Effects of cooling speed on microstructure and tensile properties of Sn–Ag–Cu alloys

The microstructures and tensile properties of three typical Sn–Ag–Cu alloys, Sn–3.0wt.%Ag–0.5wt.%Cu, Sn–3.5wt.%Ag–0.7wt.%Cu and Sn–3.9wt.%Ag–0.6wt.%Cu, prepared under three different cooling conditions were evaluated after casting. The microstructures of all rapidly cooled specimens consisted of the eutectic phase of β-Sn with fine fibrous Ag3Sn dispersion surrounding primary β-Sn grains. The slowly cooled Sn–3.5Ag–0.7Cu and Sn–3.9Ag–0.6Cu alloys exhibited additional large primary Ag3Sn platelets, while the Sn–3.0Ag–0.5Cu did not. For all alloys, both ultimate tensile strength and 0.2% proof stress increased with increasing strain-rates in tensile tests. Lowering cooling speed decreased tensile strength. Elongation increased with an increasing strain rate from 10−5 to 10−2 s−1, and decreased slightly at 10−1 s−1 for the rapidly cooled specimens. Elongation remarkably decreased for the slowly cooled Sn–3.5Ag–0.7Cu and Sn–3.9Ag–0.6Cu alloys, a degradation attributable to the formation of large primary Ag3Sn platelets.