Fig 1. Schematic drawing of the corrosion test setup and viewing directions. RD, ND, and TD stand for rolling direction, normal direction, and transverse direction, respectively.

Fig. 2. (a) The Vickers hardness-time curve of an AA7055 AlZn(Cu)Mg alloy aged at 120 °C. Back scattering electron SEM images of the alloy samples aged for 2 h (b), 24 h (c), 96 h (d), and 720 h (e), respectively, after 6 h corrosion in standard IGC electrolyte.

Fig. 3. (a) The Vickers hardness-time curve of an AA7055 AlZn(Cu)Mg alloy aged at 200 °C. Back scattering electron SEM images of the alloy samples aged for 10 min (b), 1 h (c), 8 h (d), and 24 h (e), respectively, after 6 h corrosion in standard IGC electrolyte.

Fig. 4. (a) The Vickers hardness-time curve of an AA7055 AlZn(Cu)Mg alloy aged at 400 °C. Back scattering electron SEM images of the alloy samples aged for 1 h (b), 8 h (c), and 24 h (d), respectively, after 6 h corrosion in standard IGC electrolyte.

Fig. 5. HAADF-STEM images of typical GBPs in the sample aged at 120 °C for 2 h, viewed for the sampe area but in different directions: (a) GBPs viewed edge-on, (b) GBPs viewed in an direction inclined by about 30 ° from that for (a).

Fig. 6. Bright field TEM images of the samples aged at 120 °C for 2 h: before (a, c) and after (b, d) immersion in the diluent IGC electrolyte for 3 s (b) and 31 s (d), respectively. The up-right inserts in (a, b) schematically show the 3D distribution of GBPs and the viewing direction.

Fig. 7. Bright-field TEM images of the samples aged at 200 °C for 24 h: before (a) and after (b) immersion in the diluent IGC electrolyte for 10 s.

Fig. 8. Quasi in-situ plane-view observations for the localized-corrosion of the samples aged at 400 °C for 8 h. (a, b) Back-scattering SEM images of the same area before (a) and after (b) immersion in standard IGC electrolyte for 1 min. (c, d) Bright field TEM images of the same area before (c) and after (d) immersion in a diluent IGC electrolyte (one tenth of standard H2O2 volume proportion) for 12 s.

Fig. 9. HAADF-STEM images and EDS elemental mappings of grain boundary areas for the samples aged under all different conditions.

Fig. 10. TEM and HRTEM images of the typical precipitate microstructures in various samples: (a, b) aged at 120 °C for 2 h: GP-ηp precipitates are the major internal matrix precipitates (a), and ηp-precipitates are the major GBPs (b); (c, d) aged at 120 °C for 24 h: GP-ηp and ηp precipitates are the major internal matrix precipitates (c), and ηp-precipitates are the major GBPs (d); (e, f) aged at 200 °C for 24 h: large η-precipitates are the major precipitates both in the matrix and at the GBs. (g, h) Low-magnification bright-field image and HRTEM image of the internal-grain precipitates in the sample aged at 400 °C for 8 h, respectively.

Fig. 11. Schematic illustration of the localized corrosion attacks in relation with the precipitation microstructures and Cu-element redistribution in the alloy: yellow parts indicate corroded areas, pink and red precipitates in (c) denote very high Cu-contents in these particles. The lighter the blue matrix area is the less Cu-atoms it contains. Drawings on the left illustrate cross-sectional precipitation microstructures of the samples before corrosion and those on the right represent the corresponding microstructures under corrosion attacks.