Strength and fatigue fracture behavior of Al-Zn-Mg-Cu-Zr(-Sn) alloys

The strength and fatigue fracture behavior of Al-Zn-Mg-Cu-Zr(-Sn) alloys were studied by performing tensile tests and fatigue crack propagation (FCP) tests. The microstructures of the experimental alloys were further analyzed using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM); phase analysis of these alloys was conducted with an X-ray diffraction (XRD). The results show that when Sn is included, growth of the recrystallization grains in the solution-treated Al-Zn-Mg-Cu-Zr alloy is obstructed, the precipitation-free zone (PFZ) of the overaged Al-Zn-Mg-Cu-Zr-Sn alloy becomes narrow, and the grain boundary precipitates are smaller. Consequently, the FCP resistance is higher. In addition, the overaged Sn-containing alloy has considerably higher tensile strength than the alloy without Sn.