Aging behavior of Li containing Al–Zn–Mg–Cu alloys

The aging behaviors of three Al–Li–Zn–Mg–Cu alloys were systemically studied including one-step aging, two-step aging and pre-straining aging. It is shown that the addition of Li suppressed the homogenous precipitation of MgZn2 phases in Al–Zn–Mg–Cu alloys, and the Zn-rich phases precipitate primarily on grain boundaries in the form of large particles. The two-step aging plays a key role in stimulating nucleation in the Li-free alloy, but has little effect on the hardening efficiency of Li containing alloys. The pre-deformation promotes the precipitation of Zn-rich phases in the matrix, and improves the yield strength greatly. These results are interpreted in terms of the effect of preferential binding of Li atoms and vacancies on the nucleation mechanism. Since Li atoms possess a high concentration and significant vacancy-binding energy, they preferentially trap the quench-in vacancies, then retard the diffusion of Zn, Mg atoms, and suppress the homogenous nucleation of Zn-rich phases in the matrix. The two-step aging shows little benefit on the precipitation of Zn-rich phases in the matrix because of the retarding effect of Li atom. While pre-deformation increases the defect concentration in the matrix, thereby causes the modification of size and distribution of Zn-rich precipitates.