Effect of the Interrupted Aging Heat Treatment T6I4 on the Tensile Properties and Fatigue Resistance of AA7050 Alloy

The tensile response and the fatigue behavior of age-hardenable aluminum alloys are strongly influenced by factors like the size, spacing and volume fraction of strengthening precipitates. Previous researches have shown that the interrupted aging (T6I4) could provide an improved combination of mechanical properties for some alloys. In this study, the tensile properties and high-cycle fatigue behavior of two tempers of AA7050 alloy (the commercial T7451 and the interrupted aging T6I4) were investigated. Transmission electron microscopy analyses of strengthening precipitates were performed, and the results showed that AA7050-T6I4 had a higher volume fraction of strengthening precipitates with smaller size. This microstructural feature was responsible by the higher ductility and toughness shown by T6I4 temper condition while maintaining yield stress and ultimate tensile strength similar to T7451. The smooth samples rotating bending fatigue curves of both material conditions were similar. Nevertheless, the interrupted aging leads to improved notched fatigue resistance and hence lower notch sensitivity. These results were related to the higher activity of screw dislocations and improved dislocation pinning effect during deformation promoted by T6I4 temper.