Short transverse fatigue crack propagation in 8090 AlLi alloy: re-ageing and environmental effects

Rising ΔK fatigue tests were conducted at 30 Hz on S-L oriented 8090 Al-Li alloy plate in the as-received T8771 temper and after a toughness-enhancing re-ageing treatment at 230 °C. Tests were conducted at an R-ratio of 0.5 in desiccated air, distilled water and 1 M AlCl3. Attention was directed to the near threshold and low ΔK (Stage I) region of crack propagation near and below ≈ 3 MPa m12. The fatigue behaviour of the as-received temper was compared to a previous study on the T-L orientation. The investigation included some intergranular corrosion tests in the 1 M AlCl3 environment, plus fractographic, microstructural and analytical electron microscopical analyses. It was found that S-L fatigue followed an intergranular path under all test conditions. Crack rates in the aqueous environments were demonstrably higher in the S-L orientation than in the T-L orientation, but were similar when tested in desiccated air. The re-ageing treatment had no beneficial effect on S-L fatigue behaviour in the aqueous environments and produced a slightly deleterious effect in desiccated air. The grain boundary (GB) regions exhibited Cu solute concentration gradients that influenced intergranular corrosion behaviour and localized corrosion processes during intergranular fatigue. Discussion of fatigue crack processes included the influence of slip reversibility, localized corrosion, hydrogen embrittlement, and solute distribution at the GB.