Trans. Nonferrous Met. Soc. China 29(2019) 2506-2513

Effect of Li content on microstructure and mechanical property of Mg-xLi-3(Al-Si) alloys

Cheng ZHANG^1,2, Liang WU^1,2, Zi-long ZHAO³, Zhi-hui XIE⁴, Guang-sheng HUANG^1,2,5,Lei LIU^1,2, Bin JIANG^1,2,5, Andrej ATRENS⁶, Fu-sheng PAN^1,2,5

1. State Key Laboratory of Mechanical Transmission, College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China;

2. National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China;

3. College of Civil Engineering, Chongqing University, Chongqing 400045, China;

4. Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, China;

5. Chongqing Research Center for Advanced Materials, Chongqing Academy of Science and Technology, Chongqing 401123, China;

6. School of Mechanical and Mining Engineering, The University of Queensland, Brisbane Qld 4072, Australia

Received 12 March 2019; accepted 16 October 2019

Abstract: Mg-4Li-3(Al-Si), Mg-8Li-3(Al-Si) and Mg-12Li-3(Al-Si) alloys based on the α-Mg, α-Mg+b-Li, b-Li phases, respectively, were produced to investigate the effect of alloying with Al-Si eutectic on the microstructure and mechanical properties of the three alloys. Alloying with the Al-Si eutectic of Mg-xLi (x=4, 8 and 12 wt.%) alloys caused the formation of different types of Al-Li precipitates: Al₃Li, AlLi and Li₃Al₂, respectively. Also, considerable quantities of Mg₂Si phase particles were found in the three alloys. The results of tensile tests showed that the Mg-4Li-3(Al-Si) alloy exhibited the highest ultimate tensile strength (UTS) of 249 MPa but the lowest elongation of 6.3%. The Mg-12Li-3(Al-Si) alloy had the highest elongation of 26% but the lowest UTS of 173 MPa. The different mechanical properties were attributed to the different crystal structures of the three alloys and the precipitate with different type, morphology and distribution.

Key words: Mg-Li alloy; Al-Si eutectic; second phase; tensile strength; elongation

1 Introduction

Magnesium (Mg) alloys are the lightest metallic material used for structural applications such as transportation and aerospace, owing to their low density, high specific strength, and high damping capacity [1-6]. However, Mg alloys have poor deformability and poor ductility at room temperature because of their hexagonal closed packed (HCP) crystal structure, and consequently their limited number of slip systems. Fortunately, alloying with Li has a great potential to produce Mg-Li alloys with good ductility due to the change of the HCP crystal structure to a body-center cubic (BCC) crystal structure [7,8]. Furthermore, alloying with Li can further reduce the density of Mg alloys. Mg-xLi alloys exhibit three different kinds of crystal structure with increasing Li content. Mg-xLi alloys have a HCP crystal structure and an α-Mg-based microstructure for x<5.7 wt.%, a combination of HCP+BCC crystal structures and a combination of α-Mg and β-Li in their microstructure for 5.7 wt.%10.3 wt.% [9,10].

However, Mg-Li alloys typically possess low strength despite their good ductility [11,12]. Alloying with Al is a common effective approach to improve the strength of Mg-Li alloys [13,14], but the stability of these alloys is relatively poor due to metastability of the Al-Li compounds [15,16]. Alloying with Si can improve the strength and stability of the Mg-Li alloy [17,18], but these alloys can be difficult to produce because of the high melting temperature of the Mg-Si master alloys. A practical production method is to first produce an Al-Si eutectic alloy, and to add the Al-Si eutectic alloy to the Mg-Li alloy. SHI et al [17] studied the effect of alloying with Al and the Al-Si eutectic alloy on the microstructure and the mechanical properties of Mg-8Li alloys. The Mg-8Li-6(Al-Si) alloy had a tensile strength higher than the Mg-8Li, Mg-8Li-3Al and Mg-8Li-3(Al-Si) alloys, which was attributed to the refinement of the microstructure and the precipitation strengthening by the Mg₂Si, MgLi₂Al and AlLi phase particles. Similarly, ZHAO et al [18] investigated the influence of alloying with Al and the Al-Si eutectic alloy on the microstructure and the mechanical properties of Mg-4Li alloys. The Mg-4Li-6(Al-Si) alloy had a tensile strength higher than the Mg-4Li, Mg-4Li-3Al and Mg-4Li-3(Al-Si) alloys, which was also attributed to the precipitation strengthening imparted by the Al₃Li(Mg) and Mg₂Si phase particles.

