The effect of Si accumulation in a 7108 Al–Zn–Mg alloy in the context of recycling

• The maximum strength decreases and the precipitation kinetics is reduced when Si accumulates in Al–Zn–Mg alloys, degrading the mechanical properties. • Si binds with Mg during the artificial aging and forms metastable precipitates of the Al–Mg–Si–Cu alloy system having Zn incorporation into precipitates, that co-exist with typical precipitates of the Al–Zn–Mg alloy system. • If Mn is added to the Si-containing Al–Zn–Mg alloy, AlSi(Fe + Mn) dispersoids form during homogenization that absorb part of the Si, improving the alloy performance. • If Cu is added to the Si-containing Al–Zn–Mg alloy, the precipitation corresponding to the Al–Mg–Si–Cu alloy system is enhanced, resulting in a high strength and good thermal stability, although the precipitation kinetics are still slow. Recycled 7108 alloys were simulated by additions of 0.2–0.3 wt% Si, alone or in combination with Mn or Cu. Si had a negative impact on mechanical properties by slowing precipitation kinetics and reducing the overall hardness due to the formation of L-phase needle/laths of the Al–Mg–Si-Cu alloy system co-existing with typical metastable η precipitates of the Al–Zn–Mg(–Cu) alloys, which led to a coarsening of the later. Annular dark field scanning transmission electron microscopy confirmed the presence of a new type of metastable η consisting of overlapping and laterally intertwined ORR −1 and OR −1 R sequences of stacked R and O units across the platelet thickness. Most L-phase precipitates contained C sub-units with Zn replacing Cu positions, associated with a local Zn enrichment of adjacent {1 0 0}Al planes at the interface. Adding 0.29 wt% Mn to a 7108 alloy with 0.20 wt% Si led to partial removal of Si from solid solution into α-AlSi(Fe + Mn) dispersoids. This increased the hardness, although at levels below the standard alloy with no Si added. The addition of 0.17 wt% Cu to the same alloy enhanced the precipitation of L-phase leading to the highest peak hardness and a shift of the hardening curve to longer aging times