Fundamental Study On The Control Of Solidification Microstructure Of Alloys And The Fabrication Of Graded Or Aligned Materials In High Magnetic Fields

Since the solidification microstructures of materials play the key role on the properties of these materials by directly or indirectly means, the control of the solidification process has drawn continuous attention from researchers. In this study, a series of experiments of solidification, quenching, and isothermal annealing in both the semi-solid and melting states in high magnetic fields were carried out. On the basis of measuring the phase transformation temperature and the distribution of phases in alloys and characterizing crystal orientation and phase alignment, we examined the phase equilibria, the migration behavior of particles and solutes, the relationship between crystal orientation and phase alignment and their formation mechanisms during the solidification processes of the Al-Si、Ag-Cu、Mn-Sb、Al-Li, and Al-Ni alloy systems in high magnetic fields. The main conclusions are as follows:The high magnetic field was found to show different influences on the solidification behavior of different binary eutectic alloy systems. With the application of a high magnetic field of10T, the eutectic temperature and the amount of the eutectic MnSb of Mn-Sb system were increased and the eutectic point of such alloy system was shifted to the high Mn content side. However, the liquidus and eutectic temperatures of Al-Si and Ag-Cu alloy system remained unchanged. On the basis of Lorentz force that suppresses the convection and thus weakens the solute diffusion in the melt, Si and Cu concentration in the primary Al and Ag respectively at their eutectic temperatures was increased.During the isothermal annealing of a hypoeutectic Mn-Sb alloy in the semi-solid state in a high gradient magnetic field, the MnSb particles were gathered in the side of the specimen, to which the magnetic field gradient points, with a continuous change in volume fraction. The bigger absolute product of the magnetic flux density and its gradient (|BdB/dz|) and the longer holding time, the higher degree of the graded distribution of the MnSb particles could be led to. After the isothermal annealing in the molten state and quenching of a hypoeutectic Mn-Sb alloy in a high gradient magnetic field, the primary MnSb dendrites were gathered in the side of the specimen, to which the magnetic field gradient points, with a continuous change in volume fraction. The bigger absolute|BdB/dz|value, the longer holding time, and the lower holding temperature, the higher degree of the graded distribution of the MnSb particles could be led to. The formation of the above structures was attributed to the migration of the MnSb particles for the semi-solid state and of the Mn clusters for the molten state, which were driven by the magnetic force. A novel method was proposed for fabricating functionally graded materials and separating liquid mixtures at high temperature, which based on the in-situ control on the distribution of particles and solute by the isothermal annealing process in both the semi-solid and molten states in high gradient magnetic fields.During the slow-cooling solidification process in a high gradient magnetic field, a graded microstructure, of which hypoeutectic, eutectic, and hypereutectic structures were alternately distributed, was produced. The primary MnSb dendrites in such microstructure were gathered in the side of the specimen, to which the magnetic field gradient points and their amount increased with increasing|BdB/dz|value. The formation of such structure was realized by the combined effects of the magnetic force and the Lorentz force. The former drove the Mn clusters to migrate and form a graded distribution in the melts, while the latter suppressed the convection to maintain the graded distribution of Mn solute. Furthermore, the formation of such microstructure is dependent on alloy composition, specimen size in the magnetic field gradient direction, and cooling rate, which mean that the more eutectic composition, the shorter specimen, and slower cooling rate favored such formation. A novel method was proposed for in situ fabricating graded materials by solidification process in gradient magnetic fields.During the isothermal annealing in the semi-solid state and the slow-cooling solidification of a hypoeutectic Mn-Sb alloy in high uniform magnetic fields, the primary MnSb phase particles in the alloy were oriented with their c axes perpendicular to the field direction (magnetic crystal orientation) and aligned in morphology along the field direction. The MnSb crystals in the eutectic Mn-Sb alloy were not oriented due to the absence of the space and the liquid matrix needed by these crystals to rotate during their cooperative growth. It was found that the phase alignment was on the basis of the magnetic crystal orientation; meanwhile, other mechanisms must be taken into account simultaneously. During the isothermal annealing process in the semi-solid state in a gradient high magnetic field, after the MnSb particles were oriented, the dipole-dipole interactions among the particles were induced and caused which led to the collision of the particles and form aligned structures; on the other hand, during the slow-cooling solidification process in a gradient high magnetic field, the alignment of the primary MnSb grains were produced by combined effects of preferred crystallographic, magnetic crystal orientation and heat flow directions to form aligned structures. According to the experimental results, a novel method was proposed for in situ fabricating crystal-oriented and phase-aligned materials using isothermal annealing process in the semi-solid state or solidification process in gradient magnetic fields.