Study On Atomic Diffusion In Al Alloy Melts By Magnetic Confinement

The properties of metallic materials not only depend on the specific casting process, but also are related to the liquid structures and physical properties of metallic melts. Atomic diffusion and viscosity are two important physical parameters of metallic melts, which play significant roles on the formation of crystal nucleus, crystal growth, the morphology and distribution of solidification structure. Diffusion and viscosity are highly sensitive to the concentration, temperature, structural evolution, and external field of metallic melts, which are considered as an effective route to study the metallic melts. The external magnetic field affects the various processes of formation of metallic materials by non-contact way. In just half a century, the magnetic field has been widely applied and developed into one of the most active areas of research. However, the investigations on the fundamental physical properties of metallic melts under the magnetic field are far from understood hitherto.In the present thesis, we investigated the effect of magnetic field on the interdiffusion behavior between Al-5 at.% Si and Al melts. It is found that the transverse magnetic field can effectively suppress the interdiffusion process of melt and impede the migration of Si component. The sluggish atomic diffusion was attributed to that under the magnetic field condition, the difference of chemical potential induced the occurrence of Hall Effect in the metallic melts. Finally, under the combined action of electrical field and magnetic field, atomic diffusion was suppressed. With respect to the nontrivial dynamic behavior, we set up the model of atomic diffusion by magnetic confinement and modified the Fick’s second law.Then, the macroscopic viscosity of Al-Si melt under magnetic field was investigated. Results show that with increasing intensity of magnetic field, viscosity increases gradually. Meanwhile, a quadratic B dependence of the dynamic viscosity obtained in the same confined environment was observed. Furthermore, we investigated the effects of Cu and Ni on the viscosity of Al melt. It is found that when the magnetic field intensity is less than 1000 G, the viscosities of Al-Si, Al-Cu and Al-Ni melts do not show significant difference. However, when more than 1000 G, the viscosity of Al-Ni melt dramatically increases, which may be related to strong interaction between Al and Ni atoms.Based on the understanding of atomic diffusion, we found that the TiC particle can suppress the rapid diffusional growth and aggregation of the second phase in the Al-In, Al-Bi immiscible alloys.Al-30 wt.% In and Al-20 wt.% Bi immiscible alloys show different behaviors of phase separation. In the Al-30 wt.% In alloy, a disordered distribution of Al phase and In phase was observed. In the Al-20 wt.% Bi alloy, spherical Bi phase distributed in the Al matrix.Results show that the addition of TiC particles can effectively suppress the phase separation process and result in a uniformly dispersive microstructure. It is found that the suspending TiC particles prevented the second phase (In, Bi) from diffusional growth, aggregation and sedimentation within liquid miscibility gap. The size and morphology of the second phase can be modulated by changing the content of TiC particles. When adding enough TiC particles, micron-sized second phase uniformly dispersed in the A1 matrix, then decreasing the content of TiC particle, the second phase presented big aggregations or spheres. The modified immiscible alloys by the TiC particles showed low friction coefficient that were expected to develop high performance self-lubrication bearing materials.