INTERFACE DYNAMICS AND BANDING IN RAPID SOLIDIFICATION

Rapid-solidification experiments on metallic alloys in the last decade have provided widespread observations of a novel ''banded structure.' ' We report the results of numerical and analytical studies of the int erface dynamics underlying the formation of this structure in a model of directional solidification which includes both solute and heat diff usion and nonequilibrium effects. The thrust of these studies is on th e unsteady dynamics of the planar interface and thermal effects. The m ain conclusion is that the origin of banding can be related to relaxat ion oscillations of the solidification front, characterized by large v ariations of the interface velocity, which are dramatically affected b y latent-heat diffusion. Without the latter, the oscillations are foun d to be reasonably well approximated by the phenomenological model of Carrard et al. [Acta Metall. 40, 983 (1992)], and the band spacing is inversely proportional to the temperature gradient. In contrast, with latent-heat diffusion the band spacing is insensitive to the temperatu re gradient, but is controlled instead by the interplay of solute and heat diffusion. The smallness of the solutal diffusivity to thermal di ffusivity ratio is exploited to explain analytically this effect and t o derive considerably simpler equations of interface motion that provi de an efficient numerical means to study the nonplanar interface dynam ics expected to cause dark bands. A reasonable agreement with experime nt is found for the spacing of banded structures dominated by light-ba nd microsegregation-free regions in Al-Fe alloys.