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Low-cycle fatigue behavior of Sn-Ag, Sn-Ag-Cu, and Sn-Ag-Cu-Bi lead-free solders
Authors:Chaosuan Kanchanomai  Yukio Miyashita  Yoshiharu Mutoh
Affiliation:(1) Department of Mechanical Engineering, Nagaoka University of Technology, 940-2188 Nagaoka, Japan
Abstract:Low-cycle fatigue (LCF) tests on as-cast Sn-3.5Ag, Sn-3Ag-0.5Cu, Sn-3Ag-0.5Cu-1Bi, and Sn-3Ag-0.5Cu-3Bi solders was carried out using a noncontact strain-controlled system at 20°C with a constant frequency of 0.1 Hz. The addition of Cu does not significantly affect the fatigue life of eutectic Sn-Ag solder. However, the fatigue life was significantly reduced with the addition of Bi. The LCF behavior of all solders followed the Coffin-Manson relationship. The fatigue life of the present solders is dominated by the fracture ductility and can be described by the ductility-modified Coffin-Manson’s relationship. Steps at the boundaries of dendrite phases were the initiation sites for microcracks for Sn-3.5Ag, Sn-3Ag-0.5Cu, and Sn-3Ag-0.5Cu-1Bi solders, while for Sn-3Ag-0.5Cu-3Bi solder, cracks initiated along both the dendrite boundaries and subgrain boundaries in the dendrite phases. The linking of these cracks and the propagation of cracks inside the specimen occurred both transgranularly through eutectic phases and intergranularly along dendrite boundaries or subgrain boundaries.
Keywords:Low-cycle fatigue  lead-free solder  Sn-3.5Ag  Sn-3Ag-0.5Cu  Sn-3Ag-0.5Cu-1Bi  Sn-3Ag-0.5Cu-3Bi  ductility-modified Coffin-Manson’  s relationship  crack initiation  crack propagation
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