Strain-rate effects in the environmentally assisted fracture of a commercial high-strength aluminium alloy (7049)

Specimens of this high-strength aluminium alloy, cut parallel to the short transverse direction, have been tested in tension at slow strain-rates in the T651 condition using various environments: vacuum, dry air, laboratory air and sea-water, with and without prior exposure to sea-water. Accurate measurements of the reduction in area at fracture reveal a reversible pre-exposure embrittlement due to hydrogen absorption that is observed at slow strain-rates, the critical strain-rate decreasing in the less aggressive environments. Recovery from pre-exposure embrittlement only occurs in this material when specimens are subsequently strained in an inert environment, otherwise the pre-exposure effects and the embrittling effect of the test environment are additive.Fractographic examination indicates that absorption of hydrogen leads first to transgranular and then to intergranular brittle failure and the evidence is consistent with the hypothesis that the transition occurs at lower local hydrogen concentrations (i.e. shorter pre-exposure) as the strain-rate is decreased.Stress corrosion tests carried out in tap-water under potentiostatic control reveal that a cathodic and an anodic region of embrittlement both become extended as the strain-rate is decreased. However, fractography and the observed effect of over-aging both tend to confirm that the major embrittling process at the free corrosion potential is hydrogen embrittlement.The results are discussed with reference to earlier results reported for a similar alloy (7075) and apparent discrepancies explained. In particular, the aggressiveness of laboratory air, frequently employed as a reference environment, is emphasized.