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Mg2Si precipitation sequence in alloy AA6061
Even though AlMgSi(Cu) alloys have long since become standard materials for wide-ranging applications in the field of lightweight construction, the microstructural processes that occur during their production continue to raise fascinating questions. For many years, the focus has been on cluster formation and its impact on artificial aging and the formation of hardening precipitates [1,2]. There are also new and extensive studies concentrating on the early stages of over-aging [3,4]. In contrast, however, the mechanism by which stable phases develop has been the subject of considerably less attention to date, and information on the topic remains inconclusive [5-8]. In some ways, this is hardly surprising, given that stable phases in AlMgSi(Cu) alloys are typically undesirable in the finished product, as they do not make an appreciable contribution to material strength.
Nevertheless, these phases play a significant role in process steps up to and including solution heat treatment. For one thing, it is relevant to the duration of solution heat treatment, which increases with the size of the precipitates to be dissolved at a constant phase fraction. For another, coarse precipitates with diameters of around 1 μm or more have a significant influence on recrystallization kinetics [9].
Scientific background
In the future, an improved understanding of the formation of stable phases in EN-AW 6061 - predominantly β-Mg2Si - should make it possible to recreate the cooling strategy following hot rolling or intermediate annealing more accurately in simulations and achieve optimizations.
However, the microstructural processes that occur in the transition from metastable to stable precipitation states are difficult to characterize and ideally require a combination of several experimental methods. Differential scanning calorimetry (DSC) is often used as the primary basis for the examination of precipitation processes. This involves measuring the net converted heat in a sample during heating, cooling or isothermal holding.While it does not usually allow us to make definitive statements about the development of individual phases, DSC often provides an initial point of reference for potentially interesting temperature intervals as well as relevant heating and cooling rates for targeted preparation and characterization of material states. Existing DSC measurements [10] for EN-AW 6061 alloy show that the formation of β-Mg2Si is broadly suppressed at a cooling rate of around 10 K/min. At the same time, a significant quantity of metastable precipitates also forms. Their transformation into (or replacement by) β-Mg2Si is of particular interest for the problem at hand.Examinations of heat-treated solid material (sample volume: 2 cm³) under a transmission electron microscope (TEM) confirm this impression, so solution heat treatment samples subsequently cooled at a rate of 10 K/min to room temperature (RT) were selected as the starting state for all in-situ heating tests.The phases in the starting state are primarily a combination of β’-Mg1.8Si and Q-Al3Cu2Mg9Si7 or Q’-Al6Cu2Mg6Si7 (dotted rectangles in Figure 1), as well as long plates of C-AlCu0.7Mg4Si3.3 (solid-line rectangles in Figure 1). All precipitates are located predominantly at dispersoids (dotted circles in Figure 1), which serve as heterogeneous nucleation sites. Details of the characterization of individual phases are provided in the publication upon which this article is based. [11]When this starting state (as a solid material) is heated at 1.8 K/min to 430°C, the stable phase β-Mg2Si appears almost exclusively, in different morphologies - again primarily at dispersoids.
Suitable experimental methods of capturing the transition from metastable to stable phase, which in solid materials appears to be completed at 430°C, include in-situ heating in TEM. Even though this method cannot perfectly depict the reaction kinetics in the solid material due to the small sample size, it is capable of providing a starting point for identifying potential reaction mechanisms. Figure 2 shows the development of selected metastable, copper-containing precipitates during heating as a result of bright-field TEM imaging. Initially, at 25°C the metastable precipitates (elongated, dark gray) are primarily located at dispersoids (round, dark gray to black).During the heating experiment, a new precipitate forms in all depicted cases, in some cases directly alongside the elongated precipitates (Figure 2b, c, d, e), sometimes at dispersoids (Figure 2a, f). The changes in contrast (e.g. Figure 2a: 345 to 365°C) during the heating process are very likely due to varying diffraction contrasts during heating. Given that the thin sample bends slightly under the influence of thermal stress, the orientation to the primary beam also changes, which in some cases causes the same precipitates to appear lighter or darker. Ultimately, the elongated, metastable precipitates have been replaced in all cases by new phases (within dotted outline in Figure 2, final row, at 405°C). Energy-dispersive X-ray spectroscopy (EDS) shows that the newly formed precipitates at the end of the experiment (bottom row, Figure 2) - in contrast to their precursors (top row, Figure 2) - no longer contain copper (see Figure 3, which depicts columns e and f from Figure 2). Based on the chemical composition and the measured diffraction patterns at the end of the in situ heating (see [11]), it ultimately appears that the observed reaction is an indirect transformation of the predominantly copper-containing phases from the starting status into β-Mg2Si.
