ABOUT THE BANDED STRUCTURE IN RAPIDLY SOLIDIFIED DENDRITIC AND EUTECTIC ALLOYS

A detailed analysis is presented for the formation of the so-called banded structure, consisting of a regular succession of dark and light bands, which has been found by several authors in rapidly solidified alloys. It is observed that the structures in the dark bands result from dendritic or eutectic growth depending on the alloy. Furthermore, it is concluded that the light bands are due to plane-front growth. The origin of these instabilities can be understood by considering the undercooling of the solid-liquid interface as a function of the growth rate for the two types of growth. Loss of local equilibrium and the influence of attachment kinetics at the solid-liquid interface are taken into account and quasi steady-state behaviour for growth inside each band is assumed. The proposed model is based upon a simple stability analysis leading to a periodic behaviour of the growth front when the isotherm velocity is in the range of negative slope of the interface undercooling-growth rate relationship. The predictions of the model are compared with detailed experimental results on dendritic Al-Fe, and dendritic/eutectic Al-Cu and Ag-Cu alloys. Good agreement is found between the theoretical and experimental values of the transition velocities (from a fully dendritic or eutectic structure to a banded structure and from a banded structure to a precipitation-free structure) when reasonable values are chosen for the solute-trapping and attachment kinetic parameters. The calculated widths of the dark and light bands are also in agreement with experimental data when an upper limit is used for the thermal gradient.