Liquid-liquid Structure Transition And Its Effect On Solidification: SnSb Investigated

The structures and properties of liquid metals and alloys do have a significant influence on solidification behavior, solidified microstructure and their final properties. In recent years, more and more concerns have been shown for liquid polymorphism. The liquid-liquid structure transitions (LLSTs) are observed and verified to occur with temperature or pressure, which has led a new understanding of the regularity and mechanism of liquid structure transition in liquid metals and alloys. At the same time, the facts mentioned above provide a new viewpoint to investigate the liquid metals and alloys. The overheating treatment of liquid metals, with purpose of altering the structures states of liquid alloys, has already been widely used in industry and scientific research to improve the solidification microstructure and properties. However, the essence and rule of these phenomena are not anatomized very well until now, the nature of temperature induced liquid-liquid structure transition and its effect on solidification still need a further exploration.In this paper, SnSb alloy is investigated to explore the characters, mechanism and rule of liquid structure transitions. Different melt overheating temperature, solidification condition and thermal analysis are adopted to study the effect of LLSTs on solidification behavior and solidified microstructure. The main contents are as follows:(1) The electrical resistivity of liquid SnSbx (x=3, 30, 50) alloys is investigated in three continuous heating and cooling cycles. And the resistivity changes abnormally in a relatively high temperature zone above the liquidus in several experiment, which indicates that temperature induced liquid-liquid structure transitions occur in SnSbx (x=3, 30, 50) alloys, and the transition is irreversible in first heating cycle.(2) Different pulling rates are adopted during directional solidification experiment to probe the effect of the LLSTs on solidification morphologies in SnSb3 alloy. The results show that irreversible LLSTs would result in the reduction of solute redistribution coefficient, and the enrichment of solute in front of the liquid-solid interface. Furthermore, irreversible LLSTs would obviously reduce the primary and secondary dendrite space and solute concentration along the central line of primary dendrite. LLSTs have different effects on primary dendrite space under different pulling rates.(3) Cooling curves of SnSb30 alloys are measured in self-designed sand mold and crucible to probe the effect of LLSTs on solidification with Newton thermal analysis and Fourier thermal analysis. It is found that irreversible LLSTs would lead to the increase of undercooling, nucleation rate, latent heat released by unit volume and the refining of microstructures. It also can be noted that cooling rate plays an important effect on solidification.(4) Solidification experiments of SnSb50 alloys are carried out in massive copper mold. The results show that grain size would refine when solidified from the melts experienced irreversible LLSTs, and solidified microstructure becomes more homogeneous.