Structure Research Of Typical Binary Eutectic Alloy's Melts

It is an important subject that the properties and the structures of high-temperature melt are analyzed from the point of academic research and practical application. The experiment is very difficult and the data analysis is complicated. In this present paper, the structure evolution and the resistivity with temperature in the solidification process of Al-Ge, Co-Sn and Ni-Si binary alloys were investigated by the primary means of X-ray diffraction and resistivity device, as well as other means were used to analyze the melt, such as DSC, viscosity and self-established model. The melt structures have been discussed, and the correlations between liquid structure and solid structure have been revealed. The reasons of the deep undercooled state during solidification have also been explored.With the high temperature X-ray diffraction experiment, the intensity of liquid Al_71.6Ge_28.4 alloy was measured at 1150℃, 950℃, 750℃and 550℃. The correlation radius r_c and the nearest neighbor distance r_l keep stable in the whole temperature range, but the correlation radius r_c has a sudden change at 950℃. It can be concluded that the short order range in the liquid alloy is stable, the sudden change of correlation radius r_c arises abruptly with the forming of the higher degree short range order structured by realignment of construction units. By investigating the structure correlation of Al_71.6Ge_28.4 alloy between liquid and solid states, it is indicated that the liquid Al_71.6Ge_28.4 eutectic alloy consists of 72.8%αclusters and 27.2%βclusters.αclusters structure keeps the FCC structure, butβclusters transform FCC structure into topological structure. The liquid Al_71.6Ge_28.4 structure is different from its solid structure, which is the reason of undercooling of the Al_71.6Ge_28.4 alloy during the solidification. The resistivities of different components of Al-Ge alloys have been measured by the self-developed resistivity equipment during solidification. Studies have shown: The resistivity of Al-Ge melt decreases linearly with temperature decreasing. When the composition is more than 50%, the melt is almost the same temperature coefficient of resistivity. There are two discontinuous points of the viscosity at 780℃and 600℃. The discontinuous viscosity at 780℃in the melt is for the structure change of the clusters form BCC short range to FCC short range, while the latter is the number change based on the new structure.The liquid structure of Co_79.5Sn_20.5 eutectic alloy has been investigated by means of X-ray diffraction. From 1350℃to 1150℃, the structure of melt holds quite stable, but the order increases with the temperature decreasing. The short-range order structure of the Co_79.5Sn₍20.5 alloy consists ofβCo3Sn2 clusters andβCo clusters, each which accounts for about 50% in the melt respectively. Therefore the liquid structure is consistent with its solid structure. From the point of the structure correlation between liquid and solid states, reasons of deep undercooling state are discussed further. The liquid alloy consists of two clusters with the same proportion, which adds nucleation difficulty. The Co_79.5Sn_20.5 melt was studied by the measurement of the resistivity and DSC heat. The curves of different physical properties have a discontinuous point at similar temperature during the solidification. The results of experiments indicate that there is microstructure transition in the Co_79.5Sn_20.5 melt during the solidification.The liquid structure of Ni₅₃Si₄₇ eutectic alloy was studied by the high-temperature X-ray diffractometer. "Pre-peaks" were found in X-ray patterns of Ni₅₃Si₄₇ melt, and the intensity of the "pre-peaks" increase with temperature decreasing. For the liquid alloy of Ni₅₃Si₄₇, the structure is stable but the order increases. Meanwhile, the correlation between the liquid structure and solid structure has been analyzed by "the Nanocrystal Model". It is shown that the "pre-peaks" are the reflection of liquid structure of NiSi compound. There is a big difference between the liquid structure and solid-state structure, which is the reason of deep undercooling state during solidification. The temperature coefficient of resistivity is negative and changes at about 1230℃. The result explores that there is a trend of the formation of NiSi compounds. In other words, the formation of NiSi compound in the melt lead the "pre-peak" existence and the change of resistivity temperature coefficient.