Abstract: Influence of retrogression treatment on microstructure and mechanical properties of the 7A20 aluminum alloy during RRA treatment was investigated. The mechanical properties and conductivity were tested. The microstructure at grain boundary and inner grain was characterized by using OM and TEM (HRTEM). The results indicat that the tensile strength decreases from 630 MPa to 525 MPa under 160 ℃ retrogression treatment with the increase of time. While tensile strength increases from 645 MPa to 710 MPa after reaging treatment at 120 ℃ for 12 h. The longer the retrogression time, the greater the increase of intensity caused by reaging, which reaches 185 MPa after 80 min. At a retrogression temperature of 200 ℃ and a retrogression time of 10 min, the peak strength is 715 MPa. As the retrogression temperature increases, the conductivity first increases and then decreases, which has a positive correlation with retrogression time. After retrogression at 160, 200 and 240 ℃ corresponding to the peak strength, the grain sizes of matrix are 55, 65 and 70 μm, respectively with a little difference. After retrogression treating at 200 ℃ for 10 min and reaging treatment, there is discontinuous MgZn₂ phase precipitated at grain boundaries with the size of 10-20 nm, which is incoherent with the matrix. There is dispersive nano-sized η′ strengthening precipitates inner the grains, which is coherent or semi-coherent with the matrix. There is a PFZ region with a width of 45-55 nm at the grain boundary.

Key words: 7A20 aluminum alloy, retrogression treatment, RRA treatment, microstructure, mechanical properties