Electrochemical study on preparation of Mg-Li-Yb alloys in LiCl-KCl-KF-MgCl2-Yb2O3 melts

JOURNAL OF RARE EARTHS, Vol. 30, No. 2, Feb. 2012, P. 159

Electrochemical study on preparation of Mg-Li-Yb alloys in LiCl-KCl-KF-MgCl2-Yb2O3 melts CHEN Lijun (䰜Б‫)ݯ‬, ZHANG Milin (ᓴᆚᵫ), HAN Wei (䶽 ӳ), YAN Yongde (买∌ᕫ), CAO Peng (᳍ 吣) (Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China) Received 30 June 2011; revised 15 November 2011

Abstract: This paper presented a novel study on electrochemical codeposition of Mg-Li-Yb alloys in LiCl-KCl-KF-MgCl2-Yb2O3 melts on molybdenum. The factors of the current efficiency were investigated. Electrolysis temperature had great influence on current efficiency; the highest current efficiency was obtained when electrolysis temperature was about 660 ºC. The content of Li in Mg-Li-Yb alloys increased with the high current densities. The optimal electrolytic temperature and cathodic current density were around 660 ºC and 9.3 A/cm2, respectively. The chemical content, phases, morphology of the alloys and the distribution of the elements were analyzed by X-ray diffraction, scanning electron microscopy, inductively coupled plasma mass spectrometry, respectively. The intermetallic of Mg-Yb was mainly distributed in the grain boundary of the alloys, presented as reticulated structures, and refined the grains. The lithium and ytterbium contents in Mg-Li-Yb alloys could be controlled by changing the concentration of MgCl2 and Yb2O3 and the electrolysis conditions. Keywords: current efficiency; electrochemical codeposition; Mg-Li-Yb alloys; intermetallic; rare earths

Mg-Li alloys are known as the lightest metallic materials, which have low density, high specific stiffness, high electrical and thermal conductivities. Mg-Li alloys are attracting much attention in the fields of electric appliances, automobile, military regions and aerospace industries[1–3]. However, the applications of Mg-Li alloys are limited because of high chemical activity and poor mechanical properties. It is known that high corrosion resistances of magnesium and high-temperature strength are increased by the addition of rare earth elements[4,5]. The molten salt electrolysis method has been wildly used to prepare binary or ternary alloys now, for it can control the phases of alloys by electrochemical parameters and conduct with a simple apparatus[6]. Iida et al. investigated the electrochemical codeposition of Sm-Co alloys from LiCl-KClSmCl3-CoCl2 melts, and studied electrochemical formation of Yb-Ni and Sm-Ni alloy films by Li codeposition method from chloride melts[7–9]. Kuznetsov and Masset et al. investigated the reduction of LaCl3 in LiCl-KCl melt[10,11]. Castrillejo and Bermejo et al. investigated the electrochemical behaviour of dysprosium and gadolinium in the eutectic LiCl-KCl on W and Al electrodes[12,13]. Our group has successfully exploited a relatively simpler electrochemical method in which the codeposition of Mg-Li-Zn[14], Mg-Li-Al[15,16] and Mg-Li-Ca[17] and Mg-LiSm[18] on molybdenum electrode in LiCl-KCl-MgCl2-MClx melts. But all the third alloy elements were provided by the

corresponding chloride. In this paper, multi-electrodeposition is proposed for direct preparation of Mg-Li-Yb alloys via electrochemical codeposition of Mg, Li and Yb from LiCl-KCl-MgCl2-KF-Yb2O3 melts. The effects of electrolytic temperature and cathodic current density on current efficiency were studied. The Mg-Li-Yb alloy samples obtained by the electrolysis were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and inductively coupled plasma-atomic emission spectrometry (ICP-AES).

