Mutually separated, branched segregation behavior of Mg and Ag elements in the T1 precipitates of Al-Cu-Li alloys

The segregation behavior of Mg and Ag in the T1 precipitates of an Al-Cu-Li-based alloy has been investigated using high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), energy dispersive X-ray spectroscopy (EDX) and first-principles calculation. For minimal-thickness T1 precipitates, Mg atoms display a preference for occupying Li sites at the precipitate center, while Ag atoms partially replace Al along the broad T1/Al interface. For thicker T1 precipitates, the Ag segregation behavior is correlated with the precipitate structure: standard T1 precipitates are decorated with this element only at the broad interface, whereas Ag is allowed also in interior Al-Li layers for the alternative T1 variant where such layers are paired rather than shared. This “branched” segregation behavior may serve to explain previous experimental disagreements, while further highlighting how microalloying has the potential ability to affect relative stabilities for different variants of the main hardening precipitate in the Al-Cu-Li alloy system. [Display omitted]