Precipitation, mechanical properties and early slant ductile fracture in cyclic and naturally aged Al-Zn-Mg(-Cu) alloys

Natural and artificial ageing treatments are compared with ageing by cyclic deformation at room temperature. Atom probe tomography and transmission electron microscopy revealed that a fine distribution of GP-zones, was responsible for the strength after cyclic ageing. Ten days further natural ageing allowed these early-stage clusters to grow into more stable ones, thereby improving the mechanical response. This resulted in similar tensile strength and uniform elongation as for the peak aged condition by artificial ageing. It was found that fracture of cyclic aged samples occurred earlier, in the form of a ductile slant shear fracture without necking in advance. In all cases, the slant fracture started at about the same point as reaching maximum strength, i.e., at the uniform limit. However, by an artificial ageing treatment after the cyclic hardening treatment, an ordinary necking behavior was seen, similar as for the peak aged samples. Natural aged samples had similar GP-zones as the cyclic hardened ones, and the same slant fracture mode, indicating that the precipitation structure controls the fracture type. Similar behavior was observed for two different Al-Zn-Mg alloys, with and without copper.