This paper compares the nano-scale structure of β" precipitates in a peak-aged Al-Mg-Si alloy before and after deformation. Three complementary advanced transmission electron microscopy techniques ...are used to reveal the structures and elucidate the interaction between dislocations and β" precipitates. We show that the needle-like and semi-coherent β" precipitates are sheared several times on different planes by dislocations during deformation, with no indications that they are bypassed or looped. Our results show that dislocations cut through precipitates and leave behind planar defects lying on planes inclined to 〈100〉 directions inside the precipitates. The results also indicate that precipitates are sheared in single steps, and the implication of this observation is discussed in terms of slip behaviour.
Low Cu and Ag additions (≤0.10at%) were found to strongly affect the age-hardening behavior in AlMgSi alloys with Mg+Si>1.5at%. The hardness increased during aging at 170°C and the formation of β″ ...precipitates was kinetically accelerated. The activation energy of the formation of the β″ phase was calculated to 127, 105, 108 and 99KJmol−1 in the base, Cu-added, Ag-added and CuAg-added alloys, respectively using the Kissinger method. The negative effect of two-step aging caused by the formation of Cluster (1) during natural aging was not overcome by the addition of microalloying elements. However, it was suppressed by the formation of Cluster (2) through a pre-aging at 100°C. Quantitative analysis of the precipitate microstructure was performed using a transmission electron microscope equipped with a parallel electron energy loss spectrometer for the determination of specimen thickness. The formation of Cluster (2) was found to increase the number density of β″ precipitates, whereas the formation of Cluster (1) decreased the number density and increased the needle length. The effects of low Cu and Ag additions in combination with multi-step aging are discussed based on microstructure observations and hardness and resistivity measurements.
Al-4.5Si-1Cu-0.3Mg(-1Fe) (wt%) alloys fabricated by a deformation-semisolid extrusion (D-SSE) process have been investigated by transmission electron microscopy, down to the atomic level. T5 and T6 ...heat treatments were conducted to understand the age-hardening behavior of the alloys. Disordered Mg-Si(-Cu) precipitates with strong Cu enrichments at their interfaces with the Al matrix have been observed in the overaged conditions of both heat treatments and in the peak hardness of the T6 condition, but only Cu-containing atomic clusters were detected in the peak hardness of the T5 heat treatment. Despite having a lower bulk precipitate number density at comparable precipitate size and volume fraction, hardness in the T6 condition was higher in the alloy with highest Fe content due to the extra contribution from the precipitates nucleated on fragmented β-Al5FeSi particles and grain boundaries. Many of these precipitates were Q'-phase, and two new coherent interfaces with the Al matrix are reported for this phase.
•Fragmented β-Al5FeSi particles and grain boundaries act as nucleation sites for the Q' phase.•Two new types of interfaces along the coherent Al direction of the Q' phases have been found.•Cu-containing atomic clusters have been found in the peak hardness of an artificially aged T5 condition.
In the aluminum industry, forming is an important process step that introduces dislocations in the material. To investigate the effect of dislocation retention after ageing on 6xxx-series alloys, a ...non-heat-treatable 5005 alloy was selected to measure the change in mechanical properties due to dislocation annihilation during dynamic recovery. However, the isothermal ageing treatment led to an unexpected and significant increase in mechanical properties after deformation. Increases in yield strength of 120% and tensile strength of 50% compared with the as-received material were achieved. However, this caused a significant decrease in elongation properties. The deformation start temperature did not have any impact on the final mechanical properties. TEM analysis attributed the increase in mechanical properties to an increased precipitation and dislocation density compared with the undeformed reference material. The precipitates are located along dislocation lines, showing that the solute elements are preferentially segregating to dislocations and precipitating. The precipitates were typical for the Al–Mg–Si(–Cu) system; therefore, the low amounts of Si and, to a lesser extent, Cu were responsible for the precipitation hardening in the 5005 alloy.
The mechanism behind shearing of β11 precipitates in Al-Mg-Si alloys during deformation is investigated by applying advanced transmission electron microscopy (TEM) techniques and frozen phonon ...multislice TEM image simulations on a selection of shearing configurations. In particular, the results indicate that the needle-like precipitates are sheared several times in single matrix Burgers vector steps. The multislice image simulations suggest that shearing events are most likely achieved in single Burgers vector steps, and there are some experimental evidence that the shearing planes are the matrix glide planes.
