Microstructural mechanisms controlling fatigue crack growth in Al–Si–Mg cast alloys

Fatigue crack growth behavior of long and small cracks was investigated for hypoeutectic and eutectic Al–Si–Mg cast alloys. Crack growth response 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 cracks thresholds are dictated by microstructure/roughness induced closure. 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 appearance were associated with different crack growth mechanisms at the microstructural scale. The extent of the plastic zone ahead of the crack tip was successfully used to explain the observed changes in crack growth mechanisms.