Precipitation behavior in an Al-Cu-Mg-Si alloy during ageing

The precipitation behavior and aging response of an A1-4.9Cu-0.74Mg-0.51Si-0.48Mn-0.1Cr-0.08Ti-0.02Fe (wt. %) alloy was examined in detail after aging at 170 degrees C with different dwell times up to 96 h. Deformation by 3% stretching prior to aging was used to investigate the effect of dislocations on phase and hardness evolution and compared with the undeformed material. The small pre-deformation led to a slight decrease of peak strength due to reduced homogenous nucleation. Strong interaction between different phases caused the formation of hybrid structures both in the undeformed bulk areas and on dislocations. These phases consist of different fragments of the GP zones, 8 ''- and Er-phases from the Al-Cu system, GPB zones and Sl-phase from the Al-Cu-Mg system, and beta ''-, beta'-Cu, C- and Q'-phases from the Al-Mg-Si-Cu system. A new phase named C1, isostructural to the C-phase but having a different orientation with the Al matrix - (010)(c)//(010)(Al), [001](c)//[101](Al), has been found predominantly on dislocation lines, and to a lesser extent in hybrid precipitates in the bulk. Calculations of structural stability by density functional theory (DFT) were performed on experimentally found structures, consisting of a C-phase core in two different orientations, and having Cu segregations in GP-like structures at their interfaces with the matrix.