Relationships between microstructure and fatigue crack propagation paths in Al–Si–Mg cast alloys

Fatigue crack propagation of long and small cracks was investigated for hypoeutectic and eutectic Al–Si–Mg cast alloys. Crack propagation behavior in the near-threshold regime and Regions II and III was related to microstructural constituents namely primary α-Al dendrites and volume fraction and morphology of eutectic Si. Long crack thresholds reflect combined closure effects of global residual stress and microstructure/roughness. The small crack threshold behavior is explained through closure independent mechanisms, specifically through the barrier effects of characteristic microstructural features specific to each alloy. In Regions II and III changes in fracture surface roughness are associated with different crack propagation mechanisms at the microstructure scale. The extent of the plastic zone ahead of the crack tip was successfully used to explain the observed changes in crack propagation mechanisms.