The Effect Of Melt Structure Transition On The Solidification Of Binary Sn-Bi And Sn-Sb Alloys

The overheating treatment of alloy melts has been widely studied for many years, and it has been found that the structures and properties of some alloys could be obviously improved by such proper treatment. Though the effect of overheating treatment on solidification is often ascribed to the change of the melt structure state, its nature and rules are still unclear. Prior investigation suggested that a temperature-induced liquid–liquid structure transition (TI-LLST) could occur in Sn–10%Bi ,Sn–40%Bi ,Sn–57%Bi and Sn–10%Sb alloys. The results of the TI-LLST are helpful to explore the mechanism of solidification and the effects of the TI-LLST on the solidification behavior and microstructure. In this paper, from the viewpoint of melt structure transition, the solidification behaviors and morphologies of Sn–Bi and Sn–Sb alloys under different melt overheating treatments were studied in details. The major contents and conclusions are as follows:First, Sn–Bi and Sn–Sb alloys were melted and held at the temperature above and below the TI-LLST respectively.Then the effects of TI-LLST on the solidification of Sn–Bi and Sn–Sb alloys were studied by the air cooling method. The results show that the melt experienced structure transition becomes more homogeneous and disorder. Moreover, the undercooling of the primary phase increases and the microstructure becomes finer after solidified from the melt experienced the TI-LLST. Meanwhile, the microstructure was also refined and homogeneous markedly for the sample solidified from the melt experienced the TI-LLST by improving cooling rate of the melt.Second, the directional solidification experiments of Sn-Bi and Sn–Sb alloys were carried out to investigate the effect of the TI-LLST on directional solidification microstructure by the using of self-made unidirectional solidification device. The results of experiments indicate that TI-LLST can improve their solidification microstructures obviously. The results show that the microstructure is homogeneous and micro-hardness is improved markedly for the sample solidified from the melt experienced the TI-LLST by quantitative analysis of directional solidification microstructure and detection of micro-hardness, which is meaningful for the manufacture of heavy section casting in practical engineering. Third, the sample was prepared by the liquid quench method while the directional solidification process was undergoing.Then the changes of alloy composition along the growth direction were detected by means of SEM and analysed theoretically. The TI-LLST result in the decreasing of the effective partition coefficient of solute, and further lead to the content of Sb decrease markedly for Sn–10%Sb alloy solidified from the melt experienced the TI-LLST.In conclusion, the results of solidification experiments are helpful to understand the influence of liquid-liquid structure transition on solidification behaviors and solidification microstructure. At the same time, they provide scientific basises for the development of new alloy materials and the improvement of material processing technologies.