Initial stages of solidification during semisolid processing of a transparent model material

Growth morphology of succinonitrile crystals under different cooling and stirring conditions was experimentally investigated by real time microscopic observation of the solidifying melt. It was found that under laminar fluid flow conditions, the solidified crystals exhibited equiaxed dendritic morphology immediately after the beginning of solidification. On the other hand, under turbulent fluid flow conditions the growth of solid particles followed a spherical mode with a high growth velocity from the initial stages of solidification. When the stirring was combined with rapid heat extraction from the rotor, a mushy layer was observed to form around the rotating chill. Under turbulent conditions, detachment of dendrite arms from this layer of rapidly coarsened dendrites is suggested to be the origin of the spherical particles in the microstructure. Formation of the layer was experimentally confirmed for an Al–Si alloy. The new findings can be used for more effective refinement of rheocast microstructures.

► Direct observation of the morphological evolution of succinonitrile crystals under stirring. ► Evidences of the role of turbulence on the morphological evolution of primary particles. ► Proposal of a new mechanism for the formation of spherical particles in SSR process.