On the fluid flow accelerated coarsening of the mushy zone in an AlCu30 alloy

Abstract

The transition zone between the fully liquid and the fully solid state in single phase solidification of binary alloys undergoes severe morphological transformations during the solidification process. This so-called mushy zone consists of primary, secondary and often tertiary dendrites embedded in the melt. The fraction primary solid changes from zero at the liquidus temperature to a value intermediate between a Scheil and a lever rule type solidification. The morphological transformations are commonly attributed to coarsening induced by variations of the local interfacial curvature in the mush. This local variation in curvature induces mass transport from regions of positive to those of negative curvature. We performed experiments with a near-eutectic Al-Cu30wt% alloy held in a small fixed temperature gradient of 0.4 K/mm, and allowed the mush to coarsen for times ranging from 10 to 500 min. After annealing, the samples were analyzed metallographically on sections having a constant fraction solid of 10%. We also used x-ray tomography to create three-dimensional reconstructions of the microstructures with a spatial resolution of approximately 1µm. We measured the specific surface area in 2D and 3D and the curvature distribution on the 3D reconstructed images. The microstructures clearly show that fluid flow accelerates coarsening of the mush and modifies the kinetics, i.e. the time dependence. The curvature distributions of the cases with and without fluid flow differ discernibly.