Alloys with a miscibility gap in the liquid state are a broad kind of materials. Many of them are excellent candidates to be used in industry if a finely dispersed microstructure can be obtained. These alloys, however, have an essential drawback that just the miscibility gap, poses problems. When a homogeneous single phase liquid is cooled into the miscibility gap, it transforms into two liquids. Generally the liquid-liquid decomposition causes the formation of a microstructure with serious segregation. This makes not only the manufacture of monotectic alloys very difficult, but also the solidification process rather complex. The knowledge on the mechanism of the microstructure evolution is the prerequisite for the development of the manufacturing technique of the monotectic alloys. Its study has great science and practical significance. In this paper, the solidification behaviors of Al-Pb monotectic alloy, which has good potentials in the application of automotive industry, are investigated. A model is proposed to analyze the microstructure evolution during a cooling of immiscible alloys. The microstructure development in a directionally solidified Al-Pb alloy is analyzed in details. The main researches are summarized as follows:
(1) A model is developed to describe the melt flow in front of the solidification interface in a monotectic alloy directionally solidified. The governing equations for the concentration field and temperature field during the solidification of the sample are given which take into account the common action of the diffusional transport, convective transport and the migration of the minority phase droplets. A model was developed to describe the kinetics of the liquid-liquid phase transformation in a monotectic alloy directionally solidified. The model takes into account the concurrent actions of the nucleation, diffusional growth, collision and coagulation of the minority phase droplets, spatial segregation of phases and fluid flow in the melt. The results show that the numerical model based on the melt flow, heat transport, solution transport and liquid-liquid phase transformation describes the microstructural development well and provide theoretical foundation for manufacturing production;
(2) The directional solidification experiments have been carried out with Al-Pb alloys. The Pb content and solidification rate have great effect on the microstructure formation. The average size of the minority phase particles decreases with the increase of the solidification rate and increases with the increase of the Pb content;
(3) The microstructure formation in a monotectic alloy solidified under the practical directional solidification conditions is calculated. The effects of the alloy composition, solidification rate and melting temperature on the microstructure formation are investigated. The numerical results demonstrate that for an alloy of given composition, the nucleation position moves towards the solidification interface, the nucleation rate increases and the average radius of the minority phase droplets decrease with the increase of the solidification velocity and the enhancement of the melting temperature. For a given solidification velocity and melting temperature, the nucleation position moves away from the solidification interface, the nucleation rate decreases and the average radius of the minority phase droplets increases with the increase of the Pb content;
(4) The effect of the convections on the microstructure evolution has been calculated. The numerical results demonstrate that the convections have great effect on the nucleation of the minority phase droplets. A convective flow along moving direction of the sample causes an increase in the cooling rate and, therefore, an increase in the nucleation rate of the minority phase droplets. The convections enhance the maximums of the number density, average radius and volume fraction of the minority phase droplets in front of the solidification interface. It promotes the formation of the solidification microstructure with a serious macro-segregation. The convections make it more difficult to obtain a monotectic alloy with a well dispersed microstructure;
(5) The effects of the cooling rate on the microstructure evolution during the nucleation period and growth stage of the minority phase droplets is investigated. The results demonstrate that the cooling rate during the nucleation period of the minority phase particles has an overwhelmingly strong effect on microstructure formation compared with the cooling rate after the nucleation period;
(6) The directional solidification experiments have been carried out for Al-Pb alloys in a static magnetic field. The microstructure formation of the monotectic alloy in a static magnetic field has been analyzed. The magnetic field has great effect on the solidification of the monotectic alloys. The average radius of the minority phase particles decreases with the increase of the magnetic field strength. For a given alloy and magnetic field strength, the lower the solidification rate, the stronger the effect of the magnetic field on the solidification microstructure;
(7) A model is developed describing the two-phase flow in front of the solidification interface when solidifying a monotectic alloy directionally. The microstructure evolution in a directionally solidified Al-Pb alloy in a magnetic field is investigated by solving together the controlling equations for the kinetic process of the phase transformation, the temperature field, the concentration field and flow field. The results demonstrate that the magnetic field affects the microstructure formation mainly through the suppression of convections. It causes a more uniform distribution of the nucleation rate and the number density of the minority phase droplets along the radial direction of sample. It also causes a decrease in the size of the maximum droplet in front of the solidification interface.
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