Dendrite Growth Kinetics In Undercooled Fe And FeSi Melts Under Static Magnetic Fields

Dendrite is the most typical crystal form during solidification of metallic metals.During the solidification process,the flow of the melt can affect the growth velocity of dendrites.The microgravity is the ideal method for studying the effect of convection on dendritic growth velocity,but it needs high cost and long cycle.The static magnetic field can control the melt flow effectively,and it has been widely regarded by researchers.By changing the magnetic field intensity,different convective environments can be realized.In the present thesis,dendritic growth velocities in undercooled melts of pure substances under static magnetic fields were investigated experimentally and theoretically.In the present experiments,bulk melts of pure Fe and FeSi intermetallic compound were undercooled using a combination of glass fluxing with repeated overheating.The surface temperature of pure substances was measured by single-color pyrometer,and the cooling curves were recorded in real time,and the undercoolings of the two samples were calculated.The characteristics of recalescence process were recorded by a high-speed camera.Dendrite growth velocities in undercooled melts were determined using the three-dimensional computer animation technique.The features of the cooling curves under different conditions were compared and analyzed.The measured dendrite growth velocities of pure substances under different static magnetic fields were analyzed using the LKT/BCT model.The major conclusions are drawn as follows:(1)In the solidification process of pure Fe and FeSi intermetallic compounds,the time of the recalescence platform as well as the recalescence temperatures decrease with the increase of the undercooling.The static magnetic fields have no obvious effect on the cooling curves of pure Fe and FeSi compounds.(2)The measured dendrite growth velocities of pure Fe show a power law first and then show a sudden rise at a critical undercooling of about 190K due to a solute trapping effect of a small amount of impurities.The measured dendritic growth velocities of the FeSi compound also show a power law with increasing undercooling,and then show a sudden rise at a critical undercooling of 230 K due to adisorder-trapping effect.(3)Under low and medium undercooling conditions,the static magnetic fields have obvious effect on dendrite growth velocities of pure Fe and the FeSi compound.With increasing intensity of the static magnetic fields,the dendritic growth velocities of pure Fe and the FeSi compound decrease first and increase after reaching a minimum at a critical magnetic field intensity of 4 T.Generally,the dendrite growth velocities measured with no magnetic field are higher than those measured under the non-zero static magnetic fields.Under large undercooling conditions,the static magnetic fields have no significant effects on the dendritic growth velocities of pure Fe or the FeSi compound.(4)The experimental data were modeled using the LKT/BCT model and a good agreement between the data and model was concluded.The thermal diffusivity of the two pure substances decrease first with increasing magnetic field intensity and then increase after experiencing a minimum at the critical magnetic field intensity.In contrast,the interfacial kinetic coefficients of the two substances do not vary with the intensity of the magnetic fields.(5)The effects of the static magnetic field on the thermal diffusion coefficients suggest that the imposition of a static magnetic field lower than 4 T can damp convection in the melts of the two pure substances effectively,which reduce the dendritic growth velocities.If the magnetic field intensity exceeds 4 T,the thermoelectromagnetic convection will be enhanced,and weaken the inhibiting effect,resulting in the recovery of the dendritic growth velocity.