Dendritic Growth Kinetics Of Pure Cobalt Under Uniform And Gradient Magnetic Fields

Dendrite is the most typical growth morphology of crystals in solidification of metals.World-wide scientists include the materials scientists,metallurgists,mathematicians and even the physicist have a pay great attention to investigate the process,which is a highly nonlinear self-assembly process.When a magnetic field was applied to the undercooling solidification,the temperature gradient and thermoelectric convection will be promoted,leading to changes of the tip kinetics and morphology.When a gradient magnetic was applied,the Kelvin force caused by magnetic susceptibility gradient can excite additional melt flow around the dendrites.Therefore,to analyze the mechanism of the influence of magnetic field on dendritic growth,the dendritic growth kinetics of undercooled pure Co melts both under uniform and gradient magnetic fields was investigate by experiment and theoretical analysis.In the experiments,melts of pure Co were undercooled by glass fluxing in uniform and gradient magnetic fields.Rapid solidification of the undercooled melts was in-situ observed using a single-color pyrometer and a high-speed camera.Dendritic growth velocities in rapid solidification were determined by computer-aided analysis of video images recorded by the high-speed camera.Measured growth velocities were examined as a function of undercooling and the intensity of the magnetic fields for an understanding of the influence of magnetic field on dendritic growth kinetics.Finally,the measured growth velocities were also modeled using the theoretical model of three-dimensional(Alexandrov-Galenko theory)on dendritic growth.The conclusions are drawn as follows:Both uniform and gradient magnetic fields can change the orientation of the dendrites,and the uniform magnetic field effect is more obvious.With the increase of magnetic field intensity,the dendrite was more like to growth along with perpendicular direction of the equatorial plane,but less likely to parallel or appear a certain angle with the equatorial plane under uniform magnetic fields.However,the percentage of the direction of dendrite growth which was at a certain angle with the equatorial plane does not change a lot under gradient magnetic field.The ferromagnetic of pure cobalt and the magnetic field intensity decline under the gradient magnetic field is the main reason.At low undercooling(?T?150 K),the dendritic growth velocity appeared a decrease and then increase,then follows the "sinusoidal" shape tendency to change with the rising of magnetic field intensity.The critical magnetic field intensity where the greastest change of dendritic growth velocity was happened is 5 T,1 T and 3 T respectively,and the dendrite growth velocity was maximum,minimum and maximum value respectively.At large undercooling,the growth velocity as a function of magnetic field appeared a reverse tendency to small undercooling and a smaller extent.The results of theoretical calculation of the experimental data shown that the melt flow is closely related to the magnetic field and undercooling.When the melt flow reached the dynamic equilibrium,the corresponding convection velocity point is called the extreme point of flow velocity.When flow velocity was zero,the corresponding point is called zero flow velocity point.When applying uniform magnetic fields,the melt flow enhanced first and then subdued alone the direction of dendritic growth with rising undercooling.Finally,the melt flow subdued alone the opposite direction of dendritic growth.The undercooling at extream point and zero point of flow velocity increased with rising magnetic fields,and the melt flow enhanced under low magnetic fields(B?3 T).The same change of melt flow was happenned under high magnetic fields(B?3 T),and the maximum of flow velocity was 2000 m/s under 5 T magneic field.Analysts say that there was existing transverse flow promoting dendrite growth and circular flow suppressing dendrite growth under uniform magnetic fields,and the two kinds of convection conditioned each other.Dendrite tip gradually coarsened with rising magnetic fields,which leaded to change of flow velocity.Compared with the uniform magnetic fields,the undercooling of the extreme point of flow velocity and zero flow velocity point decreased,and flow velocity increased under gradient magnetic fields,especially under the negative gradient magnetic fields.Moreover,the maximum of flow velocity was 2000 m/s under the uniform magnetic field of 5 T,while was 600 m/s under negative gradient magnetic field of 2 T.Analysts say that the damping of magnetic field intensity under gradient magnetic fields and the difference of magnetic susceptibility could generate additional convection in the melts(negative convection under positive gradient magnetic fields,positive convection under negative gradient magnetic fields).The melt flow would reached dynamic equilibrium more easily under gradient magnetic fields duo to the additional convection.