The precipitation behavior of the 7050 thick sheet during the RRA process at 185 ℃ was analyzed by tensile test

conductivity test and transmission electron microscopy（TEM）observation

and it was compared with that of 7050-T6 ones.The results indicate that 7050-T6 alloy possesses high strength

such as the yield strength of 507 MPa

and low stress corrosion resistance

such as low conductivityof 16.59 MS/m.During the retrogression process

strength of the alloy is increased firstly and then decreased with the increase of electric conductivity through the whole process.As retrogressed at185 ℃for 1h

the 7050-RRA exhibits excellent comprehensive performance

where yield strength is increased to 523 MPa and electric conductivity is 20.07 MS/m.The TEM observation reveals that the main strengthening phase is GP（I）zone andη′in the 7050-T6 alloy.During the RRA process

the GP（I）zone is transformed into GP（Ⅱ）zone andη′

and the further precipitation ofη′results in the strength increase to a peak value.Then

retrogressed to 4h

the GP zone disappears and the transition ofη′→ηcauses the decrease of strength.