SOLIDIFICATION MICROSTRUCTURE EVOLUTION IN THE PRESENCE OF INERT PARTICLES

The development of solidification microstructure can be significantly influenced by the presence of inert particles in the liquid during the processing of particulate composites by the solidification techniques. The effect of particles on the microstructural development is characterized through directional solidification experiments in a transparent organic system, succinonitrile, in which the interactions between the particles and the interface can be examined in situ. These particle-interface interactions have been examined in an alloy system in which the long-range solute field interactions between the particles and the interface are dominant. These interactions have been found to be significantly different for different interface morphologies, i.e. planar, cellular or dendritic interfaces. The effect of particles on the morphological changes and on the nature of the particle trapping processes are characterized for different particle distributions, particle sizes and interface morphologies. It is shown that the presence of particles in binary alloys can either cause the instability of the interface, such as dendrite tip splitting, or lead to morphological transitions, such as a dendritic-to-cellular transition. When the particle density is large, the presence of particles significantly influences the interface shape and thereby alters the interface growth characteristics such that an oscillatory behavior of the interface can be obtained under constant externally imposed velocity. Appropriate theoretical ideas are developed to explain the effects of particles on the morphological development of the interface.