Heterogeneous Properties Of Al-Sn-Bi Alloy Melts And Its Influence On Solidification Characteristics And Structure

In this work, the microinhomogeneity and liquid-phase separation of Al-Sn-Bi alloy melts were investigated through electrical resistivity, viscosity experiment and DSC method, the melt structure changes phenomenon and mechanism were also discussed. The method of Kissinger and Ozawa were applied to study the thermodynamic and kinetic characteristics of Sn-Bi alloy melt structure transition. The cooling curves, solidification structure and corrosion resistance of Sn₅₇Bi₄₃ alloy melt solidified from different liquid states were measured, the effect of liquid structure transition on the solidification behavior and solidified structure was investigated. The effect and mechanism of melt temperature, cooling rate and composition on the liquid-phase separation and core-shell structure formation in immiscible Al-Bi-Sn alloys was studied by copper rapidly cooling method.The liquid structure of Sn100-x Bix alloys（x=0, 7, 30, 43, 80, 100） was studied with resistivity and viscosity method. The resistivity of liquid alloy increases with the increase of the temperature and Bi content, and obvious turning point is observed on resistivity-temperature and viscosity-temperature curves of different Sn-Bi alloy, while the resistivity increases linearly with increasing temperature before the turning point. Moreover, different composition alloy shows different turning temperature. These results indicate that temperature-induced liquid-liquid structure change occurs at 973-1073 K and is reversible, which may be formed by large cooperative motions for molecular rearrangements. Based on these results explored the melt structure from different aspects, Sn-Bi melt structure can be described and the nature of discontinuity of structural phase transition can be explored.The temperature dependence of electrical resistivity, the thermodynamic and kinetic characteristics of Sn₅₇Bi₄₃ eutectic alloy melt structure transition were investigated; the activation energy of structure transition（Ea） and the continuous heating transformation curve（CHT curve） were calculated by a thermal analysis method. The results showed that, both resistivity and thermal parameters of Sn₅₇Bi₄₃ alloy changed abnormally at the range of 973-1073 K, and their characteristic transition temperatures matched well each other, which verifies the existence of liquid-liquid structure transition in Sn₅₇Bi₄₃ alloy and the effectiveness of the resistivity and DSC experimental results, and the method of Kissinger and Ozawa can be applied to study the kinetic characteristics of Sn-Bi alloy melt structure transition.The liquid-phase separation of ternary immiscible Al-Sn-Bi melts was studied with resistivity and thermal analysis methods at different temperatures. The resistivity-temperature curves appear anomalous and abrupt change as rising temperature, corresponding to the distinctive and low peak of melting process in the DSC curves, indicative of the occurrence of the liquid phase separation. The anomalous behavior of the resistivity temperature dependence is attributable to concentration-concentration fluctuations. The microheterogeneity-microhomogeneity transformation causes a large fluctuations in concentration, which make the randomness and chaos of the atoms larger, leading to the greater impediment to electron movement and the sharp rise of resistivity. The addition of tin to the Al-Bi immiscible alloys decreases the temperature of monotectic reaction. It is concluded that concentration-concentration fluctuations are responsible for the anomalous behavior of resistivity and DSC methods.The obvious turning point is observed on resistivity-temperature, DSC and viscosity-temperature curves of Sn₅₇Bi₄₃ alloy. Based on these results explored the melt structure from different aspects, the effect of liquid-liquid structure transition on solidification of Sn₅₇Bi₄₃ alloy is studied. The results show that the undercooling of the eutectic phase increases and the microstructure becomes finer after solidifying from the melt experiencing liquid-liquid structure transition. The spacing of eutectic phase decreases markedly. Its corrosion resistance increases.The effect of melt temperature, cooling rate and composition on the liquid-phase separation and core-shell structure formation in immiscible Al-Sn-Bi alloys was studied by copper rapidly cooling method. The liquid-phase separation and formation of the core-shell structure in immiscible Al-Sn-Bi alloys are acquired when alloy compositions falling into liquid miscibility gap. These results indicate that the composition of immiscible alloys plays a crucial role in the formation of the core-shell morphology. The minor volume phase always formed the core, and the components of core and shell of core-shell morphology was changed when the contents of the components were changed. What’s more, the cross-sectional structure changes from irregular, dispersed to core-type shapes with increasing melt temperature. The higher the melt temperature, the higher the temperature gradient, and the larger the heat flux will be. So, the velocity of globules migration will be faster. What’s more, the high temperature gradient of a moved droplet makes a high velocity of droplets due to Marangoni motion, inducing the formation of core-type structure rapidly. This study provides some clues for the preparation of core-shell microspheres of immiscible Al-Sn-Bi alloys via liquid-phase separation, which expand the application field of immiscible alloys.