Liquid-liquid structure transition and its effect on the solidification behaviors and microstructure of Sn75Bi25 alloy

This paper studied the liquid-liquid structure transition (L-LST) of Sn75Bi25 alloy with atom percentage via two direct methods such as a high temperature X-ray diffractometer and a modified high temperature Hall effect instrument as well as several indirect methods including resistivity, density, viscosity and thermal analysis methods. The results indicated that the reversible first-order liquid-liquid structure transition did occur in the range from 990 to 1068 K during heating and occur in the range from 865 to 945 K during cooling. From the atomic and molecular scale, the nature of the L-LST was the breaking of the residual covalent bonds and the cooperative rearrangement of atoms during heating. In addition, the breaking of the residual covalent bonds occurred not only during the L-LST but also before and after the L-LST. The L-LST reflected a qualitative change of melt structure. From the view of the electronic structure, the nature of the L-LST was the releasement of free electrons and the localized-extended transition of electron. Besides, a novel strategy combining atomic structure and electronic structure was developed to reveal the nature of the L-LST. The influences of the L-LST on the solidification behaviors and morphologies of Sn75Bi25 alloy also were investigated. The results showed that the undercooling of the eutectic phase increased after the L-LST, which was responsible for the drastic decrease in the spacing of the eutectic phase.