Effect of heat treatment on microstructure and mechanical properties of a novel Al-Zn-Mg-Cu alloy

Effects of solution and aging treatment on the microstructure and mechanical properties of a novel Al-Zn-Mg-Cu alloy by microalloying rare elements Sc and Er were studied. The results show that solution time has a visible influence on the microstructure and mechanical properties of the alloy. Specifically, as the solution time increases, the area fraction of the residual phase in the alloy decreases, and the shape of the grain becomes more spheroidal and coarser, leading to a decrease in hardness. This is attributed to the dissolution of strengthening phases during the solution treatment, which weakens the solid solution strengthening effect. The single-stage aging treatment shows an initial increase in strength and hardness of the alloy, followed by a decrease as the aging time is extended, until a steady state is achieved. The optimal single-aging conditions are found to be at 120 °C for 24 h, where the alloy exhibits an excellent combination of high strength and good ductility, with an ultimate tensile strength (UTS) of 523 MPa, yield strength (YS) of 482 MPa, and elongation (El) of 1.75%, respectively. Compared to single-stage aging, double-stage aging (120 °C for 24 h and then 150 °C for 52 h) significantly increases the elongation of the alloy (4.17%), but the UTS reduces to 465.29 MPa, and YS reduces to 410.64 MPa. Transmission electron microscopy (TEM) observations disclose that the grain size, the distribution spacing of precipitates along the grain boundary, and the width of the precipitation-free zone (PFZ) all undergo augmentation as the duration of the second stage aging process elongates.