| dc.description.abstract | In this thesis, a variety of techniques within the field of transmission electron microscopy have been applied in the study of two dense 6xxx series aluminium alloys, at their peak hardness. This is a class of age-hardenable alloys with magnesium (Mg) and silicon (Si) as main alloying elements. When subjected to elevated temperatures, these alloys experience a high increase in mechanical strength, which is credited to the precipitation of numerous metastable, nanometer-sized, needle- and lath-shaped particles, from a solid solution of Mg, Si and other added elements. The morphology and distribution of these particles are of utmost importance for the alloy's macroscopic properties, and by connecting these properties to the nano-scale characteristics, alloys can be tailor-made for specific applications.
The studied alloys are similar in composition, and both are alloyed with significant amounts of copper (Cu). It is demonstrated that at T6 the precipitate micro-structure consists of a high density of very fine needle-shaped precipitates, coexisting with a lower density of lath-shaped, Cu-containing, disordered, L-phase precipitates. In one of the alloys a small amount of Si is substituted by germanium (Ge), in order to further investigate the element's effect on precipitation. Ge was found to refine the precipitate morphologies, resulting in a high increase of the number densities of both precipitate types. The alloy retained a similar needle volume fraction, but exhibited a doubling of the L-phase volume fraction. Both alloys exhibit good thermal stability, retaining a hardness over 95 HV5 after one week artificial ageing at 200C. The Ge-containing alloy exhibit an improved thermal stability compared to the Ge-free alloy, believed to correlate with the increased volume fraction of the L-phase precipitate.
High-angle annular dark-field scanning transmission electron microscopy showed that both needles and laths were disordered, containing fragments of multiple known phases. The needles commonly had Cu-enriched interfaces. It is demonstrated that in the needles in the Ge-containing alloy, Ge is incorporated in the Si-network, rotating it along a 100Al direction, and that these needles contain a lower amount of Cu as compared to the needles in the Ge-free alloy.
With the recent advances within scanning precession electron diffraction, this technique was applied to the Ge-free alloy at the T6 condition, with the intent of exploring the possibilities of making an automated procedure for acquiring the precipitate statistics, in a less time-consuming and more objective manner than what is currently used. A semi-automated procedure was developed and preliminary results gave similar values for precipitate number densities, while at the same time providing information on the crystal structure of the different hardening precipitates. | |