However, the effects of alloying with the Al-Si eutectic alloy on microstructure and mechanical properties of Mg-Li alloys with three different crystal structures (HCP structure, HCP+BCC structure, BCC structure) have not been systematically studied. Moreover, the type, morphology and distribution of the precipitates containing Al and Si still need detailed analyses. In this work, the effects of alloying with the Al-Si eutectic alloy on the microstructure and mechanical properties of Mg-xLi (x=4, 8 and 12 wt.%) alloys were studied, and the aim was to focus on the strengthening effect and the mechanisms.

2 Experimental

Three alloys: Mg-4Li-3(Al-Si), Mg-8Li-3(Al-Si) and Mg-12Li-3(Al-Si) (wt.%) were prepared using magnetic-levitation, vacuum, high-frequency induction melting In this designation 3(Al-Si) represents 3 wt.% of the Al-12.6Si eutectic alloy. Appropriate quantities of pure Mg (99.95 wt.%), pure Li (99.95 wt.%) and the Al-12.6Si (wt.%) eutectic alloy were melted under an argon atmosphere. The melt was rapidly cooled by suction casting into a copper mold to form a cast rod with a dimension of d30 mm × 60 mm. The chemical compositions of the actual alloys are given in Table 1, which were detected by a plasma-atomic emission spectrometer (ICP-AES). The rod was cut into round plates: d30 mm × 10 mm. The plates were heated at 300 °C for 1 h and were then multi-pass rolled to a final thickness of 1.6 mm, with a reduction of about 20% for each pass, with an intermediate anneal at 300 °C for 15 min. The d130 mm-diameter rolls were preheated and maintained at 150 °C throughout the rolling. The rolled sheets were finally annealed at 150 °C for 1 h.

Table 1 Chemical compositions of actual alloys (wt.%)

Tensile specimens were prepared with a gauge length of 25 mm, a width of 5.7 mm and a thickness of 1.5 mm, according to the ASTM standards E8/E8M- 2011. The tensile tests were performed on a CMT5205 electronic universal testing machine with a 0.5 mm/min tensile speed. The microstructure was examined using a scanning electron microscope (SEM, MIRA3) equipped with an Oxford electron backscattered diffraction (EBSD) system and energy dispersive spectroscopy (EDS), and a transmission electron microscope (TEM, JEOL-2100F). The samples for TEM examination were prepared by twin-jet electropolishing using a solution of 5% perchloric acid in ethanol at about -20 °C, and finally ion-milling for 30 min on a Gatan PIPS II machine.

3 Results and discussion

3.1 Microstructure

Figure 1 presents the grain orientation and (0002) pole figure of the annealed Mg-4Li-3(Al-Si) alloy. The EBSD data were cleaned up based on grain dilation type, whose grain tolerance angle was 5° and the minimum grain size was 4 mm. There were a few large grains whose size was about 15 mm, and small grains were distributed around the large grains. The highest texture intensity for the annealed Mg-4Li-3(Al-Si) alloy was 14.23. According to ZHAO et al [18], the precipitate phase particles in the Mg-4Li-3(Al-Si) alloy were Mg₂Si and Al₃Li. After rolling, there was no basal texture, indicating that the fine dispersed precipitates produced by Al-Si effectively inhibited the activation of basal slip.

