Crystal Growth Kinetics In Undercooled Ni And Ni-Sn Melts Under Gradient Magnetic Fields

Undercooled rapid solidification includes crystal nucleation and growth process.There were deep researches about the dendritic crystal growth of the pure substance and a variety of alloys.With the development of magnet technology,it has been widely used in materials research.Magnetic fields can effectively influence convection in the melts,because of the magnetization force in gradient magnetic fields,which are different from uniform magnetic fields,and the melt flow will change when applied with different magnetic fields.Therefore,in the present thesis,the crystal growth behaviors in undercooled melts were investigated experimentally and theoretically under different magnetic fields.In the experiments,bulk melts of pure Ni and Ni81.3Sn18.7 solid solutioncompounds eutectic alloys,Ni83Sn17 hypoeutectic alloys,Ni80Sn20 hypereutectic alloys were undercooled under high magnetic fields.In each melting and solidification cycle,a high-speed camera was used to monitor recalescence of the undercooled sample.The measured crystal growth velocities of pure Ni and Ni-Sn alloys under different gradient magnetic fields were analyzed using the LKT/BCT model and LZ model to understand the effect of gradient magnetic fields on the crystal growth behaviors of undercooled Ni and Ni-Sn alloys.The major conclusions are drawn as follows:(1)The dendritic growth velocities of pure Ni under negative gradient magnetic fields are obviously larger than that under uniform magnetic fields at the same undercooled.With the increasing of undercooling,the change of dendritic growth velocities tend to be gentle,which reflect that the effect of magnetic fields on the dendritic growth velocity weakens gradually.The experimental data of pure Ni were calculated using LKT/BCT model.The interfacial kinetic coefficient μ do not vary with the change of magnetic field intensities,the effective thermal diffusivity DT vary obviously whether under a negative gradient magnetic field or a uniform magnetic field,and the former is larger than the DT value under uniform magnetic fields.The results show that the magnetic fields affect dendritic growth velocity through thermal diffusion,which lead to dendritic growth velocities vary different with the change of magnetic field intensities.(2)The growth velocities of Ni81.3Sn18.7 eutectic alloys decrease first and increase after reaching minimum at the critical magnetic field intensity of B=3 T with increasing intensities of the gradient magnetic fields.The growth velocities of Ni83Sn17 hypoeutectic alloy show three different regions,and during the medium undercooling region,the eutectic growth velocities reach local minimum at B=3 T and B=6 T.The growth velocities of Ni80Sn20 hypereutectic alloys show two different regions,and during the high undercooling region,the eutectic growth velocities reach local minimum at B=2 T and reach local maximum at B=3 T.(3)The dendritic growth velocities of α-Ni and Ni3Sn are not significantly affected by negative gradient magnetic fields.For the α-Ni growth velocity of hypoeutectic alloy,the effect of magnetic fields on the growth velocity weakens because of the interaction between the Lorentz force and the thermal electromagnetic Lorentz force.The Ni3Sn is an intermetallic compound,the effect of magnetic fields on the growth velocity decreases because of a high solute concentrate in the front of interface.(4)The experimental data of Ni81.3Sn18.7 eutectic alloys were calculated using LZ model.The effective thermal diffusivities DT vary obviously with increasing intensities of the gradient magnetic fields,which are similar to the change of growth velocities.The effective solute diffusivities DL remain constant expect at B=3 T and B=6 T.It is shown that the growth velocities are mainly affected by the thermal diffusion under negative gradient magnetic fields,and the effect of solute diffusion is relatively small.