Dynamics and instability of triple junctions of solidifying eutectics: flow-modified morphologies

Abstract

Fluid motions impressed upon a eutectic crystal front during directional solidification result in an increase of the lamellar spacing. This flow-induced change of microstructures is analyzed analytically to show the relationship between spacing and the operating conditions. The resulting system is a set of ordinary differential equations which describe the evolution of triple junctions. In a weak-flow regime lamellar widths at the minimum undercooling have a scaling similar to that of Jackson and Hunt, modified by the flows. When the flows are strong, a new scaling law shows that the width is proportional to one-fourth power of the imposed shear rate. Lamellar phases then tilt into the flow. Only at large morphological numbers does the minimum undercooling point corresponds to the marginal stability limit. Simulations show that the flows promote pinching of unstable lamellae.