Dendrite Growth Kinetics In Undercooled Ni₅₀Cu₅₀ And Ni₉₉B₁ Melts Under Gradient Magnetic Fields

Dendrite is the most common crystal form during solidification of metallic metals, and dendritic growth has also been an important research topic in the field of materials science. In recent years, there were deep and systematic researches in the dendritic crystal growth mechanism of pure substances and binary alloys. The lorentz force and the magnetization force can effectively control melt flow in gradient magnetic field, which also has been widely used in materials research. Therefore, it is necessary to study dendrite growth velocity systematically in undercooled melts of solid solutions alloys under different gradient magnetic fields both experimentally and theoretically.In the present thesis, bulk melts of Ni50Cu50 and Ni99B1 were undercooled using glass fluxing. During their rapid solidification process, a single-color pyrometer and a high-speed camera were used to monitor the surface temperature and the process of recalescence of the samples, respectively. Then, a three-dimensional computer animation technique was used to determine the dendrite growth velocities in undercooled melts. The feature of the cooling curve in different gradient magnetic fields was examined and compared. The measured dendrite growth velocities of solid solution under different gradient magnetic fields were analyzed using the recently modified LKT/BCT model. The major conclusions are drawn as follows:1. Regardless of the applied magnetic field, solute trapping phenomenon occurred in Ni50Cu50 and Ni99B1 melts with large undercoolings. The increasing of dentrite growth velocity slowed down at the beginning evident of the solute trapping phenomenon. Diffusion controlling played a leading role in dendrite growth. The critical undercooling is about 170K/200K in Ni50Cu50/Ni99B1 . When it is more than that, occurring solute trapping absolutely, dendrite growth velocity is linear growing by thermally controlling.Gradient magnetic fields had no significant effects on the critical undercooling.2. The gradient magnetic fields had obvious effects on the dendritic growth velocities under low and medium undercooling conditions. The dendritic growth velocities of Ni50Cu50 and Ni99B1 alloys decreased firstly with increasing magnetic field intensity, and then increased after reaching a minimum at a critical central magnetic field intensity of 3T. Under large undercooling conditions, the gradient magnetic fields did not have any significant effects on the dendritic growth velocities of Ni50Cu50and Ni99B1 alloys. Such effets of the magnetic field gradients on the dendrite growth velocities of solid solution alloys are similar are those on dendritic growth velocities in pure substances.3. It was suggested that the imposition of a suitable gradient magnetic field can damp convection in the melts of the two melts effectively, which reduced the dendritic growth velocities. When the central magnetic field intensity exceeded 3T, the promoting effect of the gradient magnetic fields on convection is greater than the suppressive effect on convection, resulting in the recovery of the dendritic growth velocities. A fitting of the LKT/BCT model to the measured dendritic growth velocities revealed that gradient magnetic field changed the chemical diffusivity and thermal diffusivvity in the front of the growing dendritic tips. Both diffusivities were decreased first with increasing central magnetic field intensity and then increased in Ni50Cu50and Ni99B1 alloys. When the central magnetic field intensity was 3T, they reached their mininua.4. The gradient magnetic fields imposed more significant effects on the dendritic growth kinetics in Ni99B1 alloys with a small solute distribution coefficient than those on the dendritic growth kientics in Ni50Cu50 alloys with a large solute distribution coefficient.