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In order to achieve the compatibility between excellent mechanical properties and good electrical conductivity of Al-Mg-Si alloys, detailed investigation was carried out to understand the microstructure evolution and their influences on performances of different Al-0.7Mg-0.6Si-0.2Ce-X (X=Sc, Y and Zr) alloys prepared by casting, homogenization, hot extrusion, solution and aging treatments. Under aging state, Ce can form a large number of micron-sized AlCeSi and a few AlCeFeSi phase. The strength increased to 334 MPa as a result of decreased grain size and improved precipitation effect and the electrical conductivity increased up to 53.53%IACS because of lower solute Si atoms. The β’’ precipitates were coarsened and consequently the volume fracture was largely reduced due to the presence of Sc element, leading to the decreased strength of 311 MPa. The more solute Mg, Si and Sc atoms increased the degree of free electron scattering and the electrical conductivity decreased to 51.86%IACS. It is also found that Sc significantly enhanced the elongation of the alloy from 11.8% to 18.0%, which might be attributed to the formation of Al3Sc and (Al, Si)3Sc phases. The addition of Y raised the electrical conductivity of Al-Mg-Si-Ce alloy to 55.79%IACS by ameliorating the volume fraction and distribution of β’’ precipitates and decreasing the solute Si atoms. The co-addition of Sc and Y inherited the advantages of such two alloys. The ultimate tensile strength, elongation, electrical conductivity of Al-0.7Mg-0.6Si-0.2Ce-0.2Sc-0.1Y alloy processed by aging treatments is 336 MPa, 19.4% and 53.11%IACS, respectively.

The effects of trace Sc and Y on the corrosion behaviors of cold-drawn Al-0.2Ce alloys were revealed by scanning Kelvin probe force microscopy (SKPFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Electron Probe Micro-analyzer (EPMA), and electrochemical measurements. The results showed that the Al3Sc particles not only enable the increase of subgrain boundaries in the modified alloy after cold drawing but also improve the morphology and size of the Al13Fe3Ce phase. SKPFM analysis showed that the enrichment of Y around the Al13Fe3Ce phase in the modified alloy can decrease its electrochemical inertness and thus reduce the micro-galvanic corrosion susceptibility of the alloy. The results of the electrochemical analysis showed that the individual and composite addition of Sc and Y can improve the corrosion resistance of the cold-drawn Al-0.2Ce alloy. The corrosion potential and current of Al-0.2Ce-0.2Sc alloy are −0.892 V and 0.91 μA·cm−2, while the corrosion potential and current of Al-0.2Ce-0.1Y alloy are −0.918 V and 1.08 μA·cm−2, respectively. It is obvious that Sc element improves the corrosion resistance of Al-0.2Ce alloy more effectively than Y element. The corrosion depth of Al-0.2Ce-0.2Sc-0.1Y alloy was the smallest tested by immersion under the same conditions, and its corrosion potential and corrosion current were −0.842 V and 0.79 μA·cm−2, respectively, corresponding to the best corrosion resistance. The formal mechanism of pitting corrosion in the alloys is also discussed.

Sc-containing Al alloys have excellent mechanical properties, which provide the foundation for their applications. The precipitation of the Al3Sc/Al3(Sc, Zr) is generally considered to be a key factor for the performance boost. However, the performances of the alloy in different production batches often exhibit unstable and fluctuation, which may be related to the heterogeneous of scandium. Herein, we investigated the precipitation behaviors of Al3Sc/Al3(Sc, Zr) phases in the Al-0.5Sc and Al-0.5Sc-0.25Zr alloys at 340 °C. Results shown that the high scandium‑zirconium ratio not only lead to the discontinuous precipitation, but also contribute to the abnormal continuous Al3(Sc, Zr) precipitation in Al-0.5Sc-0.25Zr alloy. The morphology of abnormal Al3(Sc, Zr) is similar to the common secondary Al3(Sc, Zr) phase with a coffee-bean like morphology. With the aid of Three-Dimensional Atomic Probe (3DAP), the core-shell structures of Al3(Sc, Zr) phases are quantitatively analyzed. In the shell layer of abnormal Al3(Sc, Zr) phase, the atomic concentration ratio of Sc to Zr is much higher than the common continuous precipitated Al3(Sc, Zr) phase. The abnormal Al3(Sc, Zr) phase may be one of the key factors for the structure heredity of Sc-containing Al alloys.

The effect of Sc content on microstructure and properties of Al-Zn-Mg-Cu-Zr alloy was studied by transmission electron microscopy (TEM), scanning electron microscopy (SEM), optical microscopy (OM), tensile tests, hardness measurements and corrosion evaluation. It is revealed that the increase in the volume fraction of the η' phase and the precipitation of Al3(Sc, Zr) particles within the alloy contribute to the significant grain refinement within the Al-7.0Zn-3.0 Mg-0.3Cu-0.15Sc-0.12 Zr (wt.%) alloy. The strength and hardness of the alloy have also been improved. Due to the addition of Sc element, the precipitates at the grain boundaries of the alloy become discontinuous, and the size of such precipitates is reduced. The population density is also decreased. Consequently, the alloy has excellent resistance to intergranular corrosion and exfoliation corrosion. Electrochemical data also shows that the alloys have a smaller tendency to corrode and slower corrosion rates.

In ultra-high strength Al-Zn-Mg-Cu alloys, traditional heat treatments such as peak aging, two-stage aging and retrogression and re-aging cannot perfectly balance the relationship between mechanical properties and corrosion resistance and usually consume more time. Based on previous studies, the aim of this paper is to establish a suitable non-isothermal aging method for ultra-high strength Al-Zn-Mg-Cu alloys. In this study, after enhanced solution treatment and aging process with the heating rate of 20 °C/h, the alloy achieves excellent hardness and ultimate tensile strength of 195.7 HV and 680 MPA, respectively. When heating rate increases to 40 °C/h, non-isothermal aging can also ensure the alloy achieve the excellent mechanical properties (195.6 HV, 675 MPA). Similar to mechanical properties, the samples have outstanding resistance to intergranular and exfoliation corrosion, which is in accordance with the results of potentiodynamic polarization and electrochemical impedance spectroscopy tests. Evidences from high resolution transmission electron microscopy show that non-isothermal aging can promote the formation of a large number of smaller GP zones, η’ precipitates and few η2 precipitates inside grains, which greatly strengthens the mechanical properties. Furthermore, the grain boundaries precipitates transform from continuous to discontinuous and precipitate free zones form gradually, which greatly enhances the corrosion resistance. Because of these precipitates inside grains and grain boundaries, the samples with enhanced solution treatment +20 °C/h aging process and both solution treatments +40 °C/h aging process can achieve good mechanical properties closing to T6 state and corrosion resistance similar to T74 state simultaneously with much less aging time.