Improving creep strain and mechanical properties of Al-Zn-Mg-Cu alloy under high stress level via pre-aging

This study delved into the influence of pre-aging on the creep behavior and mechanical properties of an Al-Zn-Mg-Cu alloy subjected to high creep level. The creep strain was improved significantly for both pre-aged (120 °C for 6 h) and T4 samples under high creep stresses (280 MPa). Notably, pre-aging endowed the alloy with a long “peak-aging strengthening zone” during high-stress creep aging. This can effectively extend the creep aging time and enhance the creep strain while maintaining high strength of the alloy. This may stem from the additional consumption of vacancies and solute atoms by the pre-aging treatment, inhibiting the transformation of the η′phase into the η phase. Moreover, the pre-aging promotes the formation of fine, high density, and uniformly distributed η′ phase. It can also reduce the diffusion of solute atoms from inter grain to grain boundaries (GBs) during high-stress creep aging. This effectively impeded the coarsening of precipitates at GBs, reducing the width of the precipitation-free zone, and consequently minimized plasticity loss induced by high creep stress. Therefore, a suitable pre-aging can make the samples still maintain outstanding mechanical properties during the high-stress creep aging, which opens a new way of thinking to enhance the creep deformation behavior and aging-strengthening behavior in a synchronized manner. In particular, this provides a new approach to address the tendency of large rib-stiffened tank plates to develop high-stress zones during creep aging, leading to over-aging.