Elsevier

Materialia

Volume 8, December 2019, 100437
Materialia

Full Length Article
Precipitation sequence in Al–Mg–Si–Sc–Zr alloys during isochronal aging

https://doi.org/10.1016/j.mtla.2019.100437Get rights and content

Abstract

6xxx-series Al alloys are the most commonly used alloys in the automotive industry as they have an appropriate balance of strength, corrosion resistance and formability. These alloys experience significant strengthening from the precipitation of coherent GP-zones and β'' precipitates. One opportunity to achieve stronger 6xxx-series Al alloys, without affecting corrosion and formability, is the use of Sc. The strengthening benefits from Sc are obtained by the formation of the Al3Sc dispersoids. The addition of Zr, together with Sc, results in the formation of hybrid Al3(Sc,Zr) dispersoids with a core–shell morphology. The microstructural development in 6xxx-series alloys containing Sc is still largely unknown. Here we show, that combining conductivity, hardness and modelling allows for the prediction of the precipitation sequence in an Al–Sc–Zr, an Al–Mg–Si and an Al–Mg–Si–Sc–Zr alloy. The evolution of conductivity and hardness during isochronal aging is discussed in terms of solute depletion and precipitate formation. APT, TEM and SEM are also conducted in key conditions to help interpret the resistivity and hardness fluctuations. Modelling allows for estimation of the precipitation kinetics of the Mg-Si precipitates and Sc–Zr dispersoids during isochronal aging. The precipitation kinetics of Al3(Sc,Zr) is found to be accelerated in the presence of Mg and Si and APT reveals that this is due to the preferred nucleation of the Al3(Sc,Zr) dispersoids on the MgSi precipitates. These results pave the way for the design of suitable heat treatments for 6xxx-series alloys containing Sc that would allow for the formation of both Al3(Sc,Zr) dispersoids and fine MgSi precipitates.

Introduction

6xxx-series are increasingly used in the automotive industry for light weighting purposes. For optimal light-weighting potential, the key challenge is to develop alloys offering enhanced strength, formability and corrosion resistance. The strength of Al–Mg–Si alloys comes mainly from the β’’ precipitates [1]. Enhancing the strength of these alloys can come from optimising the precipitation of β’’ or by adding additional strengthening phases. Modern high strength 6xxx-series alloys contain high levels of Cu (up to 1 wt%), high levels of dispersoids forming elements (e.g. Mn) and tightly controlled heat treatment schedules, allowing them to achieve a tensile strength in the vicinity of 400 MPa through the formation of Q’-AlMgSiCu precipitates [2], [3]. Although strength is improved by these additions, processability and corrosion performance are significantly impacted [4]. Contrastingly, scandium additions offer the potential of a significant strength increase, with a comparatively minor impact on processability and corrosion resistance.

Of all the elements, scandium can provide significant strengthening to aluminium alloys with up to 100 MPa strength increment per 0.1 wt% Sc addition [5]. This strength increase comes from the formation of the L12 coherent, nanosized, Al3Sc precipitates [6], [7]. The coarsening resistance of Al3Sc can be improved by adding zirconium (Zr), as it forms a L12 Al3Zr shell around Al3Sc which reduces the mismatch with the Al matrix, and prevents Sc from moving across the interface [8], [9], [10], [11]. This core–shell morphology comes from Zr being slower to diffuse in Al as compared to Sc. Sc and Zr usually supersaturate during direct chill casting [12], [13] and the core–shell particles can then be formed during a multi-step heat treatment [14]. A high number density of Al3Sc can first be formed at a temperature around 250–350 °C and the temperature is then increased to form Al3Zr.

The work on Sc additions in 6xxx-series alloys is limited and while some works report a positive impact [15], [16], [17], others report a detrimental effect on mechanical properties [18], [19], [20], [21]. These different reports come from the strong interaction between Si and Sc which requires the design of specific treatments. Indeed, all of the studies which saw a positive impact from alloying with Sc, utilised specific alterations in the heat treatment and production process of the alloys which ensured effective precipitation and utilisation of Sc (and Zr) [17], [22], [23], [24]. Furthermore, the formation kinetics of Al3Sc has been shown to be enhanced by the presence of Si, even when at impurity levels [25], [26], [27], [28]. However, the impact of Sc on the precipitation sequence in alloys containing high levels of Si, such as the 6xxx-series, is largely unknown. Recent work by Babaniaris et al. [29], showed that both solutionised and as-cast Al–Mg–Si–Sc–Zr alloys were potential starting points for a heat treatment that effectively utilises the alloyed Sc. Recent work by Dorin et al. on Al–Mg–Si–Sc–Zr alloys [30], [31], revealed the presence of solidification micro-segregations in the as-cast condition. The presence of Si also resulted in the formation of (Al,Si)3(Sc,Zr) dispersoids in the as-cast microstructure. In the present work, we conduct an initial solution treatment to solutionise the precipitates formed during solidification and remove solidification micro-segregation. An isochronal heat treatment is then conducted on the studied alloys and hardness and resistivity evolutions are monitored and interpreted in terms of depletion of solute in solid solution and precipitate formation. Isochronal heat treatment is a good way to screen the precipitation sequence [32], [33], [34].

