The relation between grain boundary precipitate formation and adjacent grain orientations in Al-Mg-Si(-Cu) alloys

The occurrence of grain boundary precipitates was investigated with respect to the crystallographic orientation of the adjacent grains in extruded AA6110, AA6063 and AA6061 alloys brought to T6 temper. It was found that the requirement for grain boundary precipitate formation is for the adjacent grains to have Al directions or {100}Al planes parallel to the grain boundary plane. The highest density of grain boundary precipitates was present when this requirement was fulfilled by both adjacent grains.publishedVersio

Hot Deformation and Die-Quenching of 6000-Series Alloys—The Effect of Quench-Interruption Temperature

The automotive industry face demands to produce lightweight vehicles and substituting steel with aluminium is a straightforward solution. Age hardenable aluminium alloys are attractive candidates when the aim is a combination of high strength and good ductility. To obtain a solution that is suitable for high-volume production, an integrated hot forming and in-die quenching process has been developed. The method involves less operations and less handling than the conventional production process. However, the developed method changes the temperature exposure of the part and hence alters the precipitation and clustering sequence during hardening. In the present work, the effect of the modified temperature sequence has been investigated and, in particular, the effect of direct artificial ageing (DAA) has been studied. The potential benefits of DAA are less time needed for production and less space needed for storage. A lab-scale set-up, with the ability to simulate the industrial scale integrated forming and quenching method, has been built. By application of a water-cooled compression tool combined with subsequent age-hardening we have explored the effect of the in-die quenching on three different age hardenable alloys. The temperature experienced by the blank has been measured throughout the process, and the effect of variations in the quench-interruption temperature has been investigated. For AA6082, the results indicate that for selected temperatures, the effect of changing the closed-die time is minimal, and that the effect of room temperature storage (RTS) is negligible.acceptedVersio

The Role of Grain Boundary Precipitates during Intergranular Fracture in 6xxx Series Aluminium Alloys

Ringdalen, I.G.; Jensen, I.J.T.; Marioara, C.D.; Friis, J. The Role of Grain Boundary Precipitates during Intergranular Fracture in 6xxx Series Aluminium Alloys. Metals 2021, 11, 894. https://doi.org/ 10.3390/met11060894During ageing, 6xxx aluminium alloys will develop a microstructure characterised by needle-shaped Mg/Si-rich precipitates in the bulk, precipitate-free zones along the grain boundaries and larger Mg/Si-rich precipitates on the grain boundary. Depending on, among other things, the size of the precipitate-free zone, these alloys are prone to intergranular fracture. The role of the grain boundary precipitates during the initiation and propagation of the intergranular fracture is still not fully understood. Transmission Electron Microscopy has been used to characterise the grain boundaries and grain boundary precipitates. The precipitates were found to be of the β′ type surrounded by a layer of U2 structure. The atomic details of relevant interfaces of Al-β′ were characterised for further investigation. Density Functional Theory simulations were performed on the bulk precipitate structures and on the interfaces obtained experimentally. The decohesion energy of these interfaces was calculated and compared to bulk values. In addition, simulated tensile tests were performed in order to find values for the tensile strength σt. The dependence of the interfacial energy and tensile strength of β′ grain boundary precipitates were found to depend on the orientation and type of interface in addition to the amount of defects on the interfacepublishedVersio

AutomAl 6000: Semi-automatic structural labelling of HAADF-STEM images of precipitates in Al–Mg–Si(–Cu) alloys

