SELECTION OF A LENGTH SCALE IN UNCONSTRAINED DENDRITIC GROWTH WITH CONVECTION IN THE MELT

Abstract

For the first time the free growth of fully developed succinonitrile, SCN, dendrites is studied experimentally with carefully controlled, well defined, forced convection velocities, U(infinity), in the ultrapure melt up to 1 cm/s, which is about 40 times larger than the velocity due to natural (thermal) convection, U(N), and is 300 times larger than the growth velocity of the dendrite, v. Therefore thermal convection and advection have a negligible effect on our experimental data. The selection parameter, sigma* = 2alphad0/vR2, increases by over 50% as the ratio, U(infinity)/v, of the forced convection velocity, U(infinity), to the growth velocity, v, increases from 3 to 300. This result is opposite to the prediction of microscopic solvability theory for a pure material. Our result also is opposite to that reported for binary experiments which support solvability theory and indicate that sigma* decreases as U(infinity)/v increases up to values of about 19.