Since the development of semisolid processing, significant studies have been conducted on understanding the solidification process in stirred melts. However, the theories proposed so far for the formation of non-dendritic microstructures appear to be inconsistent. This inconsistency is mainly due to the inability of in-situ observation of the microstructure formation at the early stages of solidification, i.e. during nucleation and early stages of growth. Most of the proposed theories, therefore, are based on indirect observations. Recently, traarent organic materials have been used to study the microstructural evolution during semisolid processing. The present work attempts to study the mechanisms of microstructural formation during rheocasting using traarent 98% pure succinonitrile model alloy and aspecially designed and built optical semisolid simulating device. The results give insight to the relative importance of nucleation rate, shear rate and flow characteristics on the formation of globular microstructure. It was found that the critical factorfor effective dendrite multiplication during the early stages of solidificationwas the presence of turbulent flow rather than shear rate as commonly believed. Combination of copious nucleation and turbulent flow can create such high density of dendrite fragments which would result in preserving of their globular growth mode up to a radius of about 30?m. Otherwise, the solid particles would choose dendritic growth mode even under very high shear rates. When the stirring is combined with rapid heat extraction from the stirrer, a mushy layer was observed to form around the rotating chill. Under turbulent conditions, detachment of dendrite arms from this layer of rapidly coarsening dendrites was suggested to be the origin of the spherical particles in the microstructure. Formation of the layer was also experimentally confirmed for an Al-Si alloy. Furthermore, a new model for coarsening of dendrites in the shear flow was presented. The model takes into account the accelerated convective solute traort due to the convection by incorporating the apparent diffusivity parameter into the existing coarsening models for stagnant melts. The model predictions were in a good agreement with the experimental results in respect to the coarsening rate under different fluid flow conditions.
