In this study we demonstrate the complementarity of transmission electron microscopy (TEM) and atom probe tomography (APT) in studying the early stages of phase decomposition in the age-hardening ...Al–Mg–Si alloy AA 6016. Samples are subjected to natural ageing at ambient temperature or artificial pre-ageing at elevated temperature in order to produce different types of atomic clusters and early stages of precipitation before age hardening commences. APT is utilized to detect clusters and identify their number density, size and compositions, whereas TEM is applied to analyse and quantify number density, sizes and types of the hardening precipitates during artificial ageing. Finally, the particle statistics derived by APT and/or TEM were utilized to predict the mechanical properties of the various samples and conditions analysed.
The crystal structure of β′, one of the metastable phases formed during precipitation hardening of Al–Mg–Si (6xxx) alloys, has been determined using electron diffraction (ED). With the aid of ...high-resolution electron microscopyimages and the composition estimated from energy dispersive X-ray analysis, an initial model for the structure of the β′ phase was obtained. The data from digitally recorded ED patterns were then used for a least-squares refinement of the atomic parameters of the model with a software program package developed in Delft (MSLS), taking into account dynamic scattering. The β′ structure has a composition of Mg
9Si
5. The unit cell is hexagonal, space group
P6
3/
m, with unit cell parameters
a
=
0.715
nm and
c
=
1.215
nm. The composition and structure were confirmed by ab initio calculations.
A numerical solution is presented, capable of handling nucleation, growth and coarsening, and likewise dissolution, of hardening precipitates in Al–Mg–Si alloys during ageing, welding and post weld ...heat treatment (PWHT). By coupling this to a separate strength model for shearing and bypassing of particles by dislocations, the evolution of the macroscopic yield stress at room temperature can be calculated at each time step. Following testing of the model against reliable hardness and transmission electron microscope (TEM) data obtained from dedicated Gleeble simulation experiments, the connectivity on a micro/macro level throughout a multistage manufacturing route is illustrated by means of a numerical example. These simulations show how a past process step influences the microstructure and strength evolution in the subsequent process steps due to interactions between different groups or classes of particles that form at various temperatures.
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″.
The plastic anisotropy of aluminium alloys is known to depend not only on the crystallographic texture but also on the heat-treatment, and this effect has been studied on various alloys both ...experimentally and numerically. However, the 6000 series of aluminium alloys is not broadly represented in these studies. In this work, an extruded profile of the AA6063 alloy was investigated. Electron backscatter diffraction (EBSD) measurements revealed a strong cube crystallographic texture with a minor Goss component, which is typical for recrystallized aluminium alloys. The plastic anisotropy was studied by uniaxial tension tests in different material directions, using digital image correlation to measure the displacement field and thus to calculate the strain field. The tensile specimens were heat-treated to three different tempers: T6, T7 and O, in addition to the as-received T1 condition. Transmission electronic microscopy (TEM) was used to characterize the precipitate structure of the heat-treated material. A crystal plasticity finite element model of the tensile test was created and calibrated using some of the experimental data. The comparison of the experimental stress-strain curves, strain ratios and flow stress ratios with their simulated counterparts revealed that the crystallographic texture is dominating the anisotropy in all tempers. The accuracy of the CP-FEM predictions varies for different material orientations, and, in general, the simulated material exhibits a sharper anisotropy than the real material. The effect of the heat treatment on the anisotropy is found to be minor compared with the texture effect.
TEM investigations of two alloys isothermally heat treated at 175°C and 260°C show how Cu additions to the Al-Mg-Si system affect precipitation. Both alloys had a solute content Mg + Si = 1.3 at.%, ...0.127 at.% Cu, but with Mg/Si 0.8 and 1.25. Cu-containing Guinier-Preston (GP) zones and three types of Q′ precursors are identified as most common phases at peak-hardness conditions, whereas β″ accounts for maximum 30% of the total number of precipitates. The precursors have needle (L and S precipitates) or plate (C precipitate) morphologies. They consist of different arrangements of Al, Mg and Cu atoms on a grid defined by triangularly arranged Si planes parallel with and having the same period as {100} Al planes. The Si grid is composed of nearly hexagonal sub-cells of a = b = 4.05 Å, c = 4.05 Å. The Cu arrangement on the grid is often disordered in the needle precursors. The plate precursor is ordered, with a monoclinic unit cell of a = 10.32 Å, b = 8.1 Å, c = 4.05 Å, γ = 101°.
► APT and HRTEM study of intergrown precipitate particles in an overaged Al–Mg–Si–Cu (6xxx) alloy. ► The particles are shown to contain the structures of both the Q′ and L hardening phases. ► A more ...accurate structure and composition is reported for the Q′ phase. ► The L and Q′ phases are shown to be connected by a common underlying Si arrangement present in both structures.
