Effect of convection on directional solidification in transparent succinonitrile–2.2wt.%(d)camphor alloy

Interaction of melt flow with solidification microstructures is of fundamental interest and technical relevance due to its effect on the selection of characteristic microstructural features and length scales. Here, we report on in-situ optical observation of both melt flow at the solidification front and microstructural evolution in polycrystalline cellular and dendritic microstructures during upwards directional solidification of a transparent succinonitrile-2.2wt.%(d)camphor alloy in a bulk rectangular sample. Melt flows at different flow intensity levels were applied and the microstructures were characterized in terms of morphology, spacing and growth direction. Under unforced or natural convection, transitions from planar to cellular and further to dendritic solid/liquid interface morphology were obtained (having cellular/dendritic spacing with similar trends as reported in literature data). Under forced convection, after a plateau, decreasing dendrite spacing with increasing shear flow intensity was observed. Also, an increasing inclination of cellular and dendritic growth in the downstream direction was established. Furthermore, for the first time a flow-mediated transition from dendritic to seaweed-type microstructures and vice-versa was observed. Finally, an oscillation of dendrite tip positions with fixed frequency was found for a certain parameter range. The results indicate the large variety of flow effects at different length scales and corresponding physical mechanism, which are discussed.