Uncovering the origin of enhanced strengthening in Li-added Al-Cu-Mg alloys

The addition of minor elements is effective in designing high-strength aluminum alloys by modifying precipitation, such that clarifying the strengthening origin is highly significant. Here, 0.5 wt% Li is added in an Al-Cu-Mg alloy (AA2024 alloy), where, obvious increases in strength without reducing the ductility are achieved in both the unstretched and stretched alloys, as compared to those in the counterpart Li free alloys. The enhanced strength arising from Li addition is mainly related to the modified precipitation behavior (rather than the solute strengthening due to Li elements), in terms of Guinier-Preston-Bagaryatsky (GPB) zones and S-phase; the strengthening sources in the unstretched and stretched alloys, however, are fundamentally different. Correlative characterizations reveal that, for the unstretched alloy in the peak-aging (PA) state, the Li-addition favors a high density of uniformly distributed GPB zones while suppressing the nucleation of S-phase precipitates. Whereas, for the stretched alloys in the PA state, the Li-addition, in combination with the possible strengthening due to the T1 phase, results in refined S-phase precipitates. These modified precipitates (GPB zones and S-phase) are decorated with Li enrichments, which are believed to be correlated with the proposed mechanism of the Li-Vacancy complex. Thus, elucidating the origin of strengthening in Li-added Al-Cu-Mg alloys under both unstretched and stretched conditions, is expected to provide basic insights into the strengthening mechanisms in other high strength Al alloys when Li is added as the minor element.