Effect of Mg content on microstructure, mechanical properties and intergranular corrosion properties of Al-Cu-Mg-Ag alloys

The effects of Mg content on the microstructure, mechanical properties and intergranular corrosion (IGC) resistance of Al-Cu-XMg-Ag(X = 0.35-1.27 wt%) alloys are investigated by a combination of Optical microscope (OM), X-ray diffraction (XRD), Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) analysis. The results show that with the increase of Mg content in the studied range, the maximum solubility of Cu decreases, resulting in the formation of more Al2Cu and Al2CuMg(Ag) intermetallic compounds (IMCs) in the as-cast alloy. At the Mg content of 0.83 wt%, the alloy has the highest mechanical properties (sigma(UTS)=521 MPa, sigma(YS)=477 MPa, delta = 10.4%) and better IGC resistance. The precipitation of Omega phases (Al2Cu-{111}(alpha)) is promoted firstly and then inhibited with the growth of Mg content. The major intergranular corrosion (IGC) mechanism is the Precipitate Free Zone (PFZ) dissolution, which has the lowest self-corrosion potential. The alloy with 0.83 wt% Mg content has the narrowest PFZ, the width of which is similar to 31.9 nm. This is because the solute in the alloy is mainly consumed by the generated high density Omega phases and the grain boundary precipitates consume less solute. The grain boundary precipitates (GBPs) have a strong solute competitive advantage in the alloy with 1.27 wt% Mg content, in which a wider PFZ (similar to 52.1 nm) exists. Meanwhile, Mg and Ag atoms are gradually segregated at grain boundaries, leading to the formation of Al2CuMg(Ag) phase in the GBPs, which makes the dealloying reaction happens and the IGC resistance worse.