Effects of Li content on precipitation in Al-Cu-(Li)-Mg-Ag-Zr alloys

The microstructure of Al-4.0Cu- (Li) -0.4Mg-0.4Ag-0.15Zr alloys changes significantly with Li content. Small additions (0.5wt.%) of Li encourage the precipitation of θ′ with a refined dispersion and a reduced density of {111}α precipitates in the matrix. The precipitation of θ′ phase occurs less frequently with increasing Li content from 0.5wt.% to 1.6wt.%. The T1 precipitation predominates in alloy 2195 with relatively high Li content of 1.0 wt.%. High Li contents (1.6wt.%) result in the δ′ precipitation with a decreased amount of T1. Grain-boundary precipitation decreases significantly with increasing Li content from 0.5wt.% to 1.6wt.%. For Al-4.0Cu-0.4Mg-0.4Ag alloys with small Li additions of 0.5wt.%, the {111}α, precipitates in the matrix seems to be a mixture of Ω and T1 phases with Ω as the predominant phase while those at grain boundaries be Ω phases exclusively. This is consistent with the previously reported experimental findings of Lee et al.