INTERFACE DYNAMICS, INSTABILITIES, AND SOLUTE BANDS IN RAPID DIRECTIONAL SOLIDIFICATION

In rapid solidification experiments on metallic alloys structures have been observed which are periodic along the growth direction. The orig in of these banded structures has been ascribed to an oscillatory inst ability of the solid-liquid interface characterized by large-variation s of the interface velocity; this instability was predicted by several authors incorporating nonequilibrium effects into the classic Mullins -Sekerka analysis. In this paper the rapid solidification of a binary alloy, directed by a moving temperature field, is studied with the pha se-field model; in a region of the parameter space an oscillatory inst ability is evidenced, which reflects in alternating low and high conce ntration solute bands. The equations of the model are numerically solv ed to show under what conditions (i.e., isotherm velocity and temperat ure gradient) the banded structure can be observed. In many respects t he results agree with the linear stability analysis of the free-bounda ry equations performed by Merchant and Davis [G. J. Merchant and S. H. Davis, Acta Metall. Mater. 38, 2683(1994)]; we detected also signific ant deviations which trace their roots to the diffuse solid-liquid int erface characteristic of the phase-field model, opposed to the zero di mension interface of the free-boundary model.