Study On Influence Of Thermoelectric Force On The Structure Of Directionally Solidified Eutectic Alloys

As a new interdiscipline major,solidification of metallic alloys under a static magnetic field is a new research area and some research results have been obtained.Present thesis has investigated the effect of a weak tranverse magnetic field(B?0.8 T)and a high axial static magnetic field(B?12 T)on the solidification behavior in metallic alloys.With the assist of advanced testing method,numerical simulation and in-situ synchrony X-ray oberservation,the effects of the magnetic field on the solidification have been investigated.Further,models are built to analyze the physical effects-the thermoelectric magnetic convection(TEMC)in the liquid,the thermoelectric magnetic force(TEMF)imposed on the solid.Some valuable conclusions have been gained.The dendriteic growth in the binary alloys under a transverse magnetic field was investigated.It has been found that the application of a transeverse magnetic field has caused the sloping of solid/liquid interface and the segreagation channal in the dendritic structure at a higher magnetic field.With the increase of the applied magnetic field intensity,the primary dendrite spacing and mushy zone length decreases.In the directional solidification of Al-7wt%Si alloy,the applied magnetic field decreases the number of fragments.Theoretical analysis and numerical simulations have been taken to discuss the thermoelectric magnetic convection(TEMC)at various typical scales.The results showed that the magnitude of TEMC increases to a maximum value and then decreases with the increasing of the magnetic field intensity.Moreover,the results also showed that for the larger typical scale,the higher critical magnetic field intensity is larger.The transportation of solute caused by TEMC should be responsible for the above experimental phenomena.The influence of a transverse magnetic field on the primary phase in the melt has been investigated by studying the growth behavior of the Al-10wt%Cu and Al-18wt%Si alloys during the directional solidification under a transverse magnetic field.It has been observed that the primary phase migrated to one side of sample and formed a graded distribution in the melts.Theoretical analysis and numerical simulations have been taken.The results show that the TEMF on the primary phase should be responsible for the migration.The TEMF increases with the increase of magnetic field intensity and temperature gradient.In-situ experimental results proved that the exsit of TEMF and the effects of magnetic field on the primary phase directly.Above results show that,the TEM flows speed up to a maximum velocity with the increase of magnetic field intensity and then slow down when the applied magnetic field intensity rise further,and TEM forces in solid increases constantly with the magnetic field intensity rising.Thus,a high axial magnetic field was applied to investigate the dendritical growth during directional solidification.It can be found that the regular dendrite structure was prone to be destroyed under a high magnetic field,and the equiaxed structure has appeared.With the appearance of CET phenomenon,the equiaxed grains were aligned with the easy magnetization axis along the magnetic field direction.Numerical simulation show that the TEMF acting on the dendrtite increases with the increase of the magnetic field intensity.When the acting force was strong enough,the dendrite will be destroyed and the aligned equiaxed will be formed.Further,the influence of the axial high magnetic field on the Al-Cu-Ag ternary alloys has been investigated by studying the growth behavior of univariant reaction(L??(Al)+Al2Cu)and invariant reaction(L??(Al)+Al2Cu+Ag2Al)during directional solidification.It has been observed that the coupled eutectic spacing decreases till disappears with the increase of magnetic field intensity.For invariant eutectic growth,the applied magnetic field decreases the ternary eutectic spacing and caused the formation of banded structure.It has been suggested that owing to the magnetization of the diffusion boundary,the solute diffusion in the diffusion boundary has been retarded.This will result in the decrease of the eutectic spacing and the formation of the banded structure.Moreover,the effects of magnetic field on the crystallography of the ternary eutectic alloys were studied.The applied magnetic field modified the orientation relationship(OR)between eutectic phases and the change of eutectic interfacial energy should be responsible for the modification.The experimental examination and theoretical study on the effect of static magnetic field on the microstructure and crystallography of the metallic alloys during directional solidification carried out in this work contribute to the enrichment of the theory of Electromagnetic Processing of Materials(EPM)and are of both theoretical significance and technical interest.The positive results on solute convection and primary grain control,eutectic growth could offer useful guidance for practical material processing techniques.