1 Experimental LiCl and KCl used in the experiments (LiCl:KCl=50:50, wt.%) were dehydrated at 573 and 873 K for 24 h before electrolysis, MgCl2 and KF were dehydrated at 403 K in vacuum desiccation box to remove excess water, and then melted in an alumina crucible placed in a quartz cell located in an electric furnace, respectively. The working electrode was molybdenum wire (d=1 mm). The lower end of the working electrode was polished thoroughly by SiC paper, and cleaned in ethanol using the method of ultrasonic cleaning. A spectrally pure graphite rod (d=6 mm) served as a counter electrode. The whole electrolysis process was performed under the protection of argon atmosphere. The experimental apparatus conclude a small cell to load the molten salt; a DC steady flow power (WYK-3010) to provide the constant current; and a crucible resistance furnace (SG2-1.5-

Foundation item: Project supported by the National Natural Science Foundation of China (21103033, 21101040 and 21173060), the Fundamental Research funds for the Central Universities (HEUCF101210002) and 863 projects (2011AA03A409, 2009AA050702) Corresponding author: ZHANG Milin (E-mail: [email protected]; Tel.: +86-451-82533026) DOI: 10.1016/S1002-0721(12)60015-5

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10) to control the temperature of the experiment. The Mg-Li-Yb alloy samples obtained by the galvanostatic electrolysis under different conditions were washed in hexane (99.8% purity) in an ultrasonic bath to remove salts. These samples were analyzed by XRD (Multi Flex TTR-III; Rigaku Industrial Corp., Ltd.) using Cu K radiation at 40 kV and 150 mA. The microstructures of these deposits were characterized with OM (DFC320; Leica Microsystems). The surface appearance and micro-zone chemical analysis were also measured using SEM and EDS (JSM6480A; JEOL Co., Ltd.). Each sample was dissolved in aqua regia (HNO3:HCl:H2O=1:3:8, vol.%) to determine Mg, Li, and Yb contents. The solution was diluted and analyzed by ICP-AES (IRIS Intrepid II XSP, Thermo Elemental).

2 Results and discussion 2.1 Effect of electrolytic temperature on current efficiency The relationship between the current efficiency and electrolytic temperature is show in Fig. 1. Mg-Li-Yb alloys were prepared by electrolysis for 60 min using Mo electrode in KCl-LiCl-KF-MgCl2-Yb2O3 melts at 630–700 ºC. The original concentration of KCl was 40 wt.%, LiCl was 40 wt.%, KF was 10 wt.%, MgCl2 was 9.5 wt.%, Yb2O3 was 0.5 wt.%, the cathodic current density was 9.3 A/cm2. From Fig. 1, we can see that the current efficiency increases with the increase of electrolytic temperature, and reaches a maximum at the temperature around 660 ºC. Then, the current efficiency decreases apparently afterwards. While it is generally considered that the increase of temperature can accelerate the spread of diffusion of metallic ions and the deposition of metallic atoms as well, which can improve the current efficiency. But following a further increase of electrolytic temperature, the volatilization and burning of the metal will make loss of lithium. That is why the current efficiency decreased at higher temperature.

JOURNAL OF RARE EARTHS, Vol. 30, No. 2, Feb. 2012

using Mo electrode in KCl-LiCl-KF-MgCl2-Yb2O3 melts at 660 ºC. The original concentration of KCl was 40 wt.%, LiCl was 40 wt.%, KF was 10 wt.%, MgCl2 was 9.5 wt.%, Yb2O3 was 0.5 wt.%, the cathodic current density varied from 7.8 to 14 A/cm2. The relationship between current efficiency and cathodic current density is shown in Fig. 2. It is known that metallic atoms deposition rate on the molybdenum cathode and formation processes of the alloy are controlled by the cathodic current density. The current efficiency increases with an increase of the current density at the beginning, reaches a maximum about 69.3% at the current density around 9.3 A/cm2. However, under excessive high current density, the metallic Mg, Li and Yb cannot deposit at higher speed because the diěusion rate is much slower than discharge rate, and these metallic atoms will dissolve into the molten salt, resulting in the decrease of current efciency. 2.3 Effect of time on current efficiency Mg-Li-Yb alloys were prepared by electrolysis for 40 min to 100 min using Mo electrode in KCl-LiCl-KF-MgCl2Yb2O3 melts at 660 ºC. The original concentration of KCl was 40 wt.%, LiCl was 40 wt.%, KF was 10 wt.%, MgCl2 was 9.5 wt.%, Yb2O3 was 0.5 w.t %, the cathodic current density varied from 9.3 A/cm2. The current eĜciency increased along with electrolytic time extension rstly, and at 60 min, it arrived at the peak, and then decreased (see Fig. 3). In the beginning, the dis-