Aluminium alloys in the 6000 series may become susceptible to intergranular corrosion (IGC) by unfavourable thermomechanical processing in combination with presence of Cu and a low Mg/Si ratio. The ...IGC susceptibility has been related to the segregation of Cu during artificial ageing to form a nearly continuous Cu film along the grain boundaries with an anodic solute depleted zone adjacent to it. In this work, extruded alloys 6005 and C28 as well as two variants of 6016 automotive sheet were subjected to various degrees of deformation by rolling or stretching before artificial aging. Accelerated IGC testing showed that pre-deformation significantly improved the IGC resistance of the Cu containing extruded alloys, while no improvement was observed for the 6016 sheet materials containing < 0.01 wt% Cu. High resolution scanning transmission electron microscopy (STEM) of pre-deformed 6005 alloy showed precipitation of Cu-containing Q' phase on dislocations. As a result, the amount of Cu available for diffusion to the grain boundaries is reduced, which might explain why a beneficial effect of pre-deformation on the IGC resistance was observed only for the Cu-containing materials.
Aging heat treatment is the most commonly used strengthening method for Al–Mg–Si alloys since high-density precipitates will be formed to hinder the movement of dislocations. In the current work, ...room temperature cyclic deformation was attempted to strengthen the alloy. We compared tensile test results of aged samples and cyclically deformed samples. It was found that cyclically deformed samples can achieve similar strength and approximately twice the uniform elongation as the peak aged samples. The high density of dislocations and nanoclusters observed in the cyclically deformed samples is thought to be the main reason for strengthening. Different cyclic deformation conditions have been tried and their effects were discussed.
Precipitation behavior in an Al–Cu–Mg–Si alloy during ageing Gazizov, Marat; Marioara, Calin Daniel; Friis, Jesper ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
11/2019, Volume:
767
Journal Article
Peer reviewed
Open access
The precipitation behavior and aging response of an Al-4.9Cu-0.74Mg-0.51Si-0.48Mn-0.1Cr-0.08Ti-0.02Fe (wt. %) alloy was examined in detail after aging at 170 °C with different dwell times up to 96 h. ...Deformation by 3%-stretching prior to aging was used to investigate the effect of dislocations on phase and hardness evolution and compared with the undeformed material. The small pre-deformation led to a slight decrease of peak strength due to reduced homogenous nucleation. Strong interaction between different phases caused the formation of hybrid structures both in the undeformed bulk areas and on dislocations. These phases consist of different fragments of the GP zones, θ”- and θ’-phases from the Al–Cu system, GPB zones and S1-phase from the Al–Cu–Mg system, and β”-, β’-Cu, C- and Q’-phases from the Al–Mg–Si–Cu system. A new phase named C1, isostructural to the C-phase but having a different orientation with the Al matrix – (010)C//(010)Al, 001C//101Al, has been found predominantly on dislocation lines, and to a lesser extent in hybrid precipitates in the bulk. Calculations of structural stability by density functional theory (DFT) were performed on experimentally found structures, consisting of a C-phase core in two different orientations, and having Cu segregations in GP-like structures at their interfaces with the matrix.
A model for vacancy annihilation during aging has been combined with a precipitation model for coupled nucleation, growth, and coarsening in AA 6xxx series aluminum alloys. The simulation results ...were compared with precipitation parameters from TEM measurements and hardness data obtained for various times during artificial aging. Both simulations and measurements indicated that a combination of an excess concentration of non-equilibrium vacancies at the start of aging and a fast vacancy annihilation rate significantly affected the resulting precipitation and strength evolution. Hence, the model reproduced the short aging time required to reach the maximum strength when direct artificial aging was applied (DAA). In contrast to the fast aging response of DAA, the hardness measurements showed a much slower aging response when artificial aging was performed after prolonged natural aging. This aging behavior was captured in the model simulations by assuming that an equilibrium vacancy concentration is present from the start of the aging.
Al–Mg–Si alloys are usually formed into their final shape by rolling or extrusion. After extrusion, the aluminium profiles are usually straightened, causing the material to be subjected to a small ...plastic deformation. This study demonstrates the positive effect on strength that can be obtained from such small deformation levels or from only elastically straining the material. Elastic straining of a lean Al–Mg–Si alloy, when performed immediately after solution heat treatment, enhances the material yield strength after artificial ageing to T6. Transmission electron microscopy shows that this effect can be attributed to a higher number density and finer dispersion of the age-hardening precipitate needles. Furthermore, introducing a small plastic deformation of 1% after solution heat treatment results in a comparable strength increase to elastically straining the material. In this case, however, the strength increase is due to the increased dislocation density, which compensates for a lower density of precipitate needles. Finally, by combining plastic deformation with a succeeding elastic strain, we demonstrate how elastic strain can cause an on-set of dislocation cell formation in this material.
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