³ is also independent of coarsening time. We find that there is more coalescence of dendrite arms during coarsening than fragmentation.