2.2 Effect of cathodic current density on current efficiency Mg-Li-Yb alloys were prepared by electrolysis for 60 min

Fig. 1 Relationship between current efficiency and electrolytic temperature

Fig. 2 Relationship between current efficiency and cathodic current density

Fig. 3 Relationship between current efficiency and temperature

CHEN Lijun et al., Electrochemical study on preparation of Mg-Li-Yb alloys in LiCl-KCl-KF-MgCl2-Yb2O3 melts

solubility of the alloy in this molten salt system was unsaturated, at the same time of deposition of Mg-Li-Yb alloys, these alloys dissolved continuously in the molten salt. In addition, it was diĜcult to collect the small amount of the alloy which was electrodeposited in short time. With the extension of time, the concentration of the metal ions reduced gradually, which caused enlargement of the electrolyte resistance. Besides, it may cause ablation if the alloy stayed in the molten salt for long time. So there was an appropriate electrolytic time (about 1 h). 2.4 Characterization of Mg-Li-Yb alloys Galvanostatic electrolysis was carried out in LiCl-KClKF-MgCl2-Yb2O3 melts at different conditions. Fig. 4 shows the XRD patterns of Mg-Li-Yb alloy samples. Sample (1) obtained by galvanostatic electrolysis with 3 A, 60 min at 660 ºC in the melts (LiCl:KCl:KF:MgCl2:Yb2O3=40:40:10:9.5:0.5, wt.%) on Mo electrodes; Sample (2) obtained by galvanostatic electrolysis with 3.5 A, 60 min at 660 ºC in the melts (LiCl: KCl:KF:MgCl2:Yb2O3=40:40:10:9.5:0.5, wt.%) on Mo electrodes; Sample (3) obtained by galvanostatic electrolysis with 4 A, 80 min at 680 ºC in the melts (LiCl:KCl: KF:MgCl2:Yb2O3=40:40:10.2:9.5:0.3, wt.%) on Mo electrodes. The results indicate that the main phases of the alloys are -Mg and Mg2Yb when the content of Li is low. When the

Fig. 4 X-ray diffraction patterns of different alloy samples obtained by galvanostatic electrolysis in the LiCl-KCl-KF-MgCl2Yb2O3 melts on Mo electrodes under different conditions (1) Mg-3.6Li-7.4Yb; (2) Mg-5.4Li-1.9Yb; (3) Mg-11.2Li-4.5Yb

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content of Li is high, the main phases are -Li and Mg2Yb. Besides, the ternary phase diagrams of Mg-Li-Yb can confirm the conclusion very well. After the phases of the samples were identified, OM was employed to observe the microstructures of the alloys. Fig. 5 shows optical micrograph of the Mg-Li-Yb alloys contained different contents of Yb. The microstructure of the Mg-Li-Yb alloys obtained by galvanostatic electrolysis with 3.5 A, 90 min at 660 ºC in the LiCl-KCl-KF-MgCl2-Yb2O3 (LiCl:KCl:KF:MgCl2:Yb2O3=40:40:10.2:9.5:0.3, wt.%) melts on Mo electrodes exhibits a dual-phase microstructure, as shown in Fig. 5(a). The result is in good agreement with the one obtained in the analysis of XRD. The bright and dark zones of the microstructure correspond to  and  phases, respectively. The  phase distributes evenly in  phase matrix in Fig. 5(a). The  phase is mainly dendritic in Mg-Li alloys. With the addition of Yb,  phase is refined and becomes spherical, as seen in Fig. 5(b). With the increase of Yb content in Mg-Li-Yb alloys, the microstructure of alloy changes from + to , as given in Fig. 5(b) and (c). The Mg-Li-Yb alloys sample b (obtained by galvanostatic electrolysis with 4 A, 80 min at 680 ºC in the melts (LiCl: KCl:KF:MgCl2:Yb2O3=40:40:10.2:9.5:0.3, wt.%) on Mo electrodes) and sample c (obtained by galvanostatic electrolysis with 3 A, electrolysis 60 min at 660 ºC in the melts (LiCl:KCl:KF:MgCl2:Yb2O3=40:40:10:9.5:0.5, wt.%) on Mo electrodes) show a gray microstructure of  phase, indicating that there is relatively high lithium content in Mg-Li- Yb alloys, and the grain of the Mg-Li-Yb alloys is refined. In order to examine the uniformity of the elements of Mg and Yb distributed in the Mg-Li-Yb alloys, mapping analysis of element was employed. Fig. 6 shows SEM morphology (Fig. 6(a)) and EPMA area analysis (Fig. 6(b) and (c)) of Mg-Li-Yb alloys prepared by galvanostatic electrolysis with 3 A, 60 min at 660 ºC in the melts (LiCl:KCl:KF:MgCl2: Yb2O3=40:40:10:9.5:0.5, wt.%) on Mo electrodes. The gray zone in the SEM corresponds to the bright  phase in the optical micrograph; the white zone in the SEM corresponds to the dark  phase in the optical micrograph. Fig. 6(b) shows that the element of Mg distributes homogeneously in the Mg-Li-Yb alloys. The distribution of Yb is not uniform in the alloy, they mainly distribute in the grain boundary of -Mg, as presented in Fig. 6(c).

