Containing Sc Ultra High Strength Al-zn-mg-cu-zr Alloy Homogenization And Corrosion Behavior

Al-8.1Zn-2.3Cu-2.05Mg-0.21Sc-0.12Zr alloy ingot was prepared by using active flux and water chilling copper mould. The phase constituents and composition distribution of the as-cast and homogenized alloy were studied by optical microscopy (OM), electronic microstructure analysis (SEM, TEM), differential scanning calorimeter (DSC) and X-ray diffraction (XRD). Then, the homogenized alloy was hot rolled, annealed and cold rolled into a sheet with thickness of 2.2 mm. The intergranular corrosion, exfoliation corrosion and stress corrosion behaviour of the alloy were studied, and the experimental results were analysized by means of Electrochemical Impedance Spectroscopy (EIS). The conclusions can be summarized as follows:(1) The main phases of the as-cast are Serious grain boundary segregation exists in the as-cast Al-8.1Zn-2.3Cu-2.05Mg-0.21Sc-0.12Zr alloy. After homogenization, the grain boundaries become sparse, and the alloying elements distributed uniformly. The T (Al2Mg3Zn3) phase dissolves completely into matrix during homogenization. The proper homogenizing process is 470℃×24 h which is consistent with the result of homogenization kinetics analysis.(2) The sequence of corrosion susceptibility of the one-stage aged alloys is: over-aged> peak-aged> under-aged> nature-aged. All EIS patterns are simulated, and the good agreement between the experimental results and the simulated results has been obtained. The polarization resistance for the alloys increases with the immersion time, which induces an increase in the electrochemical corrosion resistance of the alloy. On the other hand, the artificial aging heat treatment can provide better electrochemical corrosion behaviour than the naturally aging treatment. And the susceptibility of the over-aged alloy is the lowest.(3) The RRA or two-stage age treatment can provide the less depth of intergranular corrison and lower exfoliation corrosion grade than T6 peak-aged treatment. The results from the electrochemical impedance show that after the RRA or two-stage age treatment, the C2 to the capacitance of the pore layer decreases and the polarization resistance increases, which means that the RRA and two-stage age treatment can improve the corrosion resistance of the alloy.(4) The T6 peak-aged alloy is the most susceptible to SCC in 3.5% NaCl solution at 1×10-6s-1. The fractograph of the alloy reveals intergranular fracture, with a small amount dimples along grain boundaries and a large number of secondary cracks. While the number of the secondary cracks is relatively less for the two-step aged or RRA alloy at the same strain rate, indicating that the two-step aging or RRA treatment can improve the resistance to stress corrosion.(5) The increase of the stress corrison resistance of the alloys is mainly due to the accumulation and discontinuous distribution of grain boundary precipitates. On the one hand, the grain boundary structure block the anodic dissolution of the grain boundary precipitates; on the other hand, the coarse grain boundary precipitates are irreversible hydrogen traps, which reduce the tendency of the grain boundary embrittlement.