Micromechanisms of multistage fatigue crack growth in a high-strength aluminum alloy

This study reveals the micromechanisms of fatigue damage formation and evolution with respect to particle topology and grain size and orientation in a rolled 7075-T651 Al alloy. Systematic observations were made of the variations in the fracture surfaces and damage micromechanisms, which were characterized in three fatigue stages: fatigue crack formation, microstructurally/physically small cracks and long cracks. The fatigue crack was formed exclusively at the fractured Fe-rich intermetallic constituent particles preferably located at or near the specimen surface. Large impurities, such as metallic oxides, were also observed to influence the crack nucleation mechanisms. The presence of these impurities close to the nucleation sites was correlated with an approximate 30% reduction in fatigue life. In the microstructurally and physically small crack regimes, the crack front showed a rough localized brittle fatigue fracture along the crack propagation direction in addition to some localized ductile fatigue fracture. Changes in striation size across grain boundaries were clearly observed. In the long crack regime, the fracture surface became rougher but the overall surface tended to be perpendicular to the loading direction, indicating a Mode I fracture. The ramification of the results for a microstructure-based multistage model that comprises crack incubation, small crack growth and long crack growth is discussed in detail.