The tensile properties of a range of alloys based upon the A1 4.5% Zn 2.5 Mg system have been studied both in air and in a salt-water environment. The influence on properties of small changes (+/-0.5 wt%) in the zinc and magnesium contents and also minor additions of copper (<0.3 wt%), have been investigated as well as the effect of different precipitation ageing treatments. In addition to the mechanical tests a detailed analysis of the microstructures has been carried out. Particular attention has been paid to obtaining quantitative data on precipitate dissemination in the various alloys and to characterise the nature of fracture surfaces. Correlations are drawn between alloy composition, ageing treatment, microstructure and the influence of environment on tensile properties. A new model of stress-corrosion cracking in this alloy system is proposed and suggestions are made for improving the stress-corrosion resistance of A1-Zn-Ng alloys.
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