Study Of The Solidification Of Monotectic Alloys Under The Effect Of Microalloying

Monotectic alloys with a miscibility gap in the liquid state are widespread. When a homogeneous monotectic alloy melt is cooled into the miscibility gap, it transforms into two liquids. Many of the monotectic alloys show excellent properties if the soft minority phase disperses uniformly in a comparatively hard matrix phase. However,the solidification microstructure evolution of the monotectic alloys during the liquid-liquid (L-L) phase transformation is very complicated and it generally causes the formation of a phase segregated microstructure under regular solidification condition. The application of the monotectic alloys is, thus, very limited. It is of great significance to investigate the solidification process of the monotectic alloy in order to control the preparation of the monotectic alloys with desired structure and promote the indurstrial application of the monotectic alloys.Microalloying remarkably influences the solidification processe of metals and alloys. However, up to now, little work has been carried out on the solidification behavior of the monotectic alloys under the effect of microalloying. In this paper, both the experiments and numerical simulations have been carried out to investigate the solidification behavior of the monotectic alloys under the effect of microalloying. The influence mechanisms of microalloying on the solidification microstructure of the monotectic alloys and the feasibility to controll the solidification process and microstructure by microalloying have been explored. The main research work and results are summarized as follows:Continious solidification experiments have been carried out with the monotectic alloys under the effect of minor elements. The effect of minor Bi on the solidification microstructure of the Al-Pb alloys and the effect of minor Sn on the solidification microstructure of the Al-Pb (Bi) alloys have been investigated. The results demonstrated that the addition of minor Bi can obviously refine the Pb-rich particles in the Al-Pb alloys and the addition of Sn can obviously refine the Pb (Bi)-rich particles in the Al-Pb (Bi) alloys. With the increase of the minor elements addition,the refining efficiency shows a tendency of increasing and then keeping constant.With the increase of the volume fraction of the MPDs, the refining efficiency of the minor surface-active elements on the MPDs increases. That is to say, minor Bi can work as the surface-active element of the Al-Pb alloys and minor Sn can work as the surface-active element of the Al-Pb (Bi) alloys.A model was established to describe the solidification microstructure evolution of the monotectic alloys under the effect of the minor surface-active elements.Simulations have been carried out in accordance to the continuous solidification experiments carried out with Al-Pb alloys under the effect of minor Bi. The influence mechanism of the minor surface-active elements on the solidification microstructure of the monotectic alloys has been disscussed. It has been demonstrated that, the minor surface-active elements enrich at the interface between the liquid matrix phase and the minority phase droplets (MPDs). The enrichment cause a decrease of the interfacial tension between the two liquid phases of the monotectic alloys and an increase in the nucleation rate of the MPDs as well as a decrease of the Marangoni migration velocity of the MPDs, and thus promotes the formation of the monotectic alloys with a well dispersed microstructure.A new strategy for the fabrication of the Al-Ti-C master alloy by using aluminum melt reaction method with the carbon nanotubes (CNTs) acting as the carbon source has been proposed. The Al-Ti-C master alloy with the TiC particles well dispersing in the matrix has been successfully fabricated. The micro structure evolution of the Al-Ti-C master alloy with the CNTs acting as the carbon source has been analyzed. It has been demonstrated that: (1) the TiC particles form through the reaction between solid C and solute Ti, not through the reaction between solute C and solute Ti. (2) The large specific surface area and high chemical activity of the CNTs significantly promote the reaction between the CNTs and the solute Ti, and the fabrication of the Al-Ti-C master alloys with a well dispersed microstructure.Solidification experiments have been carried out with the Al-Pb (Bi) monotectic alloys under the effect of the TiC particles in order to investigate the effect of minor TiC on the solidification microstructure of the monotectic alloys. The experimental results demonstrated that the addition of appropriate amount of TiC can obviously refine the MPPs of the Al-Pb (Bi) alloys. With the increase of the of the TiC particles addition, the average size of the MPPs shows a tendency of keeping constant,increasing, decreasing and then keeping constant again. With the increase of the volume fraction of the MPDs, the refining efficiency of the TiC particles on the MPPs increases. It indicates that the TiC particles can work as effective heterogeneous nucleation sites of the MPDs during the L-L phase transformation of the Al-Pb (Bi)alloys, and thus improve the nucleation of the MPDs and promote the formation of the Al-Pb (Bi) alloys with a well dispersed microstructure.A model was developed to describe the kinetic behavior of the TiC particles in the melt and the microstructure evolution during the L-L phase transformation of the monotectic alloys under the effect of the TiC particles. Simulations have been carried out in accordance to the solidification experiments carried out with Al-Bi alloys under the effect of minor TiC. The effect of the kinetic behavior of the TiC particles in the melt on the solidification microstructure evolution of the monotectic alloys has been investigated and the influence mechanism of minor TiC on the solidification microstructure of the monotectic alloys has been illuminated. It has been demonstrated that the TiC particles may dissolve, coarsen during holding temperature and precipitate out during cooling process, which plays an important role on the solidification microstructure evolution of the monotectic alloys. What determines the refining efficiency of the TiC particles on the MPPs is the resultant number density of the TiC particles in the melt cooled to the beginning temperature of the L-L decomposition of the monotectic alloys. When the number density of the TiC particles in the melt cooled to the beginning temperature of the L-L decomposition of the monotectic alloys is not high enough, the addition of TiC particles causes a coarsening of the MPDs/MPPs. If the number density of the TiC particles in the melt cooled to the beginning temperature of the L-L decomposition of the monotectic alloys is high enough, the addition of TiC particles causes a refinement of the MPDs/MPPs and promotes the formation of the monotectic alloys with a well dispersed microstructure.