Assessment of low-cycle fatigue life of Sn-3.5mass%Ag-X (X = Bi or Cu) alloy by strain range partitioning approach

The fatigue lives and damage mechanisms of Sn-Ag-X (X = Bi and Cu) solder alloys under creep-fatigue interaction mode have been investigated, and the adaptability of the strain partitioning approach to the creep-fatigue of these alloys was examined. Symmetrical and asymmetrical saw-tooth strain profiles components (i.e., fast-fast, fast-slow, slow-fast and slow-slow) were employed. Application of the slow-slow strain mode did not have an effect on fatigue lives of the alloys under investigation. Transgranular fracture observed on the fracture surfaces suggests that creep damage might be cancelled under slow-slow mode. The fatigue lives of all alloys were dramatically reduced under slow-fast mode, which is attributed to intergranular cavitation and fracture during tensile creep flow. On the other hand, the compression creep component generated by fast-slow mode also significantly reduced the life of Sn-3.5Ag and Sn-3.5Ag-1Cu, while the component did not affect the life of Sn-3.5Ag-xBi (x =2 and 5). The four partitioned strain ranges (i.e., Δε_pp, Δε_pc, Δε_cp, and Δε_cc) versus life relationships were established in all alloys tested. Thus, it is confirmed that the creep-fatigue life of these alloys can be quantitatively predicted by the strain partitioning approach for any type of inelastic strain cycling.