Atomic-scale pathway of early-stage precipitation in Al–Mg–Si alloys (original) (raw)
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The influence of composition and natural aging on clustering during preaging in Al–Mg–Si alloys
Journal of Applied Physics, 2010
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 natural aging produced clusters with Mg:Si ratios more similar to those of the alloys.
Acta Materialia, 2007
The strength of Al-Mg-Si aluminium alloys depends critically on nanometre-size Mg x Si y Al z -type precipitates that have a face-centered cubic-based structure. In this work, a large number of early structures are investigated by means of first-principles calculations. Both platelet-type and needle-type precipitates are considered. Calculations show that for alloys with an Mg:Si ratio smaller than one, needle-type precipitates with Si pillars extending in the needle direction are energetically favoured. The formation of Si pillars and the low density cylinder is described. For alloys with an Mg:Si ratio larger than one, platelet-type precipitates consisting of stacked layers of Mg, Si and Al atoms are energetically favoured. Using both the information on the formation enthalpies and the calculated lattice mismatch with the Al matrix, it is discussed which structures are likely to be formed. The earliest, most favourable structures with high Al content are the needle-type initial-b 00 Mg 2 Si 3 Al 6 structure and the platelet-type structures {MgSi}
Acta Materialia, 2006
AlMgSi alloys are commonly used in the automotive industry. In the bake-hardening process of AA6xxx alloys, nanometer-size Mg x Si y Al z -type precipitates play a crucial role. A large number of structures and compositions occur between the initial supersaturated solid solution and the stable Mg 2 Si phase. The transition from pre-b 00 to b 00 phase marks the transition from face-centered cubic (fcc)-type structures to non-fcc-type structures. A new pre-b 00 phase is identified with the composition Mg 4 Si 7 that is energetically very favorable. Although the late precipitate phases (b 00 , b 0 , U1, U2 and U3) are very different in composition and crystallography, their mutual correspondence is made clear by an analysis of the orientation relationships and substructures. It is shown that they share the same substructures consisting of Mg hexagons which enclose a parallelogram of four atoms. Phase transitions are characterized by columnar and planar shifts of these substructures, and not by a total rearrangement of all the atoms.
The Effect of Cu and Cr on Clustering and Precipitation in Al-Mg-Si Alloys
13th International Conference on Aluminum Alloys, 2012
A group of alloys based on pure ternary Al-0.4 wt.%Mg-1.0 wt.%Si are used to study the effect of Cu and Cr on clustering and precipitation in Al-Mg-Si alloys. Differential Scanning Calorimetry (DSC) is performed for samples naturally aged for different times after solution heat treatment and ice water quenching. Three clustering processes are observed in all the alloys. The fundamental clustering sequence does not change by additional elements. However, the Cu containing alloy shows less clustering and the first clustering event is hindered. Kissinger analysis reveals that the 2 nd and 3 rd processes have very similar effective activation energies. Therefore, a model incorporating only two independent reactions is used to obtain kinetic parameters. It is found that the first clustering process starts with a low effective activation energy of 50 kJ/mol and has a mechanism similar to mixed nucleation while the latter two processes are governed by a higher activation energy of 79 kJ/mol and have a mechanism similar to particle growth. During precipitation, the Cr containing alloy shows a similar precipitation heat signal in DSC as the pure ternary, thus having negligible effect on precipitation. On the other hand, the formation of " is less dominant in the Cu-containing alloy while precipitation of other phases before reaching the peak-aged condition is possible.
Phase-field crystal modeling of early stage clustering and precipitation in metal alloys
Physical Review B, 2012
A phase-field crystal model is used to investigate the mechanisms of formation and growth of early clusters in quenched/aged dilute binary alloys, a phenomenon typically outside the scope of molecular dynamics time scales. We show that formation of early subcritical clusters is triggered by the stress relaxation effect of quenched-in defects, such as dislocations, on the energy barrier and the critical size for nucleation. In particular, through analysis of system energetics, we demonstrate that the growth of subcritical clusters into overcritical sizes occurs due to the fact that highly strained areas in the lattice locally reduce or even eliminate the free energy barrier for a first-order transition.
