The effect of minor solute additions on the precipitation path of an Al-Cu-Li alloy

Minor solute additions of Mg, Ag or Zn to AlCuLi alloys are known to promote the formation of the T-1-Al2CuLi phase and improve the strengthening of these alloys. In this paper, we use atomic resolution high angle annular dark field scanning transmission electron microscopy (STEM-HAADF) in conjunction with electron dispersive spectroscopy (EDS) mapping to elucidate the mechanisms by which these minor solute additions modify the precipitation path. The results show that the presence of Mg profoundly changes the nature of the precipitates formed, as early as the nucleation stage. In the presence of Mg, precursor phases containing Mg and Cu form at the dislocations, and later lead to a microstructure with GPB zones and S phase together with a high number density of T-1 precipitates. In the absence of Mg, GPI zones form in the early stages of precipitation and later lead to a microstructure dominated by theta' precipitates, with a low density of T-1. Ag and Zn additions do not change the precipitation sequence, yet they are found in the T-1 precipitates, Ag located at the precipitate/matrix interface, and Zn within the precipitate, probably substituting for Cu. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.