凝固中断実験による三元共晶点付近の Sn-Ag-Cu 系合金の凝固過程の解析

抄録

  Near-ternary eutectic Sn-Ag-Cu alloys are leading candidates for Pb-free solders. These alloys have three solid phases: β-Sn, Ag₃Sn and Cu₆Sn₅. In order to reveal the solidification process of the alloy, the technique for quenching samples during freezing was used to evaluate the freezing characteristics of near eutectic ternary Ag-Cu-Sn alloys. In the present study, Sn-3.5Ag-0.5Cu and Sn-3.5Ag-1.5Cu (mass%) samples were investigated. These samples were subjected to the thermal program to give a cooling rate of about 0.05°C/s.
   In the case of Sn-3.5Ag-0.5Cu alloy, the solidification structure almost corresponded with the estimation from equilibrium phase diagram, though the solidification process was different. In this study the Sn-Ag₃Sn eutectic nucleated immediately after primary β-Sn solidified. On the other hand, eutectic Cu₆Sn₅ did not nucleate easily, though the temperature of the specimen indicated at 217°C (ternary eutectic temperature).
   In the case of Sn-3.5Ag-1.5Cu alloy, the solidification structure as well as solidification process were quite distinct from the estimation of equilibrium phase diagram. Primary Cu₆Sn₅ was the first phase formed in the liquid. This was followed by the nucleation and growth of psud-primary Ag₃Sn. The recalescence was observed when β-Sn nucleated and grew as a halo. The ternary eutectic structure emerged after Sn-Ag₃Sn binary eutectic solidification.