Fig. 1 Grain orientation image (a) and (0002) pole figure (b) of annealed Mg-4Li-3(Al-Si) alloy

Figure 2 shows EBSD data for the annealed Mg- 4Li-3(Al-Si) alloy, which were used to analyze the grain boundary configuration and dynamic recrystallization. In Fig. 2(a), green represents low-angle grain boundaries (LAGB), and black represents high-angle grain boundary (HAGB). Figure 2(b) shows the distribution of grain size. The microstructure was mainly composed of LAGB, which was present in almost every large grain. This indicates that severe plastic deformation occurred during rolling, and a large number of dislocations accumulated in the grains and subgrain boundaries. Static recovery was predominant in the alloy during annealing, and there was not a high degree of recrystallization, which would lead to a dramatic reduction of dislocation density and grain refinement [19,20]. In Fig. 2(c), the red areas represent the deformed grains, the yellow areas represent the substructure, and the blue areas represent the recrystallized grains. The majority of grains were red deformed grains, and only a few recrystallized grains appeared at the junctions of tiny grains. Many factors can impact on dynamical recrystallization, in which precipitates could postpone or promote dynamical recrystallization, depending on the particle size, distribution and density of particles [18]. Obviously, the fine dispersed Mg₂Si and Al₃Li precipitates effectively pinned the grain boundaries of the α-Mg grains of the Mg-4Li-3(Al-Si) alloy during deformation and annealing, and restricted the grain recrystallization. In addition, the distribution of the yellow substructure was not uniform, which may be due to the uneven deformation during rolling. Figure 2(d) indicates that the red deformation grains had larger strains, while the yellow substructure had smaller strains. This shows that the substructure can relieve strain.

Figure 3 shows SEM images of the annealed Mg-4Li-3(Al-Si) alloy. Figure 3(a) shows coarse white laminar Mg₂Si elongated in the rolling direction. Higher magnifications (Figs. 3(b-d)) indicate tiny granular precipitates in addition to the white Mg₂Si. According to ZHAO et al [18], the second phase precipitates contained Al in the Mg-4Li-3(Al-Si) alloy are Al₃Li precipitates.

Fig. 2 Grain boundary and dynamic recrystallization of annealed Mg-4Li-3(Al-Si) alloy

After rolling, the smaller particles were distributed in the microstructure uniformly, which enhanced the precipitation strengthening [21-23]. These results indicated that the strengthening mechanisms of the α-Mg in the Mg-4Li-3(Al-Si) alloy were solid solution hardening, precipitation hardening and strain hardening.

Fig. 3 SEM images of annealed Mg-4Li-3(Al-Si) alloy with different magnifications

Fig. 4 SEM image (a) and EDS mapping scanning results (b, c, d) of annealed Mg-8Li-3(Al-Si) alloy

Figure 4 shows the SEM images and EDS mapping results of the annealed Mg-8Li-3(Al-Si) alloy Figure 4(a) indicates that the Mg-8Li-3(Al-Si) alloy consisted of strip-like α-phase and matrix β-phase. There were some small white granular precipitates existed on the surface. The distribution of Al was more uniform (Fig. 4(c)) than Si, because Al element had a higher solid solubility in Mg, which showed solid solution strengthening effect in Mg matrix. Point-like segregations of Al and Si elements were distributed in the Mg alloys, indicating that there were second phase precipitates. According to Ref. [24], the distribution showed characteristics of Mg₂Si brittle precipitates, which was consistent with that observed in the Mg-4Li-3(Al-Si) alloy.

SEM images and EDS analysis results were taken to obtain more detailed information in the annealed Mg-8Li-3(Al-Si) alloy (Fig. 5). Figure 5(a) shows that the white acicular Mg₂Si particles were distributed along the α/b interface and mainly in the β-phase matrix which was attributed to the β-phase is the softer phase [25]. There was also a smaller gray particulate precipitate (Fig. 5(b)), which can be identified by EDS analysis. The precipitate contained only Al without Si, so the precipitate was AlLi phase according to Ref. [25]. ZHANG et al [26] reported that Mg alloys had poor corrosion resistance and were easy to oxidize. Thus, the oxygen content in Fig. 5 is attributed to the oxidation of the sample during processing.

