Structures and mechanical properties of ECAP processed 7075 Al alloy upon natural aging and T651 treatment

Equal-channel angular pressing (ECAP) processed ultrafine grained (UFG) and coarse grained (CG) 7075 At alloys were treated by natural aging and T651 temper (annealed at 120 degreesC for 48 h in Ar atmosphere), respectively. Mechanical tests showed that for the UFG sample, the natural aging resulted in the highest strength (the ultimate tensile strength is 720 MPa). In contrast, for the CG sample, the T651 treatment resulted in a higher strength (the ultimate strength is 590 MPa) than the natural aging (530 MPa). Microstructural analyses indicated that the enhanced strength of the T651 treated CG sample was mainly caused by high densities of GP zones and metastable eta' precipitates. The enhanced strength of the naturally aged UFG sample was mainly caused by the high densities of G-P zones and dislocations. Upon T651 treatment, the dislocation density of the UFG sample deceased significantly, overcompensating the precipitation strengthening.