| Evolution of the complex solid-liquid interface morphology during a solidification process is an important issue in solidification theory since the morphology eventually dictates the final microstructure of the solidified material and therefore the material properties. Significant progress have been made in recent years in the study of the formation and development of regular dendritic growth, while only limited understanding is achieved for the irregular interface patterns observed in many industry processes. This dissertation focuses on the physical mechanisms of the development and transition of various irregular interface patterns, including the tilted dendritic, the seaweed, and the degenerate patterns. Both experimental observations and numerical simulation using the phase field modeling are performed. A special effort is devoted on the effects of the capillary anisotropy and the kinetic anisotropy in the evolution of the interface morphology during solidification.; Experimentally, a directional solidification system is constructed to observe in situ the interface morphology by using the transparent organic material succinonitrile. With such a system, both the regular interface patterns (cellular and dendritic) and the irregular interface patterns (seaweed, degenerate and tilted dendritic) are observed. The effects of the temperature gradient and the interface velocity on the development and transition of the irregular interface patterns are investigated. It is found that the interface morphology transits from the seaweed to the tilted dendritic pattern as the interface velocity increases, while the tilted dendritic pattern may transit to the degenerate seaweed pattern as the temperature gradient increases. Under certain conditions, dendrites and seaweed coexist within the same grain. The dynamic transitions among various patterns and the effect of the solidification conditions are examined in detail.; Numerically, a 2-D phase field model is developed to simulate the formation of the irregular interface patterns. Both the capillary anisotropy and the kinetic anisotropy are incorporated into the model. For the first time, the effect of the kinetic anisotropy on the formation of the irregular interface patterns is investigated in the case of a small capillary anisotropy. It has been found that the kinetic anisotropy has a profound influence on the development of the irregular interface patterns. The simulation finds that the interface morphology transits from the cellular into the seaweed and then into the tilted dendritic pattern as the kinetic anisotropy increases. The large angle tilted dendrites are also successfully reproduced. The simulation results agree well with our experimental observations. |
