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

The aging behaviors of three Al-Li-Zn-Mg-Cu alloys were systemically studie d including one-step aging, two-step aging and pre-straining aging. It is s hown that the addition of Li suppressed the homogenous precipitation of MgZ n2 phases in Al-Zn-Mg-Cu alloys, and the Zn-rich phases precipitate primari ly on grain boundaries in the form of large particles. The two-step aging p lays a key role in stimulating nucleation in the Li-free alloy, but has lit tle effect on the hardening efficiency of Li containing alloys. The pre-def ormation promotes the precipitation of Zn-rich phases in the matrix, and im proves the yield strength greatly. These results are interpreted in terms o f the effect of preferential binding of Li atoms and vacancies on the nucle ation mechanism. Since Li atoms possess a high concentration and significan t vacancy-binding energy, they preferentially trap the quench-in vacancies, then retard the diffusion of Zn, Mg atoms, and suppress the homogenous nuc leation of Zn-rich phases in the matrix. The two-step aging shows little be nefit on the precipitation of Zn-rich phases in the matrix because of the r etarding effect of Li atom. While pre-deformation increases the defect conc entration in the matrix, thereby causes the modification of size and distri bution of Zn-rich precipitates. Published by Elsevier Science S.A.