Analysis on the behavior of exceeding peak aging strength of aluminum alloy at condition of retrogression and reaging

The tensile test indicated that after preaging at 100 or 120 degrees C for 24 h and retrogressing at 200 degrees C for various times and reaging treatment, the strengths of the three Al-Zn-Mg-Cu alloys are greater than those at preaging condition, some of them even exceed the peak strength of single-step artificial aging (T6), reach up to 795 MPa. The alloys exhibit a desired combination of tensile strength and elongation after retrogression and reaging (RRA) treatment. TEM and EDS analyses show that the precipitate free zones (PFZ) formed during retrogressing in short period became narrowed and even disappeared after reaging treatment, while the second phase distributed dispersely in grain interior. Although the second phase in the alloys treated at T6 temper dispersed in grain interior, the PFZ in alloys still existed obviously. It is suggested that the exceeding peak aging strength in the RRA condition is ascribed to narrowing and even disappearing of PFZ, which reinforced the grain broundary, while the precipitates in the matrix still maintain a more dispersed distribution.