Study On Influence Of Interaction Between Thermoelectric Currents And Magnetic Field During Directional Solidification Of Binary Alloy

How to properly control the solidification process and thus achieve the metallic materialswith high performance is always the popular research topic for the metallurgists. Applying themagnetic field has been widely accepted as the efficient method to control solidification of alloyssince1950s’ because, during solidification, it can affect the flows in melt and modify the crystalorientation et al. But, how the solidification behaves under magnetic field is far from understoodhitherto. For instance, it was thought for a long time that applying the static magnetic field candamp the flows in melt during solidification, but some experiments uncovered that staticmagnetic field can produce flows in melt as well. Researches shown that such ‘unexpected, new’flows (thermoelectric magnetic flows, TEM flows) are the result of interaction between theinherent thermoelectric currents and the applied magnetic field during solidification of alloys.Although some researches about TEM flows in solidification of alloys can be found, the systemicinvestigation on the interaction between the internal thermoelectric currents and the appliedmagnetic field is lack. Considering so, this thesis chosen Al-Cu alloys as the research subject toconfirm the existence of TEM effect (which contains TEM forces and TEM flows) duringdirectional solidification of binary alloys and investigate the influence of them on the structureformation by the methods of schematic model, synchrotron X-ray radiograph, and the directionalsolidification experiments.After introduce the encounter between the thermoelectric current and the applied magneticfield in solidification of alloys, this thesis illustrates the corresponding thermoelectricphenomena in the solidification process and how the thermoelectric currents and TEM effectoccur by means of schematic model.In order to display TEM effect in directional solicitation of binary alloys more intuitively,this thesis carries the numerical simulation of thermoelectric currents, TEM forces and TEMflows at the vicinity of solid/liquid interface during directional solidification after deduces theequations governing the TEM effect. Computed results show that TEM flows speed up to the maximum velocity with increasing magnetic field and then slow down when the appliedmagnetic field rise further, and TEM forces in solid increases constantly with the magnetic fieldrising.The directional solidification of Al-Cu alloys was real-time observed by means of in situsynchrotron X-ray radiograph in order to provide more convictive evidence for the existence ofTEM effect in directional solidification of binary alloys. Based on the comparison with theanalytically calculating particle motion driven by the TEM forces only, it can be found that theobserved crystal (dendritic fragmentation) motion under transverse magnetic field is caused byTEM forces acting them. Moreover, according to the comparison of the solid/liquid interfaceshape evolutions those observed without and with transverse magnetic field, and the3Dnumerical simulations of TEM flows for the in situ experimental conditions, it can be found theoriginal tilted interface developing to nearly flat under magnetic field is just the result of TEMflows.Further, single phase Al-Cu alloys were directionally solidified under various axialmagnetic fields to ascertain whether TEM effect can affect in a relative bigger samples or not.Results show that the interface shape is always affected by TEM flows no matter it has the planar,cellular or dendritic morphology, and more, the change tendency of the interface shape with theapplied magnetic field is the same as that of TEM flows. Besides, similar to the evaluation ofTEM flows, TEM flows will reach the maximum velocity under higher magnetic field for themorphology has smaller typical length.3D numerical simulations of TEM effect reveal that TEMforces in solid under an axial magnetic field cause a torque in the solid, and the solid will besubjected to the uniform TEM forces if apply a transverse magnetic field.Furthermore, this thesis extends the investigations on the influence of TEM effect onstructure formation to the near-eutectic Al-Cu system. Results show that, for both thehypoeutectic Al-26wt%Cu and hypereutectic Al-40wt%Cu cases, the mushy zone length varieswith the magnetic field rising within1T, and the varying tendency is the same as that of TEMflows with increasing magnetic fields. In additions, applying high magnetic field enhances theformation of couple growth structure during directional solidification of hypoeutectic alloys, whilst, facilitates the growth of primary Al2Cu phase during directional solidification ofhypereutectic alloys. Differential thermal analysis shows that the former phenomenon should beattributed to the changes of nucleation temperature and velocity of different phases by highmagnetic field, which benefits the prevailing of coupled growth structure. Transition electronicmicroscope investigation finds that the faceted Al2Cu phase achieved under high magnetic fieldhas more dislocations. Because stress in solid is the direct reason for defects multiplicationduring solidification, the latter phenomenon can be attributed to the TEM forces acting on theAl2Cu phase.