Investigation Of Sn-Bi-In Phase Diagram Calculation And Alloy Properties

One obvious disadvantage of Sn-Bi alloys is the coarsened crystals of Bi.However,rapid cooling or the addition of alloying elements can be used to form a multi-component alloy to refine and disperse the Bi phase to improve the original brittleness of Bi.The addition of aluminum can improve the tensile strength of the solder alloy and effectively alleviate the coarsening of the Bi phase.The melting point of Indium is 156.6℃.Adding Indium to Sn-Bi alloy will lower the eutectic temperature and melting temperature.The biggest disadvantage of Indium is its high price and insufficient available resources.In general,the use of Aluminum to improve the structure and properties of Sn-Bi eutectic solders is seldom studied,and the research of Indium is too much.In this paper,alloying elements of 1.0 wt.%In and 0.5 wt.%A1 are added to Bi-43Sn alloy to prepare four alloys based on Bi-43Sn.Using XRD,SEM-EDS and TGA techniques,the melt characteristics,microstructure and mechanical properties of the four alloys of Bi-43Sn,Bi-43Sn-1In,Bi-43 Sn-0.5 A1 and Bi-43 Sn-1 In-0.5 A1 were investigated.At the same time,the alloy was subjected to thermal aging treatment for 35 days,focusing on the analysis of the thermal aging on the mechanical properties of Bi-43 Sn-based alloy.Using the CALPHAD method,the three binary system phase diagrams and the ternary system phase diagrams of the Sn-Bi-In ternary system were calculated in the PandatTM software,and the differences between the calculated phase diagrams were analyzed.The calculated binary phase diagrams of Sn-Bi,Sn-In and Bi-In are in good agreement with the experimental values.Based on this,the phase thermodynamic calculation of the designed alloy was carried out.The calculated results are consistent with those contained in the smelted alloy.Compositional characterization and thermodynamic analysis indicate that the melting temperature of the alloy is between 135℃ and 150℃.The addition of alloying elements will lower the melting point of the alloy.Among the four alloys,the Bi-43Sn-lIn alloy has the best melting performance,the supercooling degree is 8.22℃,and the temperature in the paste region is 18.86℃.The microstructure in the Sn-Bi based as-cast alloy is composed of most of the network binary eutectic structure and a small amount of Sn dendrite phase which is precipitated first.The eutectic structure is composed of β－Sn phase and Bi phase.A small amount of A1 and In does not form IMC in the Bi-43Sn alloy.In is mainly distributed on the Sn substrate,and A1 is distributed in the form of finely dispersed fine particles throughout the substrate.The Bi-43Sn alloy with In and A1 added at the same time has more irregular eutectic structure and increased density.Compared with the eutectic Bi-43 Sn alloy,the samples with alloying elements showed significantly better and more stable microstructure,and Bi coarsening was completely avoided during the solidification process.After heat aging,the structure becomes more stable,and the mechanical properties of the alloy have changed significantly.With the extension of the thermal aging time,the network eutectic structure becomes discontinuous,the layer-like Bi is distributed more uniformly,the plate spacing becomes larger and tends to be stable.Both In and A1 can increase the ultimate tensile strength of Bi-43 Sn eutectic alloy,and the effect of adding In is more obvious than that of A1.Indium can improve the plasticity of Bi-43Sn eutectic alloy,and the effect is more obvious.Aluminum element makes the plasticity of Bi-43Sn eutectic alloy worse,and increases the brittleness of Bi-43 Sn eutectic alloy.The extended stress-strain curves of the four alloys with aging time showed a consistent change trend.That is,the mechanical properties of the Bi-43 Sn alloy relative to the as-cast state first improved and then decreased.The fracture of Bi-43Sn alloy mainly shows the cleavage fracture characteristics,and the two phases of Sn and Bi can be clearly distinguished on the cleavage facet.There is no obvious plastic deformation of Bi phase on the cross section,but Sn phase has undergone a large degree of plastic deformation.With the extension of the aging time,the surface cleavage surface decreased,but the dimples did not increase further,and the macroscopic plasticity did not further increase.