The precipitation competition of η-series precipitates and δ′ precipitates in Li-containing Al-Zn-Mg-Cu alloys

The eta-series precipitates (i.e. eta, eta' if precipitates and their precursors) are the crucial strengthening precipitates in Al-Zn-Mg-(Cu) alloys, whereas the delta'-phase (Al3Li) is a common precipitate in Al-Li alloys. Using ab-initio calculations, we have investigated Li-containing Al-Zn-Mg-(Cu) alloys with focus on the competing formation of eta-series precipitates and delta' precipitates. It is shown that the Zn-doped delta' precipitates are energetically favorable. As such, the Zn and Mg contents and the Zn-to-Mg atomic ratio in the matrix are changed, and the formation of eta-series precipitates will no longer occur with increasing the Li-content in the alloys. Our calculated results have been verified by atomic-resolution scanning transmission electron microscopy, which demonstrates that significant amounts of Zn and Mg atoms are absorbed by delta' precipitates, occupying the Li-sites and Al-sites in the delta'-phase lattice respectively. Our study indicates a fundamental understanding at the microscopic scale on how the eta-series precipitates can be suppressed by adding Li atoms into the Al-Zn-Mg-Cu alloy.