The Interface Morphology Evolution In Near Rapid Directional Solidification Of Al-Cu Alloy

The near rapid directional solidification is defined as the cooling rate within 1-10~3K/s. In this region, the system condition is between near-equilibrium at low growth rate and far away from equilibrium at rapid solidification. During the process, it may be occurred by a series of turning effects of interface stability and morphologies. With the increase in growth of velocity, the interface with planar front evolved to cellular and dendrites at the stage of near-equilibrium and with further increase of growth rate they transformed reverse from dendrites to cell structure. It is difficult to find the experiments results in the region of growth rate closed to absolute stability limit, so that accurate experiments and theories are lacked.In this thesis, a detailed and thorough investigation on the selection of microstructure of binary single-phase Al-Cu alloy with a Bridgman apparatus under near-rapid directional solidification condition has been performed. The main results can be summarized as follows:1. Directional solidification equipment based on the Bridgman method was designed and manufactured and some problems such as specimen and crucible's broken during installation were solved. The performance of the equipment is fit for the experiment.2. The temperature gradient of directional solidification furnace is measured, and it is found that the temperature gradient decreases with the increase of withdrawal velocity and increases with the raise in furnace temperature. As the withdrawal rate increase from 5μm/sto 300μm/s, the temperature gradient at 1000℃ decrease from 137K/cm to 92K/cm.3. Detailed experiments for Al-4wt% Cu alloy was conducted on the selection of cellular/dendritic microstructures and primary spacing under near-rapid directional solidification condition .With increasing growth velocity , it was found that there exist the microstructure transitions(from planar to cell and from cell to dendrite)and a distribution range in primary cellular/dendritic spacing. The critical growth velocity of planer-cell is in the range of 3μm/s-4μm/s and the...