Scanning transmission electron microscopy studies of precipitation in Al-Mg-Ge alloys

Abstract

Precipitation in three Al-Mg-Ge(-Si-Cu) alloys has been investigated using transmission electron microscopy. The alloy compositions were chosen to be similar to previously studied Al-Mg-Si(-Cu) alloys to facilitate direct comparison. These alloys are strengthened by the precipitation of nanometresized, needle-shaped particles during heat treatment. A deeper understanding of precipitation at the atomic level is required in order to achieve greater control over alloy properties. The precipitation in the investigated Al-Mg- Ge(-Si-Cu) alloys was found to share similarities with that in Al-Mg-Si(-Cu) alloys, but there were also significant differences.

The high atomic number of Ge relative to Al, Mg, and Si made Al-Mg-Ge alloys highly suited for study by the atomic-number sensitive technique high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM). This was the most important technique employed in this thesis. The use of a state-of-the-art aberration-corrected microscope also made it possible to resolve details previously inaccessible.

A near-hexagonal network of Ge columns when viewed along the needle direction was a unifying feature of all the precipitates in these alloys, as is the case in the metastable precipitates of the Al-Mg-Si(-Cu) alloy system.

However, the β__ phase, the most important hardening phase in Al-Mg-Si alloys, was not observed. Instead, hardnesses similar to that of comparable Al-Mg-Si(-Cu) alloys were achieved through other precipitate phases.

Two Al-Mg-Ge alloys were the main objects of study in this thesis: one Mg-rich and one Ge-rich, with an addition of Mg and Ge in the relation Mg2Ge and Mg5Ge6, respectively. Precipitate phases that form in overaged Al-Mg-Si alloys were observed around peak hardness in these Al-Mg-Ge alloys, as well as disordered precipitates. The precipitate phases known from Al-Mg-Si, U1 and β_, were finer and more coherent with the Al matrix in the Al-Mg-Ge alloys than their counterparts in Al-Mg-Si. These precipitates also displayed highly interesting interface structures, consisting of Ge atoms in columns not part of the bulk precipitate structure.

The β_-like precipitate phase that was observed in the Mg-rich alloy was investigated by quantitative HAADF STEM. This method makes it possible to obtain quantitative compositional information from the specimen. It was found that the Ge-rich columns contained significantly less Ge than the Si columns of β_ in Al-Mg-Si alloys. A partial replacement of Ge by Al or vacancies might explain the smaller lattice parameter of the β_-like phase in Al-Mg-Ge compared with β_ in Al-Mg-Si alloys.

Precipitation in an Al-Mg-Si-Ge-Cu alloy was also investigated with HAADF STEM. No repeating unit cell was observed in these precipitates near peak hardness. However, these precipitates contained a hexagonal network consisting of mixed Si and Ge columns with Mg, Al, and Cu columns occurring in between the network columns at specific sites. Structural units consisting of Al, Mg, Si, and Ge were often arranged in an ordered manner.