Fig. 1. Thermo-mechanical processing schedules of different reversion treatment: (a) bake-hardened immediately after RA; (b) bake-hardened after different NSA times.

Fig. 2. Variation of Vickers micro-hardness before and after the bake-hardening as a function of reversion time in samples reverted at: (a)120 °C; (b)150 °C; (c)180 °C; (d)210 °C.

Fig. 3. Paint bake response after natural second ageing: (a) The hardness change with natural second ageing time after different reversion treatment; (b) the hardness before and after the bake-hardening treatment after reversion at 180 °C for 30 s (T4R); (c) engineering stress-strain curves of AA7075 alloy after T4R and paint-bake with different natural second ageing duration; (d) tensile properties obtained from (c).

Fig. 4. The subtracted DSC curves of AA7075 alloy samples: (a) samples reverted at different temperatures, starting with reversion at 120 °C for 7 min on top (120-7 min); (b) naturally aged samples after quenching and reversion. The curves are shifted and arranged in order of varying initial conditions. The incipient values indicate the zero level. Deviation above and below this level represents the exothermic and endothermic reaction, respectively. S1, S2 and S3 represent the areas of peak Ⅲ in different conditions, respectively.

Fig. 5. S/TEM images showing the precipitates in the (a, b) 180 °C-30 s (T4R); (c, d) 180 °C-90 s; (e, f) T4R-NA0h-PB and (g, h) T4R-NA1d-PB samples. The insets in (e) and (g) are the corresponding diffraction patterns. The inset in (f) is the high-resolution atomic image of the boxed precipitate (η2 with a Laves layered structure). (a-d) and (e-h) were recorded with the electron beam along the [110] Al and [112] Al zone axis, respectively.

Fig. 6. The fracture morphology of the tensile samples for the alloy (a, b) before reversion (T4); (c, d) after reversion (T4R); (e, f) after reversion and baking (T4R-NA0h-PB).