These observations and available literature enable us to identify a number of possible mechanisms by which the stable β-Mg2Si can occur.During cooling from solution heat treatment and rapid heating of the artificially aged state to the solvus temperature of precursor phases, iron-containing primary phases and dispersoids (inert sites) appear to serve as primary nucleation sites. This corresponds to classical heterogeneous nucleation (CHN), with the growth of β-Mg2Si (product) primarily controlled via volume diffusion through the Al matrix.In cases where relatively large metastable precipitates appear, the results show that β-Mg2Si forms at the interface between precursors (parents) and the Al matrix, along the contact line between the dispersoid, precursors and Al matrix. This case clearly differs from CHN in terms of growth: in this indirect precipitate-related transformation (PRT), tafter heterogeneous nucleation of β-Mg2Si the precursor phase is consumed by it, with element transport likely to occur predominantly along the boundaries. However, as the precursor phase is not “directly” replaced, it is important to distinguish this case from direct, nucleation-free precipitate transformation (direct PT), as defined in the literature [12]. Figure 4 shows a schematic of the three cases.
Benefits for customers
The detailed examination of precipitation processes in 6xxx alloys is basic research par excellence - but it is not all that far removed from industrial practice. The fascinating insights from the in-situ TEM experiments at TU Vienna are used directly to optimize the production of Al-Mg-Si plates on the AMAG premises. This relates, in concrete terms, to the process steps of hot rolling and solution heat treatment. Mg2Si precipitates form both during cooling following hot rolling of plates and during heating in the solution heat treatment furnace. The latest research results on the formation of Mg2Si precipitates make it possible the adjust the solution heat treatment parameters, such as the furnace temperature, based on prior hot rolling. This makes it possible to perform targeted product and process design for wide-ranging applications and customer requirements.
Licensing
The publication association with this article - [11] Robert Kahlenberg, Tomasz Wojcik, Georg Falkinger, Anna Krejci, Benjamin Milkereit, Ernst Kozeschnik, ‘On the precipitation mechanisms of β-Mg2Si during continuous heating of AA6061’, Acta Materialia (2023), doi: https://doi.org/10.1016/j.actamat.2023.119345 - was published under Creative Commons Public License (CC BY 4.0). Please note the disclaimer of liability and limitation of liability issued by the licensor. (See https://creativecommons.org/licenses/by/4.0/legalcode for further details.)
Sources
[1] Dumitraschkewitz P, Gerstl S S A, Stephenson L T, Uggowitzer P J and Pogatscher S 2018 Clustering in Age-Hardenable Aluminum Alloys Adv. Eng. Mater. 20 1800255[2] Yang Z and Banhart J 2021 Natural and artificial ageing in aluminium alloys - the role of excess vacancies Acta Mater. 215 117014[3] Ding L, Weng Y, Jia Z, Zang R, Xiang K, Liu Q and Nagaum H 2022 Interactive transformation mechanisms of multiple metastable precipitates in a Si-rich Al-Mg-Si alloy Philos. Mag. 102 1602-27[4] Sunde J K, Wenner S and Holmestad R 2020 In situ heating TEM observations of evolving nanoscale Al-Mg-Si-Cu precipitates J. Microsc. 279 143-7[5] Westengen H and Ryum N 1979 Precipitation Reactions in an Aluminium 1 wt.% Mg,Si Alloy Int. J. Mater. Res. 70 528-35[6] Eskin D G, Massardier V and Merle P 1999 A study of high-temperature precipitation in Al-Mg-Si alloys with an excess of silicon J. Mater. Sci. 35 811-20[7] Massardier V, Epicier T and Merle P 2000 Correlation between the microstructural evolution of a 6061 aluminium alloy and the evolution of its thermoelectric power Acta Mater. 48 2911-24[8] Thomas G 1961 The Aging Characteristics of Aluminum Alloys Electron Transmission Studies of Al-Mg-Si Alloys J. Inst. Met. 90 1-32[9] Humphreys F J and Hatherly M 2004 Recrystallization and Related Annealing Phenomena (Elsevier)[10] Falkinger G, Reisecker C and Mitsche S 2022 Analysis of the evolution of Mg 2 Si precipitates during continuous cooling and subsequent re-heating of a 6061 aluminum alloy with differential scanning calorimetry and a simple model Int. J. Mater. Res. 113 316-26[11] Kahlenberg R, Wojcik T, Falkinger G, Krejci A, Milkereit B and Kozeschnik E 2023 On the precipitation mechanisms of β-Mg2Si during continuous heating of AA6061 Acta Mater. 119345[12] Danielsen H K and Hald J 2009 On the nucleation and dissolution process of Z-phase Cr(V,Nb)N in martensitic 12%Cr steels Mater. Sci. Eng. A 505 169-77
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