Figure 6 presents SEM images of the annealed Mg-12Li-3(Al-Si) alloy. The microstructure was characterized by banded structure as shown in Fig. 6(a). Figures 6(b-d) show high-magnification images of fine particles which were uniformly dispersed in the β-Li matrix. The small white elongated particles were Mg₂Si phase precipitates and the grey granular particles were Al-Li phase precipitates, which were distributed along the rolling direction. These results show that the enhancement effect of Al-Si on α-phase might be different with that of β-phase, which is closely related to the crystal structure difference between α-Mg and β-Li.

To understand the interference with the matrix, a bare spherical precipitate in the annealed Mg-12Li- 3(Al-Si) alloy was analyzed by TEM, as shown in Fig. 7. The EDS spectrum (Fig. 7(b)) and SAD pattern (Fig. 7(c)) were taken from the red-dash-circled area in Fig. 7(a). The EDS spectrum (Fig. 7(b)) shows mainly characteristic X-ray of Al, indicating that the spherical precipitate is the Al-Li phase. The SAED pattern (Fig. 7(c)) revealed that the spherical precipitate contains interplanar spacings of 0.232, 0.202, 0.141, 0.117 and 0.093 nm which matched the d-values of the planes , , , (0210) and of Li₃Al₂, respectively. The error of interplanar spacings is minimal compared with the standard value. Thus, the spherical precipitate is the Li₃Al₂ phase.

3.2 Mechanical properties

The tensile stress-strain curves and histograms of mechanical properties of the annealed Mg-4Li- 3(Al-Si), Mg-8Li-3(Al-Si) and Mg-12Li-3(Al-Si) alloys are shown in Figs. 8(a) and (b), respectively. The Mg-4Li-3(Al-Si) alloy exhibits the highest ultimate tensile strength (UTS) of 249 MPa but the lowest elongation of 6.3%. The Mg-12Li-3(Al-Si) alloy possesses the highest elongation of 26% but the lowest UTS of 173 MPa. The Mg-8Li-3(Al-Si) alloy has intermediate mechanical properties, with UTS of 210 MPa and an elongation of 11.6%. In comparison with the Refs. [27-29], the addition of Al-Si eutectic resulted in the increase of UTS but decrease of elongation of Mg-Li alloys. In addition, the results of tensile experiments indicate that the strengthening effect of Al-Si on the α-Mg based (Mg-4Li) is significantly higher than that of β-phase (Mg-12Li). The strength of the alloys decreased with the increase of Li content, while the elongation increased.

Fig. 5 High-magnification SEM images (a, b) and EDS analyses (c, d) of annealed Mg-8Li-3(Al-Si) alloy

Fig. 6 SEM images of annealed Mg-12Li-3(Al-Si) alloy with different magnifications

Fig. 7 TEM image (a) of precipitate in Mg-12Li-3(Al-Si) alloy, EDS spectrum (b) and SAD pattern (c) from red-dash-circled area in (a)

Fig. 8 Stress-strain curves and histograms of mechanical properties of as-rolled Mg-xLi-3(Al-Si) alloy

The reason why the three alloys show different tensile strength and elongation can be attributed to two affects. On one hand, the three alloys have three different phase structures, which are α-phase, (α+β)-phase and β-phase with increasing Li content. The α-phase possesses the HCP crystal structure and acts as a hard phase whilst the β-phase has the BCC crystal structure functioning as a soft phase [30,31]. On the other hand, the precipitates of the three alloys are different in composition and morphology. Besides Mg₂Si phase, the different precipitations of the Al-Li phases (including the Al₃Li, AlLi and Li₃Al₂ phase particles) were formed in the corresponding Mg-xLi (x=4, 8 and 12 wt.%) alloys. That is, the origin of the strengthening can be understood from the well-established theories of dislocation interactions in metals, which is usually a result of solid solution and precipitation effects, strain hardening, grain size reduction, or a combination of these effects [32]. Besides, crystal structure is closely related to the mechanical properties owing to soft β-phase [33-36].