Intergrown Q′ and L precipitates formed in an overaged Al–Mg–Si–Cu alloy were studied by transmission electron microscopy and atom probe tomography. The composition of the ordered Q′ phase was measured to be Al
3.8Mg
8.6Si
7.0Cu
1.0, while less Mg was found in the disordered L phase. Compared to the equilibrium Q phase, Q′ contained less Cu and more Al. Similar Si concentrations were found in the Q′ and L precipitates, consistent with the presence of a common Si arrangement in both structures.
Starting from solid solution (T4) or a condition with beta" precipitates (T6), three Al-Mg-Si alloys with similar total solute content (1.3 at%), but different Si/Mg ratios (2, 1.25 and 0.8) were ...isothermally heat-treated at 250 or 260DGC and investigated by transmission electron microscopy. The result microstructure for all alloys and conditions consisted of metastable, needle-shaped precipitates growing along (100) directions in aluminium. Each of the phases beta"-Mg5Si6, beta'-Mg1.8Si, U1-MgAl2Si2 and U2-MgAlSi could be identified as main precipitate in the alloy with its solute Si/Mg ratio closest to the same ratio in the composition of that particular phase: The highest Si content alloy produced coarse needles of the trigonal U1-phase coexisting with finer precipitates of hexagonal B-phase. The most common phase in the Mg-rich alloy is coarse needles of hexagonal beta'-type. The Si/Mg ratio of 1.25 in one alloy is similar to the Si/Mg ratio in beta". Here the microstructure changes from that of fine beta" needles to fine needles of the orthorhombic U2-phase. This material remains strongest during heat-treatment. Nucleation on dislocations, mainly by the B'-phase, was observed to be significant in the case of Si-rich alloys heat-treated from T4-condition.
Summary
The image contrast of sheared needle‐like β′′ precipitates in the Al‐Mg‐Si alloy system is investigated with respect to shear‐plane positions, the number of shear‐planes, and the active ...matrix slip systems through multislice transmission electron microscopy image simulations and the frozen phonon approximation. It is found that annular dark field scanning transmission electron microscopy (ADF STEM) images are mostly affected by shear‐planes within a distance ∼6–18 unit cells from the specimen surface, whereas about 5–10 equidistant shear‐planes are required to produce clear differences in HRTEM images. The contrast of the images is affected by the Burgers vector of the slip, but not the slip plane. The simulation results are discussed and compared to experimental data.
Lay Description
Pure aluminium is too soft to be viable in most structural applications, but this may be remedied by alloying the metal with various elements. Adding small amounts of silicon and magnesium to pure aluminium allows small particles to precipitate during heat treatment. These precipitates resist plastic deformation and can increase the strength of the alloy and make it viable for a range of industrial applications, such as automotive door panels and load‐bearing profiles. However, if subjected to large loads, the precipitates are sheared and the strength of the alloy changes dynamically. Designing safe products such as cars or buildings require physically based predictions on this dynamical change. Developing models that can provide such predictions depend in turn on experimental observations of the shearing process. Because the precipitates are nm long, experimental observations must be done by transmission electron microscopy. However, understanding these results sometimes require computer simulations of atomic models. In this work, we have performed image simulations of various models of sheared precipitates and compared the results with earlier experiments. The simulations indicate that certain conditions must be met for the sheared precipitates to appear different from unsheared precipitates. These conditions are most likely to be met if precipitates are sheared several times in a relatively homogeneous manner. This is important for two reasons. First, a localized shearing process would lead to large dynamical changes in precipitate strength during deformation, and in turn drastically reduce the work hardening of the alloy. Secondly, a localized shearing process would have promoted earlier fracture and failure of the alloy during deformation. Finally, our results also show how different slip directions influences the images of precipitates. In the future, these influences can be used to further understand the shearing process of these precipitates. Hence, our results can be used to improve model predictions of strength, work hardening, and fracture. In turn, this may improve alloy design and reduce the use of prototype testing in, e.g. the automotive industry.
Precipitation behavior in an industrially extruded AA7003 alloy has been studied using Transmission Electron Microscopy (TEM) together with Differential Scanning Calorimetry (DSC). Air Cooling (AC) ...after solution heat treatment results in quench induced heterogeneous precipitation of both β-Mg2Si and η-MgZn2 phases. Detailed TEM characterisation of resulting nanoscale precipitates after AC, or Water Quenching (WQ), and subsequent artificial ageing demonstrate that η′ and η2 hardening precipitates dominate in T6, whereas the overaged T7 state contains η2 and η1, where the latter accounts for approximately 50% of the relative phase fraction. The T7 state in addition forms 6xxx-type hardening precipitates only after WQ. Results presented here are expected to be relevant for any Si containing 7xxx alloy and open new possibilities for development of hybrid 6xxx- and 7xxx series aluminium alloys. This is discussed with respect to potential influence on mechanical- and corrosion properties.
Display omitted
•Air cooling results in heterogenous nucleation of β-Mg2Si and η-MgZn2 on grain boundaries and dispersoids.•Artificial ageing temperature influences the Mg-Zn (η) hardening phases orientation relationship with the aluminium matrix.•6xxx type hardening phases are found to coexist with the 7xxx type hardening in the T7 temper if water quenched.
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