Phase Evolution in the AuCu/Sn System by Solid-State Reactive Diffusion

The interfacial reactions between several Au(Cu) alloys and pure Sn were studied experimentally at 200°C. Amounts of Cu in the AuSn₄ and AuSn₂ phases were as low as 1 at.%. On the basis of these experimental results there is no continuous solid solution between (Au,Cu)Sn and (Cu,Au)₆Sn₅. The copper content of (Au,Cu)Sn was determined to be approximately 7–8 at.%. Substantial amounts of Au were present in the (Cu,Au)₆Sn₅ and (Cu,Au)₃Sn phases. Two ternary compounds were formed, one with stoichiometry varying from (Au_40.5Cu₃₉)Sn_20.5 to (Au_20.2Cu_59.3)Sn_20.5 (ternary “B”), the other with the composition Au₃₄Cu₃₃Sn₃₃ (ternary “C”). The measured phase boundary compositions of the product phases are plotted on the available Au–Cu–Sn isotherm and the phase equilibria are discussed. The complexity and average thickness of the diffusion zone decreases with increasing Cu content except for the Au(40 at.%Cu) couple.