Precipitation in multicomponent, lean, Al-Mg-Si alloys: A transmission electron microscopy study

Abstract

Aluminium alloys have a number of versatile applications, and there are numerous alloys in production today, where a variety of solute additions and heat treatments are applied. This work focusses on lean, age-hardenable Al-Mg-Si alloys, which are light weight, have excellent corrosion resistance and a moderate strength potential. In Al-Mg-Si alloys it is the growth of nanosized needles, consisting of Mg, Si and Al itself, which leads to an increase in strength during heat treatment of the alloys. The nanosized needles are metastable and grow along the three <001> Al crystal directions. During heat treatment, several needle phases may form, dependent on parameters such as the extent of the heat treatment, its temperature and the alloy composition. A major challenge in industry, and the first motivation behind this work, is to make extrusion of lean Al-Mg-Si alloys easier and faster, while simultaneously not making any sacrifices in strength. The second major motivational point of this thesis is to connect the macroscopic material properties like strength and extrudability to the atomic scale characteristics. Only in this way tailor-made alloys can be made for specific applications. Transmission electron microscopy (TEM) serves as an excellent tool for microstructure characterization of Al. The high spatial resolution and possibility of detecting a number of different electron-sample interactions are obvious advantages. High angle annular dark field scanning TEM (HAADF-STEM) with aberration corrected lenses provides exceptionally high spatial resolution, down to the atomic scale. Aberration corrected HAADF-STEM is particularly well suited for detecting elements with different atomic numbers (Z) because of its Z contrast. However, in some cases the elements might be too close in atomic number to separate intensity differences, which is where electron energy loss spectroscopy (EELS) comes to aid. With EELS one obtains a ‘fingerprint’ of each element based on the energy losses of the electrons, which are dependent on the elements they have interacted with while passing through the sample. For surface-specific dynamics in the material, information can be attained by photoelectron spectroscopy (PES). PES enables us to do in-situ investigations of core levels during heat treatment of the material. By making use of this method, the complications of detecting elements with overlapping EEL edges, like e.g. Li and Mg, can be solved. Two main approaches have been chosen to solve the extrudability vs strength issue here. The first approach was based on lowering the amount of added Si and Mg, while adding back a smaller or equal amount of Li, Cu, Ge or Ag. In the second approach, ‘disturbed ageing’ was applied at various times during heat treatment. This was accomplished by elastic straining or introducing small plastic deformations to the material. It has been demonstrated how strength loss in an Al-Mg-Si alloy, caused by reduction in solute, can be compensated by adding back smaller quantities of Li, Cu, Ag or Ge. These solute additions were added to Al-Mg-Si alloys alone or in different combinations. Ge was discovered to be the most effective solute addition, significantly refining the precipitation and strengthening the material in spite of lower total solute. Cu, Ag and Ge additions have strong influence on the main hardening precipitate, β”. Ge in particular changes its structure and promotes disorder, which seems to be favourable for increased material strength. Cu and Ag both participate in the precipitation of needles and change the precipitation sequence. HAADF-STEM investigations indicated that Li causes modest structural changes to the main hardening precipitate β”. DFT has been used to support the experimental results and better explain the observed features in each alloy. Bonding energies, volume misfits, and formation enthalpies have been calculated for solute additions, vacancies and structural variants of the β” phase. DFT and HAADF-STEM support intensity variations suggesting Li to occupy Mg sites, and in particular the Mg3 sites in β”. The potential of atomic resolution EELS has been demonstrated by detailed investigation of the elemental distribution in a precipitate cross section in a multicomponent Al alloy. Furthermore, a correlative analysis of the EELS data was performed and connected to the results. The EELS technique alone was able to resolve both the face centred cubic (fcc) Al lattice along the <001> Al zone axes and the hexagonal Si-network in a precipitate cross section. Some atomic columns were revealed to contain a mix of elements after combining EELS and HAADF-STEM. It has been demonstrated how a commercial Al-Mg-Si alloy can get enhanced strength at peak hardness conditions when elastic strain is applied at the beginning of natural ageing. The strengthening effect is explained by enhanced formation of clusters, causing a higher number density of precipitates at peak hardness. Applying 1 % plastic deformation increased the material strength as compared to an un-deformed reference alloy; the strengthening effect is in this case attributed to the introduction of dislocations to the material during deformation. X-ray photoelectron spectroscopy (XPS) and X-ray photoemission electron microscopy (XPEEM) were used to study Li, Mg and Si in the surface of an Al-Mg-Si-Li alloy. The surface oxide layer was sputtered to a negligible thickness, and the relative abundance of alloying agents (Li, Mg and Si) was recorded for different time-stamps during annealing. All three elements were recorded to appear with a significant increase in surface concentration. The concentration decreased again with further increase in annealing temperature and duration of annealing. Si and Li both occur everywhere on the alloy surface, while Mg migration is mostly restricted to grain boundaries. Li occurs at much higher temperature than the two respective alloying agents.