Effects of Ca on the formation of LPSO phase and mechanical properties of Mg-Zn-Y-Mn alloy (original) (raw)

Highlights

•
Ca induced the formation of LPSO phase while discouraged that of eutectic structure phase, which can strengthen the alloy performance.
•
Strength together with ductility is improved significantly by addition of Ca.
•
Adding moderate calcium to the Mg–Y–Zn LPSO structure may reduce the Y content of the alloy, thus greatly increasing the volume fraction of LPSO phase in the case of a low Y/Zn atomic ratio.

Abstract

The influence of Ca addition on the microstructure and mechanical properties of Mg94Zn2.5Y2.5Mn1 alloy were investigated. Results showed that Ca induced the formation of long-period stacking ordered (LPSO) structure while discouraged that of Mg3Zn3Y2 eutectic structure. With Ca addition, the microstructure were refined, and the mechanical properties were enhanced.

Introduction

Magnesium alloys have a great potential for improving the fuel efficiency of vehicles because of their high specific strength and stiffness [1]. In the last decade, magnesium alloys with long-period stacking ordered (LPSO) structures, exhibiting unique microstructures and excellent mechanical properties, have been developed in the ternary Mg–Y–Zn systems, received progressive interest.

Depending on the Y/Zn atomic ratio, three kinds of main ternary equilibrium Mg–Zn–Y phases can be formed. They are W-phase (Mg3Zn3Y2, cubic structure), I-phase (Mg3Zn6Y, icosahedral quasicrystal structure), and X-phase (Mg12YZn, long period stacking ordered structure). Recently, a series of Mg–Y–Zn alloys with an atomic Y/Zn ratio of 2:1 have attracted considerable attention due to the presence of large long period stacking ordered (LPSO) structures that provide excellent mechanical properties [2], [3], [4]. Generally, X-phase is easier to be formed when the Y content is higher than Zn content. Thus, a major issue that limits the commercial application of the rare-earth containing Mg alloys is their cost.

The Ca is a relatively inexpensive element. It is pointed out that the addition of Ca to Mg alloy has beneficial influence on the microstructure refinement, and it can increase strength and ductility of alloys obviously [5]. What’s more, Saal [6] examined the thermodynamic stability of variant LPSO precipitates with density theory and predicted the Ca element is promising in terms of forming stable LPSO phases, particularly with Zn element. Thus, it is highly desirable to reduce the cost of employing LPSO precipitate strengthening on an industrial scale.

However, up to now, there was little study on Ca-containing LPSO Mg alloy. Especially, no details about the effect of Ca on the formation of LPSO phases were reported. In this work, Mg94–x_Zn2.5Y2.5Mn1Ca_x (at%) (_x_=0, 0.17, 0.34, 0.51, and 0.67) alloys were investigated systematically, so as to make clear the effects of Ca on the microstructures and mechanical properties of Mg–Zn–Y–Mn alloys, especially on the formation of LPSO phases.

Section snippets

Experimental procedures

The five groups Mg–Zn–Y–Mn–Ca alloys were melted by using high-purity Mg, Zn, Y, Mn and Mg-30

wt% Ca master alloy in an electric resistance furnace under the protective atmosphere of N2 (97.4

vol%)+CH2FCF3 (2.6

vol%) gas mixture at 1023

K. Then they were cast into a preheated mould at 993

K. Phase constitution analyses were performed with Y-2000 X-ray diffraction (XRD), using monochromatic Cu-Kα radiation. The microstructures and compositions of different phases of the alloys were investigated by

Microstructure analysis

Fig. 1a shows the microstructure of as-cast Mg93.66Zn2.5Y2.5Mn1Ca0.34 alloys. It can be indicated that the microstructure of Mg94–x_Zn2.5Y2.5Mn1Ca_x alloys consisted of α-Mg matrix, block second-phases and fishbone-like eutectic structure. The selected area electron diffraction (SAED) patterns (Fig. 1b), the XRD patterns (Fig. 2) and EDS results (Fig. 1c and d) reveal that the second-phase is X-phase (Mg12YZn, with 18R-LPSO structure), the eutectic structure is W-phase (Mg3Zn3Y2). X-ray phase

Conclusions

(1)
The investigated Mg–Zn–Y–Mn–Ca alloys are consisted of α-Mg phase, X-phase (Mg12YZn) and W-phase (Mg3Zn3Y2). With 0.34
at% Ca addition, the grains have been greatly refined.
(2)
Adding moderate calcium to the Mg–Zn–Y–Mn alloy can greatly increase the volume fraction of LPSO phase in the case of a low Y/Zn atomic ratio.
(3)
Due to the strengthening from grain refinement and the formation of large LPSO phase, the mechanical properties of as-cast Mg–Zn–Y–Mn alloy were significantly improved by 0.34
at% Ca

Acknowledgements

The authors wish to acknowledge the financial support of the National Natural Science Foundation of China (Nos. 51474153 and 50571073), Ph.D. Programs Foundation of Ministry of Education of People's Republic of China (20111402110004) and Natural Science Foundation of Shanxi Province (Nos. 2009011028-3 and 2012011022-1).

References (15)

et al.

Energy

(2007)

J.S. Zhang et al.

Mater. Sci. Eng. A

(2012)

M. Yamasaki et al.

Acta Mater.

(2011)

E. Oñorbe et al.

Scr. Mater.

(2011)

Q. Wen et al.

Mater. Sci. Eng. A

(2014)

J.E. Saal et al.

Acta Mater.

(2014)

T.V. Larionova et al.

Scr. Mater.

(2001)

There are more references available in the full text version of this article.

Cited by (64)

View full text