Effects of the interface curvature on cellular and dendritic microstructures

We study the solidification of succinonitrile-based alloys contained in cylindrical glass crucibles. In such samples, the solid-liquid interface is curved as the latent heat generated upon solidification is mainly evacuated through the crucible wall of higher thermal conductivity. This creates radial thermal gradient which drives some important modifications in solute macrosegregation and of the microstructure array. A facility dedicated to the in situ and real time characterization of solid-liquid interface morphology during directional solidification of bulk samples has been developed by CNES (French Space Agency) in the frame of the DECLIC project. The effects on microstructure dynamics of fluid flow driven by the radial thermal gradient associated to curvature have already been pointed out, but other effects of curvature - function of the type of structure developed - are here presented. For a cellular microstructure, curvature is at the origin of advection that manifests in observation by the gliding of cells from the border to the centre of the interface. For a dendritic microstructure, a strong dependence of dendrite growth direction on the curvature appears that alters the columnar structure.