Effects of microstructure on the fatigue crack initiation and propagation behavior of 7075 aluminum alloy

Fatigue crack initiation and propagation behaviors of high-strength 7075 Al alloy were fully investigated in this study. Fatigue cracks mainly initiated from hard and brittle Fe-rich particulates that are randomly distributed at grain boundary (GB) and within the grains. Dislocations cannot pass through those particulates, resulting in dislocation accumulation and stress concentration at the particulate/matrix interfaces. When the stress concentration reaches a critical value, local cracking of micrometer-sized large particulates induced crack propagation. The branching and deflection of fatigue cracks are influenced by grain orientation, GBs and particulates. When the crack tip extends at a small angle from a grain close to (101) orientation across the GB to another grain with a significantly different orientation, the cracks tend to deflect at GBs. Particulates with microcracks enhance the strain release nearby the crack tip and promote crack deflection and branching to some extent. Furthermore, the branching direction of cracks within a grain is generally consistent with the projection direction of the maximum slip plane. This study provides valuable insights for the development of high-strength 7075 Al alloys with resistance to fatigue crack initiation and propagation.