Sigmund J. Andersen

The crystal structure of the {beta}{double_prime} phase in Al-Mg-Si alloys

The crystal structure of β″, one of the strengthening phases in the commercially important Al–Mg–Si alloys, is determined by use of high resolution electron microscopy (HREM) and electron diffraction (ED). A trial structure was established from exit wave phase reconstructed HREM images. A least-square refinement of the model coordinates was done using data from digitally recorded ED patterns. A recently developed computer program (MSLS) was applied, taking into account dynamic scattering. The atomic unit cell contains two units of Mg5Si6. It is C-centred monoclinic, space group C2/m, a=1.516±0.002 nm, b=0.405 nm, c=0.674±0.002 nm, β=105.3±0.5°. The atomic packing may be regarded as a hard ball packing using clusters, the clusters being (1) centred tetragons of Mg atoms and (2) so-called twin icosacaps where Mg atoms are centred above and below pentagonal rings of four Si atoms an one Mg atom. A growth related stacking fault in the structure is explained by a deficiency of Mg atoms. A model for the β″/Al interface is given.

Modelling of the age hardening behaviour of Al-Mg-Si alloys

Abstract In the present investigation a special control volume formulation of the classical precipitation model for coupled nucleation, growth and coarsening has been adopted to describe the evolution of the particle size distribution with time during thermal processing of Al–Mg–Si alloys. The analysis includes both isothermal and non-isothermal transformation behaviour. Well established dislocation theory is then used to evaluate the resulting change in hardness or yield strength at room temperature, based on a consideration of the intrinsic resistance to dislocation motion due to solute atoms and particles, respectively following heat treatment. The model is validated by comparison with experimental microstructure data obtained from transmission electron microscope examinations and hardness measurements, covering a broad range in the experimental conditions. It is concluded that the model is sufficiently relevant and comprehensive to be used as a tool for predicting the response of Al–Mg–Si alloys to thermal processing, and some examples are given towards the end.

The influence of temperature and storage time at RT on nucleation of the β phase in a 6082 Al-Mg-Si alloy

Abstract The age hardening process of a 6082 Al–Mg–Si alloy by isothermal ageing at 100, 125 and 150 °C for 0 to 40 days has been investigated by both conventional and high-resolution transmission electron microscopy. Changes in sizes and densities of the pre-β″ and β″ phases during ageing can explain the evolution of hardness, in particular a plateau in the curve of the hardness versus ageing time. Fully coherent pre-β″ particles (GP-I zones) are formed from atomic clusters that were created upon annealing or at natural ageing. Room temperature (RT) storage reduces the number of pre-β″ particles by a factor of 5. Ageing at 125 and 150 °C causes the phase pre-β″ to transform to the phase β″. An ageing temperature of 100 °C was found to be too low to initiate the transformation of pre-β″ to β″.

Atomic model for GP-zones in a 6082 Al-Mg-Si system

Abstract Heat treatments of 6082 Al-alloy with 0.6 wt% Mg, 0.9 wt% Si, 0.5 wt% Mn and 0.2 wt% Fe can lead to a considerable increase in hardness. This increase is due to the presence of several metastable phases (in particular β″). To determine the structure of the phase formed before the β″ phase, a detailed high resolution electron microscopy (HREM) study was performed. The pre-β″ phase is needle like, as is the β″ phase. Based on reconstructed exit waves, two models were possible, one of which could be rejected because of the interatomic distances. The model resembles that of the β″ but with different positions for some of the Mg atoms along the needle direction. The structure is more similar to the Al matrix than that of the β″ phase. The Mg sites, and to a lesser extent also the Si sites are partly occupied by Al atoms. The composition is therefore less Mg-rich than β″ (Mg5Si6). The content of Al in the structure of the precipitates increases with the degree of coherency in the Al matrix. The space group of the new phase is C2/m, as for β″.

Composition of β″ precipitates in Al–Mg–Si alloys by atom probe tomography and first principles calculations

The composition of β″ precipitates in an Al–Mg–Si alloy has been investigated by atom probe tomography, ab initio density functional calculations, and quantitative electron diffraction. Atom probe analysis of an Al-0.72% Si-0.58% Mg (at. %) alloy heat treated at 175 °C for 36 h shows that the β″ phase contains ∼20 at. % Al and has a Mg/Si-ratio of 1.1, after correcting for a local magnification effect and for the influence of uneven evaporation rates. The composition difference is explained by an exchange of some Si with Al relative to the published β″-Mg5Si6 structure. Ab initio calculations show that replacing the Si3-site by aluminum leads to energetically favorable compositions consistent with the other phases in the precipitation sequence. Quantitative electron nanodiffraction is relatively insensitive to this substitution of Al by Si in the β″-phase.

