Real Time Imaging On Microscopic Behavior During Alloys Solidification Under Electric Field

Macroscopical mechanical properties of alloys mainly depend on its microstructure. The solidification process plays a dominate role in the evolution of microstructure. Electric field is an advanced and effective methold to vary thermodynamics and kinetics during solidification of alloys by affecting the force and heat in alloy melt. It is helpful to study the kinetics mechanism of microstructure under electric field based on probing microscopic behavior of alloys with electric field treatment during solidification. Accordingly, it can provide theoretical proofs to scientifically and reasonably work out electric field parameters.The conventional experimental methods are based on scanning electron microscope (SEM) and transmission electron microscope (TEM) to observe and analyze the final solidification structure after complete solidification or interrupting solidification by quenching. It can be only obtained the characters of microstructure in the particular moment and the particular section. It will lead to the missing of some crucial dynamic information during solidification process. Synchrotron radiation real-time in situ imaging technology raises a new research idea and opens a novel window for the study on dynamic solidification process of alloys. This method, which utilizes the high brightness, energy and resolution synchrotron radiation X-ray source combined with a high fast-readout charge coupled device (CCD) could capture many dynamic process of microstructure evolution during solidification process of alloys and real-time record the whole solidification process of alloys. The real-time imaging results can offer the direct proofs to investigate microscopic behavior and intrinsic mechanism during solidification of alloys. In present work, the study of the microscopic behavior in Sn-Pb, Sn-Bi alloys and the liquid-liquid phase transformation and separation in Al-Bi alloys during solidification process with electric field treatment has been achieved by synchrotron radiation X-ray real-time in situ imaging technology. The kinetics mechanism of dendrites growth and two-phase separation under eceltric field has been understood in depth.The modification of dendrite growth morphology and growth rate in Sn-Pb alloy with direct current (DC) treatment has been studied. The results show that the Lorentz force generated by DC washes the sides and underpart of the dendrites, which decreases the solute concentration nearby. The dendrites in every direction grow with the same rate and the dendrite "self-poisoning" is thus weakened. Finally, a nearly regular equiaxed dendrite appears. Meantime, it is clear that the electric current can vary solute distribution ahead of dendrite growth front. The neighbouring dendrites growth will be affected by solute diffusion layer. Consequently, the growth rate decteases and dendrite growth stops ahead of time.The influence of DC on the solidification process of Sn-Bi alloy is observed. The results demonstrate that the electric current will gather on the dendrite tip due to the difference of conductivity between solid and liquid phase. The sharp tip tends to be round or flat with an ability of self-adjustment to scatter the electric current for reducing Joule heat in order to keep growing. When the melt with temperature gradient subjected to the case of direct current, an electrical body force appears which can disturb the melt intensively, together with natural convection and density difference caused by solute segregation, leading to the floating dendrite rotate obviously.The effect of electric current pulse (ECP) on microstructure of Sn-Bi alloy during different solidification stages has been investigated. When ECP was imposed on the whole solidification process, dendrites break out to grow with a large growth rate in such a short time after the long nucleation incubation period. It can be seen that the refinement of Sn-Bi alloy is clear. When ECP was imposed only on the stage of dendrite growth, the grown dendrites are melted due to the heat effect of ECP. As the solidification proceeding, the melted dendrites reappear based on the previously growing trajectories and present hereditary character. However, no refined dendrites can be found in this experiment. The imaging experiments confirme the importance of time to impose ECP.The microscopic behavior of second-phase droplets in Al-Bi immiscible alloy during solidification process is in situ observed. It is demonstrated that the droplets with little difference in radius size collide with each other to form a new ellipse-like droplet. While the Swald coarsening occurred among droplets with extreme difference in radius size. The bigger droplet gradually engulfs a great number of droplets around it and form a nearly rounded droplet with tremendous radius.The influence of ECP on microscopic behavior of second-phase droplets in Al-Bi immiscible alloy has been investigated. It is clear that the separated Bi-rich droplets form lots of big clusters which are no longer the sphere in shape but the cloud morphology. Due to the effect of Marangoni force and electromagnetic pinch force, the second-phase droplets move along the direction of their resultant force and form an inverted triangle shape in the Al matrix finally.Based on synchrotron tomography imaging technology, the three-dimensional morphology and distribution of second-phase particles in Al-Bi immiscible alloy have been studied. Meanwhile, the effect of A1-3B, Al-5Ti-B and Al-3Ti grain refiner on Al-Bi immiscible alloy has been probed. After restructuring the solidification microstructure of the alloys, it is found that A1-3B and Al-5Ti-B can refine Al-Bi alloy, but the second-phase particles distribute inhomogeneously in matrix and the larger particles still exist. After Al-3Ti grain refiner added, not only second-phase particles appear as spherical morphology with tiny radius size, but also distribute homogeneously in matrix. The refining effect of Al-3Ti on Al-Bi immiscible alloy is excellent.