The Structure Transition Of Sn-Bi Melts And The Effects On Solidification And Wettability

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's structure state, its nature and rules are still unclear. In recent years, the discovery and researches of liquid structure transition provide the new chance and road to recognize the intrinsic nature of the liquid-solid correlation. But the characteristic and mechanism of the melt structure transition and its effect on solidification still need to be explored further. In addition, as an example of applied field of liquid-solid correlations, Sn-Bi-based alloys, a kind of lead-free solder, were also selected in the investigation of this paper.In this paper, the kinetics of temperature-induced liquid-liquid structure transition (TI-LLST) process in Sn-40%Bi melt was investigated in isothermal and continuous heating experiments with electrical resistivity method. Also the characteristic and mechanism of the transition are discussed using kinetic theory of phase transition and thermodynamics. In addition, the relationship between melt structure transition and solidified structure, wettability was further investigated through solidification and wetting experiments. The contents and results are shown as follows:First, using kinetic theory of phase transition, we find that the time evolution pattern of the electrical resistivity suggestes the transition mechanism of TI-LLST for Sn-40wt%Bi melt in good accordance with the autocatalytic reaction model, which is an indication of nucleation-growth type. Using Kissinger's equation, the apparent activation energy at transition beginning point was also calculated; its small value indicated that TI-LLST is mainly caused by localized atomic structurerearrangement.Second, from the viewpoint of melt structure transition, the solidification behaviors and morphologies of Sn-40%Bi alloy under different melt overheating treatments were studied in details. The results show that the alloy solidification structure could not be effectively improved unless by proper overheating degree and holding time. The melt structure transition caused by different holding time at the same temperature has a significant effect on the solidification behavior and microstructure. The general rule is that the undercooling degree increases, the solidification microstructure becomes finer, and the obvious change of microstructure morphologies takes place when solidifying from the melt-experienced structure transition.The reason why the solidified structure becomes finer with melt superheating is that the melt experienced structure transition becomes more homogeneous and disorder and needs higher degree of supercooling during solidification.Third, the electrical resistivities of liquid Sn-57%Bi and Sn-57%Bi-l%Ag alloys were measured, which indicated that the structure transitions existing in their melts are partially reversible. Based on the results of electrical resistivity, corresponding solidification experiments have been carried out on the two alloys, i.e., they are melted and held at the temperature above and below the TI-LLST respectively, and then cooled from the same temperature. Then by wetting them on copper plate, we explore the effect of TI-LLST on their wettabilities. The results show that TI-LLST can improve their solidification microstructures and wettabilities obviously.In conclusion, the results of this paper are helpful to understand the mechanism of liquid-liquid structural change, and to improve the technologies of materials processing.