Effect of microstructural features on fatigue behavior in A319-T6 aluminum alloy

The low cycle fatigue behavior of A319-T6 aluminum alloy under the influence of microstructural heterogeneities has been studied. SEM and EDX have been used to examine the fracture surface and to measure element concentrations, respectively. The dominant fatigue crack nucleates from porosity near the specimen outside surface in all examined specimens. Quantitative measurements of the fatigue-crack-initiation site were conducted, and it shows that the decrease in fatigue life is directly correlated to the increase of defect size. A fracture mechanics approach has been used to fit the experimental data. After nucleation, the hard and brittle silicon particles show great influence on fatigue-crack propagation. Whether they act as obstacles or weak paths mainly depend on the value of crack-tip driving force. Large and elongated particles with their major axes parallel to the tensile axis are easier to crack than small ones. As fatigue crack grows, the favorite crack path change from α-Al dendrite cells into silicon accumulated areas, results in different silicon concentrations and surface roughness in various crack-propagation regions on the fracture surface.