There is a direct relationship between microstructure and an alloy's bulk properties. In particular, nano-size precipitates are known to have a significant impact on most key properties such as mechanical properties, processability and corrosion resistance. A common way of indirectly characterising precipitation is by measuring its impact on hardness [35] and conductivity/resistivity [36] and using existing models to obtain information on precipitation kinetics, precipitate size and volume fraction. Whilst precipitation hardening is directly related to the precipitate size and volume fraction, conductivity variations are mainly associated with solid solution variations which allows to inversely compute the precipitate fraction if precipitate stoichiometry is known. The difficulty in using these techniques to obtain quantitative information on the precipitates is that these methods can become challenging to interpret and deconvolute when different reactions are occurring simultaneously (i.e., nucleation and precipitation of dispersoids at the same time as dissolution and coarsening of strengthening phases). In the present paper, we use a range of model Al–Sc–Zr and Al–Mg–Si–(Sc)–(Zr) alloys with the aim of deconvoluting the complex and multifaceted precipitation sequence that occurs during an elevated temperature homogenisation or ageing treatment. APT and TEM are also used in key conditions to help interpret the hardness and resistivity evolutions.

In this work, we interpret resistivity and hardness evolution to study the precipitation sequence in an Al–Sc–Zr, an Al–Mg–Si and an Al–Mg–Si–Sc–Zr alloy. These results are supported by targeted electron microscopy and atom probe tomography characterisations. We find that the formation of the Al3Sc/Al3Zr dispersoids is accelerated in the Al–Mg–Si–Sc–Zr compared to the Al–Sc–Zr alloy, and APT revealed that this is due to preferred nucleation of the dispersoids on the MgSi precipitates. Transition in precipitate coherency have a different impact on hardness and resistivity and we use it to characterize the transition between fully coherent β’’, semi-coherent β’ and incoherent β phases in the Al–Mg–Si and Al–Mg–Si–Sc–Zr alloys. Sc and Zr are found to have limited impact on the transition between β’’, β’ and β. From these results, we conclude on the necessity to design a specific heat treatment for Al–Mg–Si–Sc–Zr alloys.

Section snippets

Alloy composition and preparation

Three alloys were prepared for this investigation and the alloy compositions are reported in Table 1 in wt%. Both the measured and nominal compositions are given. The alloys were cast using master alloys to ensure maximum composition homogeneity. The master alloys compositions were Al-2wt%Sc, Al-5 wt%Zr, Al-25 wt%Mg and Al-20 wt%Si. The alloys were melted in a small induction furnace containing approximately 4 kg of aluminium. The melt was kept at 720 °C for 30 min before being poured in a

Efficiency of solution treatment

Adding Mg and Si to an Al–Sc–Zr alloy has been reported to result in the formation of fine (Al,Si)3Sc dispersoids in the as-solidified alloy [30], [31]. In the present work, we used an initial high temperature solution treatment with the aim of dissolving these as-cast features. Furthermore, this solution treatment also helps homogenising the solidification micro-segregations. Transmission electron microscopy was used to evaluate the efficiency of the solution treatment to dissolve the

Precipitation sequence

The variations of resistivity and hardness are directly related to the precipitation process. The resistivity generally decreases during the precipitation process as the solute is driven out of solution. The only exception to this rule is the increase or plateau in resistivity which can occur when fully coherent precipitates form, as they impose additional lattice strain on the matrix which impede movement of electrons in manner similar to solute [42]. Keeping this in mind, plotting the

Conclusions

We presented an interpretation of resistivity and hardness measurements to study the precipitation sequence in an Al–Sc–Zr, an Al–Mg–Si and an Al–Mg–Si–Sc–Zr alloy during isochronal aging. Owing to specific impact of each of these phases on resistivity and hardness evolution, the following conclusions were obtained on the precipitation kinetics in these alloys:

  • The maximum strengthening from Al3Sc/Al3Zr dispersoids was ∼25 MPa in both the Al–Sc–Zr and Al–Mg–Si–Sc–Zr alloys. The formation

Declaration of competing interest

None.

Acknowledgments

Dr Thomas Dorin is the recipient of an Australian Research Council Australian Discovery Early Career Award (project number DE190100614) funded by the Australian Government. The authors would like to acknowledge Clean TeQ for providing in-kind Al–Sc master alloys. Dave Gray is warmly thanked for casting the alloys used in this project. Deakin University's Advanced Characterisation Facility is acknowledged for use of the SEM JSM 7800F, TEM-FEG JEOL 2100F and LEAP 5000X instruments.

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