Flaw Growth in High Strength Al-Zn-Mg-Cu Alloys Exposed to Stress Corrosion Environments

Abstract

Marine atmosphere and synthetic sea water stress corrosion tests on smooth and precracked specimens from 7075, 7050, and 7049 alloy plates (32 and 76 mm thick) show that for a given strength level, alloys 7050-T7X and 7049-T7X have superior short-transverse stress corrosion resistance (SCR) to 7075-T7X. At typical strength levels above the minimum of 7075-T6, for example, SCR of these alloys is considerably better than that of 7075-T76, and approaches that of 7075-T73. Alloys 7050-T73651 and 7049-T7351 have SL threshold stress intensities (Klscc) of about 24 MPa √m at a yield strength of 450 MPa. The corresponding Klscc value for 7075-T7X at this strength level is about 19 MPa μm. In both environments, Klscc is independent of plate thickness, per se; absolute strength level appears to be the controlling factor, Klscc being inversely proportional to yield strength. Examination of fracture surfaces and metallographic cross sections showed a considerable amount of crack blunting, crack jogging, crack branching, unfailed ligaments, and mechanical fracturing in the precracked specimens. Apparent (measured) stress intensities can therefore be higher than effective values at the crack tip; hence, they should be considered as phenomenological parameters only, useful for comparing alloys and tempers under a particular set of experimental conditions, and for qualitatively estimating crack velocities that could be expected under known environmental conditions.