Elsevier

Journal of Alloys and Compounds

Volume 701, 15 April 2017, Pages 660-668
Journal of Alloys and Compounds

Temporal evolution of precipitates in multicomponent Al–6Mg–9Si–10Cu–10Zn–3Ni alloy studied by complementary experimental methods

https://doi.org/10.1016/j.jallcom.2017.01.183Get rights and content

Highlights

  • Structural and chemical evolution of clusters and precipitates are clarified.

  • During aging, two resistivity peaks are followed by their related hardness peaks.

  • Zn and Cu clusters are detected via the first and second resistivity peaks.

  • Zn cluster and hcp Zn contain Cu atoms, but θ'' phase does not have any Zn atoms.

  • Maximum alloy hardness is achieved when both the θ'' phase and hcp Zn are formed.

Abstract

The precipitation behavior of the multicomponent Al–6Mg–9Si–10Cu–10Zn–3Ni (wt%) alloy was investigated during artificial aging at 120 °C through complementary experimental methods including hardness and electrical resistivity measurements, compression testing, differential scanning calorimetry, and transmission electron microscopy. The solution treatment causes the dissolution of the preexisting θ-Al2Cu and hcp Zn phases, thereby increasing the concentrations of Zn and Cu species in the Al matrix. During aging, two resistivity peaks (IR, IIR) become visible followed by the appearance of the related hardness peaks (IH, IIH). Spherical Guinier-Preston (GP) zones are formed first and then subsequently transformed into ellipsoidal GP zones. The former process results in the appearance of the IR peak, while the latter process produces the IH peak. During transformation of the ellipsoidal GP zones to hcp Zn, GPI zones are additionally formed, leading to the appearance of IIR. The following transformation of the GPI zones into the θ''-Al3Cu phase leads to the appearance of the IIH peak. Both the ellipsoidal GP zones and hcp Zn phases contain large quantities of Cu atoms, while the θ''-Al3Cu phase does not have any Zn atoms. Based on these insights into the structural and chemical compositional evolution of complex precipitates, the strengthening mechanism of the multicomponent alloy during aging has been established.

Introduction

The strength of Al alloys strongly depends on the size, distribution, and composition of nanosized precipitates, which are partially coherent with the Al matrix [1]. The optimal mechanical properties of Al alloys are usually achieved via aging-induced precipitation [2], [3], and the resulting age-hardened Al alloys are generally classified as Al–Cu (2xxx), Al–Mg–Si (6xxx), and Al–Zn–Mg–(Cu) (7xxx) alloys [4]. In Al–Cu alloys, the main precipitates correspond to the transition phases of θ-Al2Cu, which considerably increases their strength [5]. The strength of Al–Cu alloys can be further improved by the addition of various elements (such as Mg, Ag, and Li) due to their ability to efficiently form precipitates characterized by different phases and orientations [6], [7], [8]. The strength of the Al–Mg–Si system can be increased by the formation of a metastable precipitate of β-Mg2Si [9], while the Al–Zn–Mg system is strengthened by precipitating precursors of the equilibrium η-MgZn2 phase [10].

The concurrent formation of different types of precipitates in mixed Al alloys (such as 2xxx + 6xxx and 2xxx + 7xxx) can be used to further improve their mechanical properties. Since the presence of one precipitate type affects the behavior of other precipitates, several researchers have studied the interactions between different precipitate species in mixed Al alloys. Haeffner and Cohen [11] examined the formation of Guinier-Preston (GP) zones in Al–1.5Cu–2.2Zn (at%) alloy by conducting wide-angle X-ray diffuse scattering measurements. They found that Cu atoms formed monolayered {100}Al planes, while Zn atoms were arranged in 〈110〉Al strings adjacent to the Cu zones, which relaxed the strain between Cu and Al atoms. Wenner et al. [12] recently investigated the precipitation behavior of the mixed Al–Cu–Mg (2xxx)/Al–Zn–Mg (7xxx) alloy system via transmission electron microscopy (TEM) and density functional theory calculations and determined that it was possible to substitute Al with Zn atoms in the S-Al2CuMg phase as well as Zn with Cu atoms in the η-MgZn2 phase.

More recently, several members of our research group [13] investigated the microstructure and mechanical properties of the multicomponent Al–8Mg–9Si–10Cu–11Zn (wt%) alloy, which was designed via mixing various precipitate-forming elements. In contrast to commercial Al alloys, it exhibited excellent strength at temperatures lower than 200 °C, owing to the presence of microsized secondary phases (such as Mg2Si and Q-Al5Cu2Mg8Si6), nanosized precipitates (such as θ′-Al2Cu and Q-Al5Cu2Mg8Si6), and Zn clusters [13]. Yang et al. [14] reported that Al–4.1Mg–1.2Li–10.7Cu–11.0Zn (wt%) alloy, which contained large amounts of precipitate-forming elements exhibited higher room-temperature strength and ductility than those of high-entropy AlLiMgZnSn alloy. It is certain that various types of precipitates are formed in multicomponent Al alloys, which interact with each other and affect their mechanical properties. However, the precipitation behavior of such multicomponent Al alloys has not been examined in detail.

In this study, the precipitation behavior of the multicomponent Al–6Mg–9Si–10Cu–10Zn–3Ni (wt%) alloy was investigated during artificial aging. The structural and chemical compositional evolutions of the precipitates were characterized using correlative experimental techniques including hardness, electrical resistivity, and compression measurements as well as differential scanning calorimetry (DSC) and TEM. Based on these insights into the evolution of complex precipitates, the strengthening mechanism of the studied multicomponent alloy was suggested.

Section snippets

Experimental procedure

One kilogram of alloy melt was prepared by heating a mixture containing high-purity (99.9%) Al, Mg, and Zn metals and Al–25Si, Al–30Cu, and Al–10Ni master alloys to 800 °C in an induction melting furnace followed by degassing via ultrasonic melt treatment [15] for 1 min in the temperature range of 750–700 °C. The degassed melt was poured into a Cu mold with dimensions of 245 × 70 × 200 mm3, which was pre-heated to 150 °C. The dimensions of the resulting rectangular ingot were 180 × 30 × 50 mm3,

Thermodynamic calculations

Fig. 1 shows the alloy temperature plotted as a function of the solid fraction. The obtained curve corresponded to Scheil–Gulliver solidification and was acquired using Thermo-Calc software [17] and the TCAL3 database (the contents of all constituent elements and compositions of all possible phases were taken into account during calculations). The resulting curve suggests the formation of the δ-Al3CuNi, Mg2Si, and Si phases prior to the formation of the fccAl matrix (#2–4). Further, different

Conclusions

  • (1)

    The as-cast Al–6Mg–9Si–10Cu–10Zn–3Ni alloy consisted of various secondary phases, including δ-Al3CuNi, Mg2Si, Q-Al5Cu2Mg8Si6, Si, θ-Al2Cu, and hcp Zn. The solution treatment at a temperature of 440 °C caused the dissolution of the θ-Al2Cu and hcp Zn phases and transformed the Mg2Si phase into the Q-Al5Cu2Mg8Si6 phase. The chemical composition of the Al matrix after the solution treatment was Al–0.33Mg–0.91Si–3.53Cu–14.14Zn–0.33Ni (wt%).

  • (2)

    The matrix hardness of the studied alloy was increased with

Acknowledgement

This work was supported by the Main Research Program (PNK4560) funded by the Korea Institute of Materials Science and the Industrial Strategic Technology Development Program (10062304) funded by the Ministry of Trade, Industry and Energy.

References (34)

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