When the Al–Mg–Si(–Cu) alloy system is subjected to age hardening, different types of precipitates nucleate depending on the composition and thermomechanical treatment. The main hardening precipitates extend as needles, laths or rods along the directions in the aluminium matrix. It has been found that the structures of all metastable precipitates may be generalized as stacks of columns, where most of these columns are replaced by solute elements. In the precipitates, a column relates to neighbour columns by a set of simple structural principles, which allows identification of species and relative longitudinal displacement over the (100) cross-section. Aberration-corrected high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) is an important tool for studying such precipitates. With the goal of analysing atomic resolution HAADF-STEM images of precipitate cross-sections in the Al–Mg–Si(–Cu) system, we have developed the stand-alone software AutomAl 6000, which features a column characterization algorithm based on the symbiosis of a statistical model and the structural principles formulated in a digraph-like framework. The software can semi-autonomously determine the 3D column positions in the image, as well as column species. In turn, AutomAl 6000 can then display, analyse and/or export the structure data. This paper describes the methodology of AutomAl 6000 and applies it on three different HAADF-STEM images, which demonstrate the methodology. The software, as well as other resources, are available at http://automal.org. The source code is also directly available from https://github.com/Haawk666/AutomAl-6000.publishedVersio

Precipitate/matrix incompatibilities related to the {111}Al Ω plates in an Al-Cu-Mg-Ag alloy

The atomic structure of Ω plates forming on {111}Al planes in an Al-Cu-Mg-Ag alloy has been investigated by Z-contrast atomic-resolution scanning transmission electron microscopy imaging and ab initio density functional theory calculations. Ω plates with different thicknesses have been studied in two peak-aged conditions: 150 °C for 24 h and 190 °C for 1.5 h. Volumetric and structural incompatibilities as unrelaxed misfit strains and shear components, respectively, between the Ω plates involving orthorhombic θ-phase fragments and Al matrix were found to be in the plates with thicknesses from 0 to 2.5 cθ (a normal direction to {111}Al). Two types of shear components: [−101]Al // [0−10]θ (τI) and [1−21]Al // [100]θ (τII) related to precipitate/matrix structural incompatibilities have been predicted by calculations. The shear components τI and τII have been found to be energetically favorable in the plates with different thicknesses. Comparing τI and τII absolute values in supercells involving the plates with different thicknesses, 2 cθ thick plates have a shear component close to zero. All the plates analyzed have precipitate/matrix volumetric incompatibilities with Al matrix as misfit strains along [111]Al // [001]θ, which distribute non-uniformly across the plate thickness. Large misfit strains concentrate at the broad plate interfaces, i.e. in Ag2Cu and Cui layers, and cause a prohibitively high barrier to thickening of the Ω precipitates.acceptedVersio

Local mechanical properties and precipitation inhomogeneity in large-grained Al–Mg–Si alloy

Al–Mg–Si (6xxx series) alloys show excellent mechanical properties due to the precipitates formed during heat treatment. However, heat treatment of these alloys results in a soft precipitation free zone (PFZ) close to grain boundaries that weakens them and promotes fracture, and thereby reduces the ductility of the material. This study provides quantitative insights into the mechanical properties and underlying plasticity behavior of Al–Mg–Si (6xxx series) alloys through combined nanoindentation hardness measurements and in-depth characterization of the microstructure adjacent to the PFZ region and in the grain interior. Experimental nanoindentation, transmission microscopy (TEM) and electron channeling contrast imaging results confirm the weakening effect from PFZ by means of a reduced hardness close to grain boundaries. The nanoindentation hardness mapping also revealed an increase in hardness a few micrometers from the grain boundary with respect to the grain interior. Precipitate quantification from TEM images confirms that the hardness increase is caused by a locally higher density of precipitates. To the authors’ best knowledge, this harder zone has not been recognized nor discussed in previously reported findings. The phenomenon has important implications for the mechanical properties of large-grained ( µm) aluminium alloys.publishedVersio

