The solution and room temperature aging behavior of Mg–9Li–xAl(x = 3, 6) alloys

doi:10.1016/j.jallcom.2012.05.021

Journal of Alloys and Compounds

Volume 536, 25 September 2012, Pages 145-149

https://doi.org/10.1016/j.jallcom.2012.05.021 Get rights and content

Abstract

As-cast Mg–9Li–3Al(LA93) and Mg–9Li–6Al(LA96) alloys were prepared under the ambient of pure argon, and the effect of solution treatment and room temperature (RT) aging on the microstructure and microhardness of the alloys were investigated. The results show that the main phase compositions of the two alloys both consist of α(Mg), β(Li), AlLi phase, besides that there is θ(MgLi₂Al) phase in LA96. After solid solution treatment, AlLi phase can completely be dissolved into the matrix, while θ phase is not fully dissolved into the matrix. In LA93 and LA96 alloys, AlLi phase precipitates from matrix during the 24 h RT aging. When the aging time increases to 48 h, more AlLi phase turns up and appears to aggregate which results in the decrease of hardness, and the transformation of θ phase into the equilibrium phase (AlLi) in LA96 also causes the hardness decrease.

Introduction

During the past decade, increasing attention has been paid to Mg alloys for their applications in automobile, aviation and electronic industries [1], [2]. As the lightest metallic engineering materials, Mg–Li alloys have some impressive advantages, such as high specific strength and stiffness, excellent electromagnetic shielding and damping properties [3]. Mg–Li alloy has a density of 1.3–1.6 g/cm³, which is twenty-five percent lighter than that of conventional magnesium alloys [4], [5]. Moreover, when Li content is between 5.7 and 10.3 mass%, Mg–Li alloys possess two-phase structures that consist of the α-Mg (hcp) and β-Li (bcc) phases at room temperature [6], [7], [8]. However, Mg–Li alloys have the drawbacks of low strength, poor corrosion resistance, and thermal instability, which are not beneficial for their application. To overcome these drawbacks, various third elements (such as Al, Zn) have been added to Mg–Li alloy systems [9], [10], [11], [12], [13], [14].

From previous works, it is known that Mg–Li–Al alloys are typically aging hardening alloys at room temperature (RT). RT aging behavior of Mg–(11–14)Li–Al alloys (with the matrix of β-Li) has been reported by many authors [15], [16]. As for the Mg–5Li–3Al–2Zn alloy (with the matrix of α-Mg), it has been reported that the Ag addition can cause the formation of θ-MgLi₂Al and reduce the amount of AlLi, resulting the better aging hardening [17]. In this paper, Mg–9Li–xAl alloys (with the matrix of α-Mg and β-Li) with a high Al content (3 and 6 wt.%) were prepared, and the solution and RT aging behavior of the alloys were investigated.

Section snippets

Materials preparation

Two alloys with nominal compositions of Mg–9Li–3Al(LA93) and Mg–9Li–6Al (LA96) were prepared by melting purity metals, magnesium(99.95 wt.%), lithium (99.90 wt.%), pure aluminum (99.90 wt.%), in an induction melting furnace under the ambient of argon gas. After melting, the melt was poured into a steel mold to obtain as-cast specimens. Then the specimens were cut into small ones and were solid solution-treated for 1 h under an argon atmosphere. The solution temperatures are 350 and 400 °C,

Microstructure of as-cast alloys

Fig. 1 shows the light microstructure of as-cast Mg–9Li–xAl(x = 3, 6) alloys. It reveals that Mg–9Li–xAl alloys are mainly composed of α-Mg and β-Li. In LA93, the shape of α phase (white) is block-like shape, and there exist a few small spheric particles inside the β phase (gray), as shown in Fig. 1a. In LA96 (Fig. 1b), both block-like α phase (white) and block-like compounds (black) can be seen. In more details, some white particles distributes evenly in the β phase (gray).

The XRD patterns of

Conclusions

In the as-cast alloys, Mg–9Li–3Al is composed of α(Mg), β(Li) and AlLi, and Mg–9Li–6Al is composed of α(Mg), β(Li), AlLi and θ(MgLi₂Al). After solution treatment, AlLi phase is completely solutionized into the matrix, while the θ phase in LA96 alloy cannot be fully dissolved in the matrix. In the RT aging process, plentiful AlLi phase precipitates from matrix and aggregates together after 48 h. The morphology of AlLi phase is small spheric particles in β phase in as-cast alloy. However, in RT

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. 51001034), the Fundamental Research funds for the Central Universities (HEUCF201210004), the Research Fund for the Doctoral Program of Higher Education (20092304120020) and the Project for Heilongjian Province General Institution of Higher Education Young Scientific Key Teacher (1252G018).

References (17)

Z.K. Qu et al.
Mater. Sci. Eng. A
(2010)
F. von Buch et al.
Mater. Sci. Eng. A
(1999)
Y.W. Kim et al.
Scr. Mater.
(1998)
R.Z. Wu et al.
Mater. Sci. Eng. A
(2009)
S. Schumann
Mater. Sci. Forum
(2005)
SAE International TM. Magnesium for automotive components. Warrendale (PA), Society of Automotive Engineers,...
M.M. Avedesian et al.
Magnesium and Magnesium Alloys-ASM Specialty Handbook
(1999)
R.Z. Wu et al.
Rev. Adv. Mater. Sci.
(2010)

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

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Citation Excerpt :
Besides, the solid solubility of aluminum in magnesium is large, and the solid solubility changes significantly as the temperature decreases. Therefore, the effect of solid solution strengthening is obvious [24–26]. Li et al. [27] found that the microhardness of Mg–35Li alloy increased by 50% to 1.63 GPa with the addition of Al element.
To further improve the mechanical properties of Mg–Li alloys, the microstructure evolution and mechanical properties of Mg–6.2Li–3.5Al–3Y alloy after different heat treatment processes (holding at 250 °C, 300 °C, 350 °C and 400 °C for 30 min, 60 min and 120 min, respectively) have been investigated in this study. It is found that the Mg₂Y eutectic disappears after heat treatment, and the volume fraction of Al₂Y precipitated phase decreases first and then increases with increasing heat treatment time. In addition, The tensile test results reveal that the tensile strength of the alloy increases first and then decreases with the increase of heat treatment time. The tensile strength and ductility of the alloy reach 182 MPa and 12.7%, respectively, after heat treatment at 400 °C for 30 min, which are 35.8% and 36.5% higher than those of as-cast alloy. The evolution of Mg₂Y eutectic and Al₂Y phase during heat treatment processes at different time and temperatures are analyzed in detail. It can be concluded that the tensile strength of the alloy is improved by the combined effects of precipitation hardening due to the formation of Al₂Y phase, and the solid solution strengthening due to the decomposition of Mg₂Y eutectic and Al₂Y precipitated phase.

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LetterThe solution and room temperature aging behavior of Mg–9Li–xAl(x = 3, 6) alloys

Abstract

Introduction

Section snippets

Materials preparation

Microstructure of as-cast alloys

Conclusions

Acknowledgments

Mater. Sci. Eng. A

Mater. Sci. Eng. A

Scr. Mater.

Mater. Sci. Eng. A

Mater. Sci. Forum

Magnesium and Magnesium Alloys-ASM Specialty Handbook

Rev. Adv. Mater. Sci.

Letter
The solution and room temperature aging behavior of Mg–9Li–xAl(x = 3, 6) alloys