The effects of heat exposure treatment at 175℃for 50 hand 100 hon the microstructure and mechanical properties of T6-7A99 alloy were investigated by means of X-ray diffraction

transmission electron microscopy and room temperature tensile testing.The results reveal that the precipitate phases in the 7A99 alloy still consist ofη′andηphases after being exposured at 175℃for 50 hand 100 h.Theη′phases in the grains are transformed intoηphases at 175℃for 50 hand 100 hheat exposure treatment

meanwhile the size of precipitate phase is increased with the decrease of density of precipitate phases and the increase of PFZ width.The tensile strength

yield strength and elongation of 7A99 alloy treated by T6 reach 505 MPa

406 MPa and 12%

respectively

while those of ones exposured at 175℃for 50 hare 330 MPa

250 MPa and 16.8%

and those of T6-7A99 alloy exposured at 175℃for 100 hare 305 MPa

225 MPa and 20%

respectively.The mechanism is attributed to theηphases transformed fromη′phases reducing the distoration of the lattice severely.