Stress-corrosion resistance of high-strength Al-Zn-Mg-Cu alloys with and without silver additions

The separate and combined effects of adding silver, substituting other elements for chromium, increasing zinc, or varying the copper content on the tensile properties and resistance to stress-corrosion cracking of 7075-type alloys were investigated. Adding silver produced no increase in strength and marginal increases in the resistance to stress-corrosion cracking. Substituting either zirconium or manganese for chromium increased strength and decreased the resistance to stress-corrosion cracking after a given precipitation heat treatment; at equal strength levels, the resistances to stress-corrosion cracking of the chromium-free alloys below 3 in. (76 mm) thickness were comparable to those of 7075. Increasing zinc increased the resistance to stress-corrosion cracking but increased quench sensitivity. Decreasing copper decreased the resistance to stress-corrosion cracking in the atmosphere. Increasing copper in chromium-free alloys provided the best combination of strength and resistance to stress-corrosion cracking. Longer atmospheric exposures are needed to firmly establish the stress-corrosion resistance. Formerly Assistant Director, Alcoa Research Laboratories, is now retired.