Acta Materialia, 2011
Atom-probe tomography, transmission electron microscopy, X-ray diffraction and first-principles calculations are employed to study: (i) compositional evolution of GPII zones and h 0 precipitates; and (ii) solute segregation at a-Al/h 0 interfaces in Al-1.7 at.% Cu (Al-4 wt.% Cu) alloys. GPII zones are observed after aging at 438 K for 8 h, whereas higher aging temperatures, 463 K for 8 h and 533 K for 4 h, reveal only h 0 precipitates. Most GPII zones and h 0 precipitates are demonstrated to be Cu-deficient at the lower two aging temperatures; only the 533 K treatment resulted in h 0 stoichiometries consistent with the expected Al 2 Cu equilibrium composition. For alloys containing $200 at. ppm Si we find evidence of Si partitioning to GPII zones and h 0 precipitates. Significant Si segregation is observed at the coherent a-Al/h 0 interface for aging at 533 K, resulting in an interfacial Si concentration more than 11 times greater than in the a-Al matrix. Importantly, the Si interfacial concentration undergoes a transition from a non-equilibrium delocalized profile to an equilibrium localized profile as the aging temperature is increased from 463 to 533 K. Consistent with these measurements, first-principles calculations predict a strong thermodynamic driving force favoring Si partitioning to Cu sites in h 0 . Silicon segregation at, and partitioning to, h 0 precipitates results in a decrease in interfacial free energy, and concomitantly an increase in the nucleation current. Our results suggest that Si catalyzes the early stages of precipitation in these alloys, consistent with the higher precipitate number densities observed in commercial Al-Cu-Si alloys.
Revealing the atomistic nature of dislocation-precipitate interactions in Al-Cu alloys
Journal of Alloys and Compounds, 2019
Despite significant gains on understanding strengthening mechanisms in precipitate strengthened materials, such as aluminum alloys, there persists a sizeable gap in the atomistic understanding of how different precipitate types and their morphology along with dislocation character affects the hardening mechanisms. Toward this, the paper examines nature of precipitation strengthening behavior observed in the Al-Cu alloys using atomistic simulations. Specifically, the critical resolved shear stress is quantified across a wide range of dislocationprecipitate interactions scenarios for both θ' and θ phase of Al2Cu. Overall, the simulations reveal that the dislocation character (edge or screw) plays a key role in determining the predominant hardening mechanism (shearing vs. Orowan looping) employed to overcome the θ' Al2Cu precipitate. Furthermore, the critical shear stress and mechanism to overcome the precipitate is sensitivity to the position of the glide plane with respect to the precipitate and its orientation. Interestingly in our findings, the θ Al2Cu precipitate conventionally regarded as unshearable particle was overcome by shear cutting mechanism for small equivalent precipitate radius, which agrees with recent TEM observations. These findings provide necessary information for the development of atomistically informed precipitate hardening models for the traditional continuum scale modeling efforts.
MSF, 2002
TEM study of deformed samples complemented by a new approach combining image analysis and simulation of the dislocation motion have been carried out to study the precipitate /dislocation interaction in Al-Mg-Si alloys (AA-6XXX). The analysis of HRTEM images allows a direct measurement of the strain field around precipitates and is further introduced in the simulation of dislocation propagation. In the case studied here, the simulation indicates that for an applied stress close to the yield stress, dislocations' motion in the matrix occurs by both the bypass of precipitates through the activation of the Orowan mechanism and the shearing of precipitates. This is in agreement with TEM observations on deformed samples showing numerous dislocations loops along with laths shearing
Multi-component solid solution and cluster hardening of Al–Mn–Si alloys
Materials Science and Engineering: A, 2015
Tensile tests on Al-Mn-Si ternary alloys show that a small amount of Si increases significantly the strength compared to Al-Mn binary alloys with the same concentration of Mn. This cannot be explained by classical theories for multi-element substitutional solid solution hardening under the assumption of no interaction between different alloying elements. A new simplified cluster strengthening model which addresses both the chemical and size misfit effects of atom dimers is proposed this work. The binding energies and misfit of dimers were estimated by first principles atomistic simulations. The prediction results of the model are reasonably consistent with the experimental results. It shows that the main strengthening contribution is due to the misfit of dimers.