The Effects of Low Cu Additions and Predeformation on the Precipitation in a 6060 Al-Mg-Si Alloy

Effects of low Cu additions (≤0.10 wt pct) and 10 pct predeformation before aging on precipitates’ microstructures and types in a 6060 Al-Mg-Si alloy have been investigated using transmission electron microscopy (TEM). It was found that predeformation enhances precipitation kinetics and leads to formation of heterogeneous precipitate distributions along dislocation lines. These precipitates were often disordered. Cu additions caused finer microstructures, which resulted in the highest hardness of materials, in both the undeformed and the predeformed conditions. The introduced predeformation led to microstructure coarsening. This effect was less pronounced in the presence of Cu. The precipitate structure was studied in detail by high-resolution TEM and high angle annular dark-field scanning TEM (HAADF-STEM). The Cu additions did not alter the respective precipitation sequence in either the undeformed or the predeformed conditions, but caused a large fraction of β″ precipitates to be partially disordered in the undeformed conditions. Cu atomic columns were found in all the investigated precipitates, except for perfect β″. Although no unit cell was observed in the disordered precipitates, the presence of a periodicity having hexagonal symmetry along the precipitate length was inferred from the fast Fourier transforms (FFT) of HRTEM images, and sometimes directly observed in filtered HAADF-STEM images.