Effects of the β1′ precipitates on mechanical and damping properties of ZK60 magnesium alloy

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Abstract

A new approach to improve the damping capacity and mechanical properties of ZK60 magnesium alloy at the same time is discovered by introducing the β1′ (MgZn2) precipitates. The β1′ precipitates are formed in the alloy experienced solution treatment and subsequent aging at 140 °C for 18 h in this study. The synchronous improvement in mechanical and damping properties of the alloy is mainly due to the coherent orientation relationships between the β1′ phase and the α-Mg matrix, and the shearing or bypassing of dislocations on the β1′ precipitates depending on their sizes and spacing. The modified Granato-Lücke (G-L) model is presented in this work to analyze the favorable damping capacity of the as-aged ZK60 alloy and the interaction between the β1′ precipitates and the mobile dislocations is a new source of energy-dissipation.

Introduction

Magnesium and magnesium alloys are considered as the promising structural metallic materials for modern automotive, aerospace and electric applications due to their low density, high specific strength, good machinability, recyclability and so on [1]. Besides, pure Mg has the best damping capacities among the metallic materials and can be a prominent candidate to satisfy the increasing demands for controlled vibration and noise in the dynamic mechanical systems in the above fields [2]. The strength of pure Mg can be enhanced by alloying in order to apply in the more modern industries, but the damping capacity would be reduced at the same time [1,3]. Under the normal conditions, the strength is in contradiction with the damping in Mg [4]. Thus, there are usually two main ways to balance the mechanical properties and the damping capacity of Mg alloy for the engineering applications. One method is to enhance the strength of high damping Mg alloys by alloying [5], heat treatment [6] and deformation process [7], and the other is to improve the damping capacity of high strength Mg alloys by adding the reinforcements [8].

Many researchers have reported that the addition of Zr can refine grains and increase dislocation density, thus Mg-0.6%Zr alloy has high specific damping capacity and good mechanical properties [5,9,10]. More excellent comprehensive properties can be achieved in Mg-0.6%Zr alloy by alloying, heat treatment or other methods. It has already been confirmed that Zn is an important alloying element for strengthening of Mg alloys through grain refinement, solid solution strengthening and precipitation strengthening [11]. In addition, ZK60 alloy presented in the late 1960s, which is a high strength commercial wrought Mg alloy, has been widely applied as the engineering material [12]. Precipitation strengthening is one of the normal ways to enhance the strength of Mg alloys, which exerts a great influence on ZK series Mg alloys [13]. The decomposition of the supersaturated solid solution (SSSS) can generate a number of the intermediate phases in the Mg–Zn based alloys. The precipitation process above 150 °C has been reported in the following sequence [14]: SSSS → pre-β1′/Guinier Preston (GP) zones → β1′ (rods and blocky precipitates ⊥ {0001}Mg; similar to Mg4Zn7) → β2′ (coarse plates ǁ{0001}Mg and laths ⊥ {0001}Mg; Mg4Zn7) → β equilibrium phase (MgZn or Mg2Zn3). In addition, the β1′ phase is the optimal strengthening precipitate in the Mg–Zn alloy system, which can inhibit the basal slip in Mg [13,15]. According to the G-L theory, the second phase is a strong pinning point and thus can reduce damping capacities of Mg alloys [16,17]. The precipitation of W (Mg3Zn3Y2), β1′ and β2′ phases increases the number of the strong pinning points and severe accumulation and entanglement of dislocations occur at the interface between the W-phase and α-Mg, which make the damping capacities of the as-aged Mg–Zn–Y–Zr alloy inferior to the as-solutionized alloy with the strain amplitude higher than 0.1% [6]. After the aging treatment, the Mg17Al12 particles are precipitated in the AZ91D and AZ31 alloys, and these particles act as the strong pinning points to pin the dislocations, which reduce the damping capacities of the alloys. The impact is greater with the prolonged aging time [18,19]. However, an unusual phenomenon occurs. The long period stacking ordered structure can increase the damping capacities and the yield strength of the Mg–Cu–Mn–Zn–Y alloys at the same time [20]. The Mg12Ce phase is a special parallel second phase structure, which can also improve the damping capacity of the Mg–Ce alloy [21]. The comparison in damping capacities of the above-mentioned Mg alloys is shown in Table 1. Therefore, whether the β1′ phase as a strengthening phase can also improve the damping capacity of the alloy or not, and the effect of the β1′ phase on the damping capacity of ZK60 alloy are expected to be investigated by solution treatment and aging in this study. The influence mechanism of the β1′ phase on the damping capacity of ZK60 alloy is deduced and expected for the design of magnesium alloys with wonderful damping capacities and mechanical properties by adopting the optimal second phases in the near future.

Section snippets

Experimental

The alloy used in this study was the Mg–6Zn-0.6Zr alloy (wt.%, named as ZK60) prepared from high-purity Mg, pure Zn and Mg-30% Zr master alloys. All the master alloys were added afterwards at a temperature of 780 °C in an electric resistance furnace under an argon-shield atmosphere, and then poured into a steel mould at 720 °C. Solution treatment was performed for two steps (330 °C for 24 h and 420 °C for 4 h), followed by quenching in water at room temperature. Thereafter, the artificial aging

As-cast and as-solutionized microstructures

Fig. 1 shows the optical microstructures of ZK60 alloy in the as-cast and the as-solutionized states. As seen from Fig. 1a, it is clear that some dendrites of the interdendritic eutectic phases grow from the grain boundary to the center of the grain, and the average grain size of the alloy is 58 μm. On the basis of the previous study [22], the as-cast ZK60 alloy consists of four phases, i.e. α-Mg, MgZn, MgZn2 and Mg7Zn3. After solution treatment, the ZK60 alloy consists of the small polygonal

Conclusions

  • (1)

    Almost all the second phase particles in the ZK60 alloy formed during solidification are dissolved after solid solution treatment, which improves the strength and the plasticity of the alloy. However, the solid solution treatment increases the weak pinning points in the alloy, which in turn increases the critical strain amplitude, and thus the strain-amplitude-dependent damping of the alloy is reduced.

  • (2)

    After the aging treatment, the β1′ precipitates significantly improve the mechanical

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

CRediT authorship contribution statement

Xiongpeng Zhou: Investigation, Data curation, Writing - original draft, Software. Hongge Yan: Conceptualization, Formal analysis. Jihua Chen: Supervision, Writing - review & editing, Funding acquisition. Weijun Xia: Investigation, Methodology. Bin Su: Validation. Xinyu Li: Validation. Wensen Huang: Software. Min Song: Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are grateful to National Natural Science Foundation of China (51871093).

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