Control Of The Motion Behaviors Of Particles In Metallic Melt By High Magnetic Fields And The Relevant Morphological Evolution

High Magnetic Fields (HMFs) play an obvious role in the preparations of materials both with and without phase change because of the non-contact operation and their high-density energy. By using HMFs, some causes may be suppressed and some effects may be amplified during the material processes. So, the complicated process can be changed into an easy one, which makes the physical essence be easily understood. The material’s research under the imposition of HMFs has been a hot point recently. Especially in the solidification processes of alloys, the final solidified structures can be modified by using the effects of HMFs on the phase transition and the differences of physical properties between the solid particles and the melt matrix, and thus the ultimate preferred structure and properties can be obtained.In this dissertation, the main research topics include: under an imposed HMF, the theoretical analysis and experimental study on the motion of the solid particles in melt as well as the effect of the process parameters on the transition of solidified structure, and the influence of the magnetic force and magnetization energy impelled by HMFs on externally added and in situ precipitated particles in a conductive melt. The main contents and conclusions were as follows:(1) Based on the research of the migration and rotation of a single particle in a melt under HMFs, theoretical analysis of the influence of the interaction among multiple particles on the motion behavior and morphological transition was conducted. The interactions have an important influence on the distribution and alignment of the multiple particles in the melt, which includes either attraction or repulsion between particles. The inter-grains attraction is useful to form a rod-like aggregation, in which the long axis is parallel to the direction of the imposed field, and the inter-grains repulsion is favorable to control the spacing of such aggregations vertical to the direction of the field. The rotation of the particle in the melt would be hindered due to the interactions among the multiple particles when the fraction of the solid particles in the melt is higher than a critical value, so that the morphology change depends on the subsequent crystalline growth.(2) The experiments about the motion of added particles in melt were carried out and the impacts of high magnetic fields on the solidified structures were examined. The unmelted TiAl3particles resulting from the excess addition of Al-Ti-B master alloy into Al-Si alloys was used in this study. The experimental results showed that the distribution of TiAl3particle in solidified structure can be altered by adjusting the parameters of HMFs. With the increase of intensity of uniform HMF, the settlement of TiAl3particle was damped, and the gradient of HMF has the enhanced or damped effect on the settlement of the TiAl3particles. Simultaneously, the long axis of TiAl3particle aligns parallel to the imposed field regularly, which results from the rotation of TiAl3particle in the melt to reduce free energy.(3) The experiments of the motion of in-situ precipitated particles from melt with different fraction were carried out and the impacts of high magnetic fields on the solidified structures were examined. By taking the solid MnBi grains in semi-solid state as the research object, the motion and growth of the endogenic crystals in the melt as well as the impact of obtained solidified structure on magnetic properties of Bi-Mn alloys were investigated. The results showed that the difference of the alignment of the long axis and the imposed field associated to the fraction of the MnBi phase. However, there is the same orientation of the MnBi phase in different fractions, in which the easy magnetization c axis of MnBi is parallel to the imposed field. The above results are caused by the hindrance of the multiple particles which restricts the rotation of the solid particles in the melt. But the subsequent growth of MnBi grains relies on the changed solute concentration around the particles induced by the local magnetic field. The preferentially growing direction of MnBi grain is c axis. The saturation magnetization of specimen through semi-solid process is higher than that of the conventional conditions.(4) The experiments of the motion of in situ precipitated MnBi phase grains in dilute alloy under gradient high magnetic fields were carried out and the impacts of high magnetic fields on the solidified structures were examined. The composition of experimental alloy is Bi-4.36wt%Mn. The experimental results showed that MnBi grains moved to the centre of field due to the magnetization force induced by field gradient in the initial stage of solidification, which resulted in the aggregation of MnBi grains in the end of specimens and the fraction of MnBi was changed continuously. At the same time, the spacing of aggregation in the end is more uniform and smaller than that in center of specimens. Uniform orientation and the high degree of crystal ensure the improvement in the magnetic property of composite materials.(5) The experiments of the motion and growth of primary-precipitated MnBi phase grains in high concentrated alloy under HMFs were carried out and the impacts of high magnetic fields on the solidified structures were examined. The composition of experimental alloy is Bi-8.25wt%Mn. The experimental results showed that large banded MnBi grains occurred in the center with a uniform field and in the ends with gradient magnetic fields. The XRD results are the same with those of the dilute alloys, e.g. the c axis is parallel to the imposed field. The growth of MnBi with NiAs type structure in the basal plane increases rapidly, which is due to the fact that the solute concentration reduced in the vertical front and increased in the top end induced by the locally imposed magnetic field. Therefore the large flake MnBi morphology formed.