Effect of solution treatment on microstructure and properties of Mg-6Gd-3Y-1.5Zn-0.6Zr alloy

https://doi.org/10.1016/j.matchar.2020.110295Get rights and content

Highlights

  • Co-doping with two distinct rare earth elements.

  • Ensuring the alloy performance while reducing the cost (RE content is <10 wt%).

  • After solution treatment, forming a new lamellar long-period stacking ordered phase.

  • The partial T4 treatment results in better mechanical properties; after 5 h of treatment at 500 °C.

Abstract

The effect of solution treatment on the microstructures and mechanical properties of the Mg–6Gd–3Y–1.5Zn–0.6Zr alloy was investigated using optical microscopy, scanning electron microscopy, transmission electron microscopy, and tensile tests. After solution treatment, the growth of a new lamellar long-period stacking ordered phase from the grain boundary to the inner grain was observed. The alloy exhibited improved mechanical properties after 5 h of treatment at 500 °C and reached its peak aging hardness after 4 h at 200 °C.

Introduction

At present, Mg alloys, which have advantages such as low density, high specific strength and stiffness, good damping properties [[1], [2], [3]], and easy recyclability are the lightest metal structural materials. Mg and its alloys are extensively applied in fields such as aerospace, automotive, computer, and electronic communications.

The addition of a certain amount of Zn, Cu or Ni into Mg rare earth alloys (RE = Y, Gd, Tm, Ho, Dy, Er, Tb) can form a long-period stacking ordered (LPSO) structure [4], which has abundant structural morphology, special structure, and excellent properties. The LPSO phase is a structurally and chemically ordered phase in which the structural and chemical orders indicates the periodic arrangement of atoms along the [0001]α direction and the neat distribution of RE/Zn atoms on two or four densely packed surfaces [10], respectively. Mg-RE-Zn alloys [[4], [5], [6], [7], [8], [9]], are divided into two categories based on whether the LPSO structure can be formed only by high-temperature heat treatments (Mg-Tb-Zn alloys) or under the as-cast conditions, (all the other Mg-RE-Zn alloys) [5].

The LPSO phase is a hard phase with high strength and elastic modulus, that can effectively inhibit dislocation movements and deformation twin growth. Moreover, this phase can refine the alloy structure by stimulating particle nucleation, strengthen deformed alloys via the short-fiber strengthening mechanism, and even coordinate the deformation during tensile processes. Furthermore, the α-Mg/LPSO interface is a coherent one. All these features contribute to strengthening the Mg alloys [[17], [18], [19], [20]]. The properties of the LPSO phase depend on its different morphologies. After heat treatment, the LPSO phase can show several morphologies such as reticular, block, fine lamellar, and rod-shaped ones [11,12]. The presence of a reticulated and massive LPSO phase often decreases the alloy's plasticity; however, a diffusely distributed and layered LPSO phase can effectively improve it along with the alloy strength [12]. Wu et al. [13] studied the Mg-10Gd-1Zn-0.5Zr alloy under peak aging conditions and reported a better ductility for the samples with an LPSO phase than those without it; in particular, alloy samples with a block LPSO phase exhibited the best mechanical properties, an ultimate tensile strength of 303.5 MPa, and an elongation of 7.7%. Generally, the 18R-LPSO and 6H-LPSO phases are observed in as-cast alloys, but they are converted into the 14H-LPSO one after solution treatment. However, Gao et al. [14] reported the presence of fine stripes of the 6H-LPSO phase in the Mg-10Y-5Gd-2Zn-0.5Zr alloy even after solution treatment at 535 °C for 16 h. To examine the aging precipitation sequence of Mg-RE alloys containing LPSO phases, Honma et al. [15] treated the Mg-2.0Gd-1.2Y-1.0Zn-0.2Zr alloy at 225 °C and determined the presence of the SSSS → β” → β’ → β1 → 14H-LPSO structure. Wen et al. [16] examined the effects of homogenization and isothermal aging on the Mg-12Gd-2Er-1Zn-0.6Zr alloy; the 14H-LPSO structure appeared after the homogenization treatment and formed a β’ phase after isothermal aging, thus considerably the mechanical properties compared to the as-cast condition.

Recently, Mg-RE alloys with LPSO phases have attracted considerable attention because of their high strength at room temperature and above, good plasticity, and creep resistance [[14], [15], [16]]. Because of the in-depth study of Mg-RE alloys, the addition of individual rare earth elements can no longer meet the development requirements of Mg alloys but can help to gradually obtain quaternary Mg-RE1-RE2-Zn alloy systems [[14], [15], [16],21]. Therefore, co-doping with various RE elements has important significance and research value for designing and applying Mg alloys. Thus, Gd and Y have higher solid solubility in the Mg matrix, and their solid solution strengthening effect is very good; moreover, the solid solubility in Mg decreases considerably as the temperature decreases such that the second phase can be uniformly dispersed and precipitated, thus achieving a significant aging precipitation strengthening effect.

According to a previous study [21], the total content of rare-earth elements used in the present study was <10 wt% to ensure a suitable alloy performance while reducing the cost. Mg–6Gd–3Y–1.5Zn–0.6Zr alloy samples were solution-treated during different times and subsequently aged so as to investigate the effect of LPSO evolution on their microstructure and mechanical properties and the age hardening law for the alloy.

Section snippets

Experimental

Pure Mg (99.9 wt%) and Zn (99.9 wt%) ingots were used to prepare the Mg–6Gd–3Y–1.5Zn–0.6Zr alloy; the Gd, Zr, and Y elements were added in the form of Mg-30wt.%Gd, Mg-30wt.%Zr, and Mg-30wt.%Y master alloys. CO2 and SF6 gases were used to protect the molten alloy. The Zn ingots and the master alloys were added to the molten Mg alloy at 750 °C and this temperature was maintained for 30 min to ensure the complete dissolution of the alloying elements. The resulting molten alloy was poured at 720 °C

Microstructure and phase composition

Fig. 1 shows the XRD patterns of the Mg–6Gd–3Y–1.5Zn–0.6Zr alloy before and after solution treatment. The figure shows that the heat-treated samples were primarily composed of α-Mg, Mg12(Gd,Y)Zn(LPSO), and Mg24(Gd,Y,Zn)5 phases.

Fig. 2 shows the microstructure of the as-cast alloy; it shows that the Mg matrix and lamellar and blocky structures precipitated along the grain boundaries and within the grains. The EDS analysis (Table 1) of the lamellar structures labeled as A and B in Fig. 2(b),

Conclusions

  • (1)

    With increase in the solution treatment time, the grain size change of the Mg–6Gd–3Y–1.5Zn–0.6Zr was not obvious, and the structure became gradually homogeneous. After the solution treatment, a new lamellar 14H-LPSO phase was formed at the grain boundaries and extended from into the grains.

  • (2)

    After different times of solution treatment, the mechanical properties of the Mg–6Gd–3Y–1.5Zn–0.6Zr alloy showed a downward trend, but the difference was insignificant. After 5 h of solution treatment at

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

This study was supported by China Postdoctoral Science Foundation (2017M611748), Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJCX19_0586) and Priority Academic Program Development of Jiangsu Higher Education Institutions.

Data availability statement

The raw/processed data required to obtain these results cannot be shared at this moment since they are also part of an ongoing study.

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