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

The micro structures and tensile properties of three typical Sn-Ag-Cu alloys. Sn-3.0wt.%Ag-0.5wt.%Cu, Sn-3.5wt.%Ag0.7wt.%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 beta-Sn with fine fibrous Ag3Sn dispersion surrounding primary beta-Sn grains. The slowly cooled Sn-3.5A-0.7Cu and Sn-3.Ag-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. (C) 2002 Elsevier Science B.V. All rights reserved.