Study Of Microstructure Evolution During Solidification Process Of Zn-Bi Immiscible Alloys By Superimposing Static Magnetic Fields

As a prospective alloy, immiscible alloys have been widely used in industrial application. However, the alloys with many fine properties can not be obtained for their special character of metallurgy. Recent years, with the development of the technology of super magnet and Bitter magnet, people can obtain a magnetic field of 10 T-30 T for a long time. In such a strong magnetic field, the extreme strong Lorentz force, magnetic field force and magnetic energy have a profound effect on the solidification process of metal. Such as, orientation of phases, grains migration, refinement of dendritic. However, in terms of the solidification process of immiscible alloys, the effect of the above magnetic effects and especially the super high static magnetic field(HSMF) on the nucleation, growth, migration, coalescence and orientation of the second phases is lack of deeply investigation. Thus, by using Zn-Bi immiscible alloy, the thesis makes a deep investigation on the regular of microstructure evolution in Zn-Bi immiscible alloys by superimposing HSMFs from 0-30 T and tries to explore a new method to process a homogeneous immiscible alloy under a common gravity and cooling rate. The main contents are described as follows:Firstly, the thesis has investigated the effect of HSMF on the segregation behavior of the second Bi-rich phase during the solidification process of Zn-Bi immiscible alloys. The results show that both Stokes sedimentation and Marangoni migration can be suppressed by HSMF. In addition, HSMF has a dramatic effect on both the nucleation and growth process of the second Bi-rich phase in Zn-Bi immiscible alloy. The liquid droplets grew as a way of pure diffusion when B is larger than 17.4 T. It is beneficial to the refinement and dispersion of Bi-rich droplets by imposing HSMF.Secondly, the thesis have investigated the effect of cooling rates on the solidified microstructure of Zn-6wt.%Bi immiscible alloys under the effect of HSMF. The results show that a chain-like microstructure will be formed inside the equaxed grains of surface layer of Zn-6wt.%Bi immiscible alloys solidified under both 0T and 18 T. Thermoelectric magnetic force(TEMF) acted an important role in modifying the solidified microstructure in the ingot solidifying with a natural cooling rate.Thirdly, thesis have investigated the effect of a weak magnetic field on the distribution of the second Bi-rich particles in the microstructure of Zn-6wt.%Bi immiscible alloys. Based on the experimental results carried out under a traverse magnetic field, we known that the TEMF and thermoelectric magnetic flow(TEM-flow) were the main reasons leading to the change of spatial and size distribution of Bi-rich particles.Fourthly, the thesis have investigated the effect of HSMF on the orientation behavior of solidified microstructure in Zn-95 wt.%Bi immiscible alloys. The physical orientation under HSMF was established. The results show that the c-axis of Zn plate was highly aligned to the direction of HSMF at 6 T. The alloys showed a near-magnetic isotropic behavior at 0 T and 6 T whereas that showed a magnetic anisotropic behavior at 2 T and 4 T. The alloy solidified at 6 T showed an easiest demagnetization property in the direction perpendicular to the HSMF.Lastly, the effects of HSMF compounding with alternative current on the microstructure of Zn-10 wt.%Bi and Zn-20 wt.%Bi. The experimental results indicated that the second Bi-rich phases were dispersed in matrix and its dimension were significantly refined when electromagnetic body force was 5×105 N/m~3, frequency of alternative current was 50 Hz and magnetic flux density was 10 T. The overlarge or overlow frequency of alternative current will lead to the segregation and even layered microstructure in Zn-Bi ingots.The thesis systematically studied the effects of magnetic field on the microstructure of Zn-Bi alloys. Many kinds of Zn-Bi alloys with different characterization of solidification were fully investigated. The results fulfill enrich the solidification theory of immiscible alloys under the effect of magnetic field. In addition, this thesis offers some new idea and methods to manufacture immiscible alloys without any segregation.