Effects Of Static Magnetic Fields On Dendrite Growth Kinetics Of Undercooled Melts Of Pure Substances

Dendrites are the most common crystal occurring during solidification of metallic metals. Microstructures and mechanical properties of the solidified material are determined by its dendrite growth. Hence, studies on dendritic growth kinetics are of great importance both theoretically and technically. Due to the fact that convection exists in metallic melts inevitably, dendritic growth velocities predicted by the LKT/BCT model are not in good agreement with those measured experimentally. A static magnetic field introduces different Lorentz forces to affect convection flow, which can be used to get controlled convection conditions. It is thus necessary to perform investigations of dendritic growth kinetics in undercooled melts of pure substances under different static magnetic fields, which may provide reliable experimental data for the development of the dendrite growth model.In the present thesis, bulk melts of pure Ni and Ni3Sn2intermetallic compound were undercooled using a combination of glass fluxing with repeated overheating, and their rapid solidification process were in-situ observed using a high-speed camera and a single-color pyrometer to measure the recalescence characteristics and temperatures of the sample surfaces. Dendrite growth velocities in undercooled melts were determined using the three-dimensional computer animation technique. The feature of the cooling curve as well as the macroscopic liquid/solid interface were examined and compared. Dendritic growth kinetics of the two pure substances was analyzed within the framework of the LKT/BCT model. The main conclusions are as follows.1. The static magnetic field did not have significant effects on the cooling curves and the macroscopic morphology of the recalescence front of the pure substance samples. The in-situ observations on pure Ni revealed a transition of the macroscopic liquid/solid interface from an angular one to a spherical one over a critical undercooling regime ranging between171K and192K. But, the same transition did not occur to the Ni3Sn2compound.2. The increase of the dendrite growth velocity of pure Ni with the undercooling showed a power law first and then a linear law above a critical undercooling. The critical undercooling was determined to be192K. Within the attained undercooling range, the dendritic growth velocity of the Ni3Sn2compound showed a power law with increasing undercooling, and did not show any sudden rise related to disorder-trapping.3. Under large undercooling conditions, the static magnetic fields did not have any significant effect on the dendritic growth velocity of pure Ni, but had a noticeable effect on that of the Ni3Sn2compound. Under low and medium undercooling conditions, the dendrite growth velocities of pure Ni and the Ni3Sn2compound decreased firstly with increasing magnetic field intensity, and then increased after reaching a minimum at a critical magnetic field of3T. The effect of the static magnetic field on the dendritic growth velocity of pure Ni was more significant than the effect on the dendritic growth velocity of the Ni3Sn2compound.4. A fitting of the LKT/BCT model to the measured dendritic growth velocities revealed that the thermal diffusivity of pure Ni and Ni3Sn2compound decreased first with increasing magnetic field intensity and then decreased, as their dendrite growth velocities did. However, their interface kinetic coefficients did not vary with the magnetic field.5. It was suggested that the imposition of a suitable static magnetic field can damp convection in the melts of the two pure substances effectively, which rendered the dendritic growth velocity reduced. If the magnetic field intensity exceeded3T, the thermoelectromagnetic convection would be enhanced, resulting in the recovery of the dendritic growth velocity.