Effects Of Thermoelectric Magnetohydynamics Convection On Dendritic Growth Kinetics Of Pure Fe And Pure Pd

Dendrite is the most typical growth morphology of crystals in solidification of metals.It is of great importance to investigate the process of dendritic growth for developing theories of solidification and for control of microstrucure of castings.In undercooling solidification,high thermal and chemical gradients are developed ahead of growing dendrites.When a static magnetic field is applied to the undercooling solidification,thermoelectric magnetohy-drodynamics(TEMHD)convection will be excited around the dendrites,leading to changes of the tip kinetics and morphology.Because the Seebeck coefficient is a material-dependent property,the intensity of the TEMHD convection around the dendrites will vary with metals.To verify this viewpoint,dendritic growth kinetics of pure Fe and pure Pd under static magnetic fields were investigated experimentally in this thesis work.In the experiments,melts of pure Fe and pure Pd were undercooled by glass fluxing in static magnetic fields of the intensities up to B=6T.Rapid solidification of the undercooled melts was in-situ observered using a single-color pyrometer and a high-speed camera.Dendritic growth velocities in rapid solidification were determined by computer-aided analsysis 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 dendritic growth kinetics in the two metals under the influence of the TEMHD convection.The measured growth velocities were also modelled using the Alexandrov-Galenko theory on denditic growth.The conclusions are drawn as follows:(1)The dendritic growth velocities of pure Fe in the undercooling range AT=30-270K are influenced by the static magnetic fields significantly.An analysis of the reduced growth velocities of pure Fe with respect to the data measured in the zero-magnetic field showed that there are a low and a high critical magnetic field,at which the dendritic growth kinetics of pure Fe changes its tendency.The reduced growth velocities show minima at the lower critical magnetic field of B=2T.But,the reduced growth velocities at the higher magnetic field of B=6T become comparable to those measured at the zero-magnetic field.(2)The dendritic growth velocities of pure Pd in the undercooling range AT=30-250K are also influenced by the static magnetic fields significantly.An analysis of the reduced growth velocities of pure Pd with respect to the data measured in the zero-magnetic field showed that there are three critical magnetic fields,at which the dendritic growth kinetics of pure Fe changes its tendency.The reduced growth velocities show minima and maxima at the two critical magnetic fields of B=2T and B=5T,respectively.The reduced growth velocities rise up at the third critical magnetic field of B=6T.(3)Theoretical modeling showed that the Alexandrov-Galenko theory can give a reasonable explanation of the measured dendritic growth velocities of pure Fe and pure Pd undercooled by the glass fluxing technique.A regression of the dendritic growth velocities measured under static magnetic fields of B=1-6T revealed that the TEMHD convection alters the effective thermal diffusivity in the melts bringing about the changes of the dendritic growth kinetics of pure Fe and pure Pd.Because of a difference in the Seebeck coefficient between the two metals,the extent and mechanism of the influence of the TEMHD convection on their dendritic growth kinetics are much different.