The effect of deformation and natural ageing in an Al-Mg-Si-Cu alloy with high Cu-content: A transmission electron microscopy study

Abstract

The effect of the combination of natural ageing and deformation to 80 % on precipitation in an Al-Mg-Si-Cu alloy has been investigated by different transmission electron microscopy (TEM) techniques. Similar research is sparse caused by the difficulty in obtaining quantitative information about the microstructure of samples deformed at that rate in TEM. This is due to the large density of dislocations. Different techniques were tried to evaluate how the deformed materials microstructure could be characterised in the best way. The techniques included bright field imaging, energydispersive spectroscopy mapping, low- and high resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and scanning precession electron diffraction (SPED). Of all these techniques, the combination of HAADF-STEM and SPED proved to be most suitable.

It was found that the order of the deformation and natural ageing had an influence on the final hardness: samples deformed prior to natural ageing were less hard than samples deformed after natural ageing the difference being 5.2 HV. Initial investigations by bright field imaging, low resolution HAADF-STEM and energy dispersive spectroscopy mapping gave qualitative information. The microstructure consisted of precipitates with habit plane {100}Al in-between dislocations and a continuous decoration of the dislocation lines by precipitates for the deformed samples. The un-deformed sample consisted of a homogeneously distributed precipitates with habit plane{100}Al. By these techniques, it was not possible to state any significant difference in the microstructure of the deformed samples other than the observation of one additional precipitate type in the sample deformed prior to deformation.

The high resolution HAADF-STEM investigations revealed that the precipitates in-between dislocations were mainly the L-phase, while the precipitates nucleated on dislocation lines were either C-phase, disordered structures or an ordered precipitate with habit plane{110}Al. The same was found by SPED and the phase fractions were estimated by both techniques. The estimated phase fraction from both techniques found that deforming after natural ageing favoured more precipitation along dislocations than the case where the deformation was applied prior to natural ageing.