Annealing Effects in Cast Commercial Aluminium Al–Mg–Zn–Cu(–Sc–Zr) Alloys

Precipitation reactions of the cast Al–3.4 at%Mg–2.7 at%Zn–0.80 at%Cu–0.10 at%Fe–0.05 at%Si alloy with and without addition of 0.14 at%Sc and 0.06 at%Zr were characterized by electrical resistometry, electron microscopy, X-ray diffraction, thermal analysis, microhardness testing, and positron annihilation. The AlMgZnCuScZr alloy contains a grain boundary T-phase (Mg₃₂(Al,Cu,Zn)₄₉) with a cubic and/or quasicrystalline structure. The AlMgZnCu alloy contains a mixture of MgZn₂- and the T-phase. Primary multilayer Al₃(Sc,Zr) particles precipitated during casting and subsequent cooling. The particles have a layered Al₃(Sc,Zr) + α-Al + Al₃(Sc,Zr) structure, i.e. consist of regions enriched with both Sc and Zr. Small atomic Mg,Zn(,Cu)-rich clusters coherent with the matrix were formed during the cooling of both alloys and/or in the course of their storage at ambient temperature. Their dissolution enables precipitation of the transient η′- and/or stable η-phases of the AlZnMgCu system in both investigated alloys. The effective activation energy for the dissolution of the clusters was calculated as ~ 103 kJ/mol. Annealing of the AlMgZnCuScZr alloy above 300 °C leads to a formation of the secondary Al₃(Sc,Zr) particles which cause precipitation hardening and guarantee thermal stability of mechanical properties. Addition of Sc and Zr micro alloying elements resulted in a substantial grain refinement. The grain size remains unchanged up to isochronal annealing at 390 °C.