Solidification microstructures in Ag3Sn–Cu3Sn pseudo-binary alloys

The solidification microstructures in three alloys from the Ag3Sn–Cu3Sn pseudo-binary section in the Ag–Cu–Sn system have been studied using a combination of X-ray diffraction, electron microscopy, differential scanning calorimetry, and quenching experiments. The three alloys have Ag:Cu ratios of 50:50, 40:60, and 30:70. In each case, the as-cast structures exhibit the equilibrium phases θ-Ag3Sn and e1-Cu3Sn, with a little η-Cu6Sn5. There is no evidence of the metastable high-temperature phases that are so prevalent in as-cast structures of the corresponding binary alloys. The differential scanning calorimetry data obtained on heating the alloy samples are consistent with the transformations expected on the basis of the published ternary Ag–Cu–Sn diagrams. It is proposed that the solidification microstructures observed experimentally in such alloys must correspond to the nucleation of the high-temperature phases being kinetically limited upon cooling for these compositions. This leads to the direct formation of the equilibrium low-temperature phases by eutectic-type co-operative growth.