Melt Structure Transition And Its Effect On Solidification Of Sn/Pb-Bi And Pb-Bi/Sb Alloys

Liquid-solid transition is involved in casting and the process to produce non-crystalline, quasi-crystalline and nanocrystal material. It was noted that the structure and properties were depended on the state of liquid forming the solid. In order to improve the structure and properties of solid, the melt treatment was advanced, but processing parameters of melt treatment were mainly fixed by many experiments due to less knowledge about liquid structure and its changing pattern. In recent years, the temperature induced structure transition of melt was proved by a lot of research work, but this is not enough for the theory development and engineering practice. So in this paper, the melt structure state and its pattern of some alloys were researched through the relationship between the structure sensitive parameter and temperature. Effects of melt structure transition on undercooling degrees, solidification time, latent heat, solute redistribution and microstructure were investigated during air cooling solidification, directional solidification and semi-solid holding method. Key points of this paper were listed as below:(1) The resistivities of Sn-1%Bi, Sn-6%Bi, Pb-6%Bi and Pb-7.3%Sb were measured by the method of direct current four probes during heating and cooling. The results show that the melt structure transition occurred in these four alloys, the transition of Sn-1%Bi and Sn-6%Bi melts included reversible and irreversible part, while the transition of Pb-6%Bi and Pb-7.3%Sb melt structure was totally irreversible, irreversible melt structure transition was caused by the decomposition of Bi or Sb atom clusters, reversible melt structure transition was attributed to the cracking of Sn atom clusters.(2) The effect of irreversible melt structure transition in Pb-6%Bi and Sn-6%Bi on solidification was investigated during air cooling process. It was shown that the melt structure transition increased the undercooling degrees, lowed the solidification rate and refined the solidification microstructure. The results calculated by Newton Thermal Analysis showed that the melt structure transition increased the latent heat of Pb-6%Bi while decreased the latent heat of Sn-6%Bi.(3) The compositions of Pb-6%Bi directional solidification sample at different height were measured by EDS and the actual solute redistribution coefficient k E were derived from the fitting of composition and height by the equation of ln(C s /Co)= (kE?1)ln1(?fs)+N. The result show that the melt structure transition increase k E.(4) The result of Pb-Sb%7.3 semi-solid holding experiment showed that melt structure transition decreased k E.Through above, solidification path and microstructure can be effectively controlled by melt treatment if melt structure state information is gotten, and it is beneficial for the development of material forming technology. The method which was used to quantitatively analysis the effect of melt structure transition on the solute redistribution and the results are very valuable for people to insight the melt structure and treatment.