Experimental study of free growth of equiaxed NH4Cl crystals settling in undercooled NH4Cl-H2O melts

The present study deals with experiments carried out to assess the influence of the movement of equiaxed crystals on their growth kinetics. The principle of the experiments is based on the settling of crystals growing in melts of transparent alloy undercooled at constant temperature. Ammonium chloride-water mixtures have been chosen for their convenient properties. The experimental set-up is composed of a settling tube and a video-device which allowed us to record continuously the images of dendritic NH4Cl equiaxed crystals all along their fall. On these images two kinds of morphology have been observed of which the difference seems to be related to the spinning movements of the crystals. Furthermore time evolutions of the vertical position and apparent sizes of the crystals have been measured. A careful examination of these measurements was necessary because of the uncertainties of the undercooling and of the actual shapes and sizes of the crystals which are inherent in the set-up and in the procedure. Average growth velocities were determined so as to identify some general trends concerning the effect of the settling on the growth of the equiaxed crystals. Finally, a short comparison between these average growth velocities and the corresponding theoretical ones without any convection has shown a strong influence of the convection on the equiaxed growth kinetics.