Study of Decomposition of Supersaturated Solid Solution upon Quenching of Al–Mg–Si Alloy Sheets at Different Cooling Regimes

Results of investigation of the stability of supersaturated solid solution under different quenching conditions of sheets of the heat-hardenable Al–Mg–Si aluminum alloy with the small copper addition (Al‒0.6Mg–1.0Si–0.2Cu) are reported. Samples were subjected to isothermal or continuous quenching at different quenching cooling rate; after that, the artificial aging at 170°С is performed. Results of thermodynamic simulation of the equilibrium phase composition of the alloy allow us to find that, for a temperature range of 300 to 530°С, the presence of the β phase (Mg 2 Si) is most probable. Transmission electron microscopy and electron microprobe analysis allow us to find that, upon quenching, the decomposition of supersaturated solid solution leads to the precipitation of undesirable coarse particles of metastable β-type and equilibrium β phases. The precipitates nucleate in the form of rod-like particles via the heterogeneous mechanism mainly at the surface of α-phase (Al 15 (Mn,Fe) 3 Si 2 ) dispersoids which, in such a way, increase the quenching sensitivity of the alloy. The formation of above precipitates at the low quenching rate determines the decrease in the fraction and formation density of strengthening -phase particles upon subsequent aging and leads to the increase in their sizes and nonuniformity of distribution in the aluminum matrix. This decreases the precipitation strengthening potential upon aging and the corrosion resistance of the material.