The influence of composition and natural aging on clustering during preaging in Al–Mg–Si alloys

This work provides a detailed atom probe tomography study of clustering in the Al–Mg–Si system. Focus is on separating and understanding the influence of natural aging, preaging, and alloy composition on the clustering behavior of solute atoms. Two dilute alloys with the same total solute content have been studied, one Mg-rich and one Si-rich. The detrimental effect of natural aging for these alloys is investigated by comparing directly preaged samples to samples stored at room temperature before the preaging treatment. Clusters were identified in the atom probe datasets by the maximum separation method employing heuristically determined input parameters. It was found that seven days of intermediate natural aging gave a five times lower number density of clusters as compared to direct preaging for both alloy types. The clusters were of comparable size but their compositions depended on heat treatment history. Preaging promoted the formation of clusters with an Mg:Si ratio close to 1 in both alloys, while ...

The effect of Zn on precipitation in Al–Mg–Si alloys

Effects of addition of Zn (up to 1 wt%) on microstructure, precipitate structure and intergranular corrosion (IGC) in an Al–Mg–Si alloys were investigated. During ageing at 185 °C, the alloys showed modest increases in hardness as function of Zn content, corresponding to increased number densities of needle-shaped precipitates in the Al–Mg–Si alloy system. No precipitates of the Al–Zn–Mg alloy system were found. Using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), the Zn atoms were incorporated in the precipitate structures at different atomic sites with various atomic column occupancies. Zn atoms segregated along grain boundaries, forming continuous film. It correlates to high IGC susceptibility when Zn concentration is ~1wt% and the materials in peak-aged condition.

Quantification of the Mg2Si β″ and β′ phases in AlMgSi alloys by transmission electron microscopy

A method has been developed to determine the density of the Mg2Si β″ and β′ phases in 6xxx aluminum alloys using transmission electron microscopy (TEM). Age-hardened extruded profiles of an AlMgSi alloy, having different coarseness in the precipitate structure, were examined. By darkfield (DF) imaging, β″ needles aligned in a 〈100〉 viewing direction were brought into contrast. Thickness was determined within 10 pct using a spot contamination method. After subsequent corrections, the number density of precipitates could then be quantified, usually within 5 pct. As expected, the average number density of the β″ precipitates correlates well with the material’s tensile strength. A density of β ′ was obtained by a similar method. The densities represent maximum values since precipitate-free areas are not considered. Tensile strength is quite sensitive to theratio between the number densities of β′ and β″. By determining the volume of an average β″ or β′ needle, corresponding volume fractions of the phases can be determined. Quantification of these phases could benefit calculations involving mechanical properties of such alloys.

HRTEM study of the effect of deformation on the early precipitation behaviour in an AA6060 Al–Mg–Si alloy

The effect of 10% pre-ageing deformation on the early precipitation behaviour in an AA6060 Al–Mg–Si alloy aged 10 min at 190°C was investigated by high-resolution transmission electron microscopy (HRTEM) in ⟨100⟩Al projections. The precipitate nucleation was heterogeneous since all precipitates were found to grow on dislocation lines. The pre-ageing deformation suppresses growth of Gunier–Preston zones and β″ phase. The resulting precipitates are still largely coherent with the aluminium matrix. They appear with two main morphologies; one consists of independent, small cross-sections arising from needles with disordered β′ and B′ structures. The other morphology is a much more continuous decoration where precipitates have elongated and conjoined cross-sections and where a particular precipitate phase could not be determined. All precipitates in this work were found to contain a common near-hexagonal sub-cell (SC) with projected bases a = b ≈ 0.4 nm. This strongly indicates that they are built over the same Si network, which recently has been demonstrated to exist in all precipitates in the Al–Mg–Si(–Cu) system. For the discrete morphology type the network has one hexagonal base vector parallel to or very near a ⟨510⟩Al direction. For the continuous type, one base vector falls along a ⟨100⟩Al direction. This orientation of the network is different from previous studies of ternary Al–Mg–Si alloys and must be a direct consequence of the deformation.

Aberration-corrected HAADF-STEM investigations of precipitate structures in Al–Mg–Si alloys with low Cu additions

Precipitates in a lean Al–Mg–Si alloy with low Cu addition (~0.10 wt.%) were investigated by aberration-corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Most precipitates were found to be disordered on the generally ordered network of Si atomic columns which is common for the metastable precipitate structures. Fragments of known metastable precipitates in the Al–Mg–Si–(Cu) alloy system are found in the disordered precipitates. It was revealed that the disordered precipitates arise as a consequence of coexistence of the Si-network. Cu atomic columns are observed to either in-between the Si-network or replacing a Si-network column. In both cases, Cu is the center in a three-fold rotational symmetry on the Si-network. Parts of unit cells of Q′ phase were observed in the ends of a string-type precipitates known to extend along dislocation lines. It is suggested that the string-types form by a growth as extension of the B′/Q′ precipitates initially nucleated along dislocation lines. Alternating Mg and Si columns form a well-ordered interface structure in the disordered Q′ precipitate. It is identical to the interface of the Q′ parts in the string-type precipitate.