4 Conclusions

(1) Alloying with the Al-Si eutectic causes different precipitates in the three alloys with α-phase, α+β dual-phase and β-phase. Besides Mg₂Si phase, the Al-Li phases of Mg-xLi-3(Al-Si) alloys (x=4, 8 and 12 wt.%) are Al₃Li, AlLi and Li₃Al₂ phase, respectively.

(2) The tensile strength of the Mg-xLi-3(Al-Si) alloys (x=4, 8 and 12 wt.%) decreases with increasing Li content, whilst the elongation increases. The Mg-4Li- 3(Al-Si) alloy exhibits the highest UTS of 249 MPa and the Mg-12Li-3(Al-Si) alloy possesses the highest elongation of 26%.

(3) The strengthening effect of the Al-Si eutectic alloying on the α-phase (Mg-4Li) is higher than that on the β-phase (Mg-12Li), which might be related to the different crystal structures and the different category, morphology and distribution of the precipitate phases.

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Li含量对Mg-xLi-3(Al-Si)合金显微组织和力学性能的影响

张 诚^1,2，吴 量^1,2，赵子龙³，谢治辉⁴，黄光胜^1,2,5，刘 磊^1,2，蒋 斌^1,2,5，Andrej ATRENS⁶，潘复生^1,2,5

1. 重庆大学 材料科学与工程学院，机械传动国家重点实验室，重庆 400044；

2. 重庆大学 国家镁合金材料工程技术研究中心，重庆 400044；

3. 重庆大学 土木工程学院，重庆 400045；

4. 西华师范大学 化学化工学院 化学合成与污染控制四川省重点实验室，南充 637002；

5. 重庆市科学技术研究院 重庆市先进材料研究中心，重庆 401123；

6. School of Mechanical and Mining Engineering, The University of Queensland, Brisbane Qld 4072, Australia

摘  要：基于α-Mg、α-Mg+β-Li和β-Li三种相结构，制备Mg-4Li-3(Al-Si)、Mg-8Li-3(Al-Si)和Mg-12Li-3(Al-Si)三种合金，用于研究Al-Si共晶体对其组织和力学性能的影响。在Mg-xLi (x=4%, 8%和12%，质量分数)合金中添加Al-Si共晶体分别形成以下的Al-Li析出相：Al₃Li、AlLi和Li₃Al₂。此外，在这三种合金中还发现大量的Mg₂Si相颗粒。拉伸试验结果表明，Mg-4Li-3(Al-Si)合金的极限抗拉强度最高，为249 MPa，其伸长率最低，为6.3%。Mg-12Li-3(Al-Si)合金的伸长率最高，为26%，但极限抗拉强度最低，为173 MPa。这三种合金力学性能的差异归因于晶体结构的不同以及析出物类型、形态和分布的不同。

关键词：Mg-Li合金；Al-Si共晶体；第二相；抗拉强度；伸长率

(Edited by Xiang-qun LI)

Foundation item: Projects (51971040, 51701029, 51671041, 51531002) supported by the National Natural Science Foundation of China; Project (2016YFB0301100) supported by the National Key Research and Development Program of China; Projects (2018T110943, 2017M620410) supported by the China Postdoctoral Science Foundation; Project (Xm2017010) supported by the the Chongqing Postdoctoral Scientific Research Foundation, China; Project (2018CDGFCL005) supported by the Fundamental Research Funds for the Central Universities, China

Corresponding author: Liang WU; E-mail: wuliang@cqu.edu.cn

DOI: 10.1016/S1003-6326(19)65158-7