Effect of pre-deformation on age-hardening behaviors in an Al-Mg-Cu alloy

The effects of 3%–50% pre-deformation following solution heat treatment on the age hardening of an Al-3Mg-1Cu alloy have been investigated by Vickers microhardness measurement, tensile tests, differential scanning calorimetry, scanning electron microscopy, and transmission electron microscopy. Pre-deformation has a strong effect on subsequent age-hardening behavior. The precipitation was accelerated, hardness peaks appeared earlier, formation of clusters was inhibited, and a larger fraction of precipitates was observed along the dislocation lines. The contribution of the precipitates to the hardness was evaluated by dissolution tests. It was found that pre-deformation followed by artificial aging resulted in a good strength-elongation balance. The results are significant for the development of combined mechanical deformation and heat treatment processes.publishedVersio

Enhanced nucleation and precipitation hardening in Al-Mg-Si(-Cu) alloys with minor Cd additions

This work reports a novel effect of impurity element Cd on enhancing the precipitation kinetics and increasing the peak hardness of Al–Mg–Si(–Cu) alloys during artificial ageing. It is found that the number density of age hardening Mg–Si(–Cu) precipitates is greatly increased by Cd addition (~0.06 at.%) at both the under-aged and peak-aged stages. A systematic study on the precipitation behaviour by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) shows that most Mg–Si(–Cu) precipitates in the Cd-containing Al–Mg–Si alloys are associated with Cd-rich precipitates and have highly disordered structures. It is also found that the formation of Q'/C-like sub-units in Mg–Si(–Cu) precipitates is significantly promoted by Cd additions. To explore the nucleation mechanism under the influence of Cd addition, atom probe tomography (APT) is applied to study the solute clustering behaviour in the early stages of artificial ageing, and density functional theory (DFT) calculations are used to evaluate the binding energies of different solute-vacancy complexes and therefore the formation kinetics of Mg–Si–Cd clusters.acceptedVersio

The effect of low Cu additions on precipitate crystal structures in overaged Al-Mg-Si(-Cu) alloys

This study concerns the effect of low Cu additions (<0.1 at.%) on the precipitate crystal structure evolution in three Al-Mg-Si(-Cu) alloys during overageing. The evolution was assessed through a combination of atomic resolution scanning transmission electron microscopy, scanning electron diffraction, and differential scanning calorimetry. It was found that relatively small changes in the Cu level and the Si:Mg ratio had significant effects on the resulting distribution of precipitate phases, their structural evolution, and their thermal stability. Two Si-rich alloys formed hybrid β′ phase and Q′ phase precipitates on overageing. A third Mg-rich alloy primarily formed L phase precipitates, which exhibited superior thermal stability. Three distinct Cu-containing sub-units that form the basis for all Al-Mg-Si-Cu precipitate phases were identified: the three-fold symmetric Q′/C and the β_Cu′ sub-units, in addition to a newly discovered C sub-unit. The formation of each sub-unit was discussed, and the atomic structures and connections to other precipitate phases in the Al-Mg-Si(-Cu) system were elaborated. The work presented provides new insights into the complex precipitation of Cu-added Al-Mg-Si alloys, with implications for material properties. The results obtained will be of importance in future alloy and process development, and are thought to be of high value in modelling work on the quaternary Al-Mg-Si-Cu system.publishedVersio

A transmission electron microscopy study of precipitate phases that form during operation in a heat exchanger alloy

During manufacturing of heat exchangers, the core material is cladded with a lower-melting point alloy, rolled into thin strips before being formed and finally brazed at an elevated temperature. After a period of natural aging, the final product is operated at two different temperatures depending on the application: about 95 °C for radiators, and peaks up to about 250 °C for charge-air-coolers. For an Al-Mg-Si-Cu alloy type core material, this process translates into solution heat treatment, natural aging and aging during operation. High-resolution imaging with aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) revealed the presence of a complex mix of precipitates after 58 days at 95 °C, including a never-before-reported phase which structurally is a mix between Al-Cu and Al-Mg-Si type precipitates. The stability of this phase is investigated with density functional theory (DFT). Q' is the main phase observed after 5 h at 250 °C, with most precipitates incorporating a certain type of stacking fault.publishedVersio