Microstructure evolution and mechanical properties of an ultrahigh strength Al-Zn-Mg-Cu-Zr-Sc (7055) alloy processed by modified powder hot extrusion with post aging

A modified powder hot extrusion including gas atomization, pre-compaction and hot extrusion was used to fabricate an ultrahigh strength Al-Zn-Mg-Cu-Zr-Sc (7055) alloy. The results reveal that a homogeneous microstructure containing fine grains and tiny second phases is formed after extrusion. The solution-treated alloy aged at 393 K for 24 h achieves peak hardness of 177 HB due to the common effect of GPI zone and eta' phase. The peak aged alloy exhibits an ultrahigh ultimate tensile strength of 734 MPa and a good elongation at fracture of 9.8%, which is mainly attributed to fine grains and high-density nanoscale second phases (GP, eta', and Al-3(Sc/Zr)). Micropore aggregation fracture is the main fracture mechanism in all the tested alloys states. The cracking or debonding of coarse second phases is the main cause of the limited strength of as-extruded alloy, while the coarse phases and PFZs at the grain boundaries are responsible for the decrease in the elongation of peak-aged sample. Compared with the 7055 alloys fabricated by other techniques, it is suggested that this modified powder hot extrusion may be an effective approach to develop high strength Al alloys.