Fig. 5 Optical micrographs of Mg-Li-Yb alloys obtained by galvanostatic electrolysis in the LiCl-KCl-KF-MgCl2-Yb2O3 melts on Mo electrodes under different conditions (a) Mg-7.8Li-2.4 Yb; (b) Mg-11.2Li-4.5Yb; (c) Mg-11.8Li-8.4Yb

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JOURNAL OF RARE EARTHS, Vol. 30, No. 2, Feb. 2012

Fig. 6 SEM and EPMA mapping analysis of the Mg-11.8Li-8.4Yb alloy obtained by galvanostatic electrolysis with 3 A, 60 min at 660 ºC in the melts (LiCl:KCl:KF:MgCl2:Yb2O3=40:40:10:9.5:0.5, wt.%) on Mo electrodes (a) SEM; (b) and (c) Area analysis of Mg and Yb in Mg-Li-Yb alloy

Fig. 7 SEM and EDS quantitative analysis of the Mg-11.8Li-8.4Yb alloy obtained by galvanostatic electrolysis with 3 A, 60 min at 660 ºC in the melts (LiCl:KCl:F:MgCl2:Yb2O3=40:40:10:9.5:0.5, wt.%) on Mo electrodes (a) SEM; (b) and (c) EDS patterns of points 002 and 003 Table 1 ICP analyses of samples obtained at different electrolysis conditions Samples

Temperature/ºC

Current/A

Time/h

MgCl2 concentration/wt.%

Yb2O3 concentration/wt.%

Yb content/wt.%

Li content/wt.%

1

660

3

60

9.5

0.5

7.4

3.6

2

660

3.5

60

9.5

0.5

1.9

5.4

3

680

4

80

9.5

0.3

4.5

11.2

4

660

3

60

9.5

0.5

8.4

11.8

5

660

3.5

90

9.5

0.3

2.4

7.8

6

680

3

40

9.5

0.3

3.6

8.3

However, it is difficult to estimate the main location of the distribution of the element Yb. Therefore, a SEM equipped with EDS quantitative analysis was applied to further investigate the distribution of the element Yb. Fig. 7 shows the SEM (Fig. 7(a)) and EDS quantitative analysis of the Mg-LiYb alloy obtained by galvanostatic electrolysis with 3 A, 60 min at 660 ºC in the melts (LiCl:KCl:KF:MgCl2:Yb2O3= 40:40:10:9.5:0.5, wt.%) on Mo electrodes. The EDS result of the points labeled 002 (Fig. 7(b)) and 003 (Fig. 7(c)) taken from grain boundaries and grains indicates that the distribution of Yb is more at grain boundaries than within the grains. EDS is a qualitative and semi-quantitative estimate of elemental concentration, and element Li cannot be detected in the EDS. Therefore, we carried out the ICP analysis of alloys. The ICP analyses of all samples obtained by galvanostatic electrolysis are listed in Table 1. The ICP results show that the chemical compositions of Mg-Li-Yb alloys are consistent with the phase structures of the XRD patterns, and the

Mg-Li-Yb alloy used for the analysis of SEM and EDS are identified as Mg-8.3Li-3.6Yb alloy. Under galvanostatic electrolysis, the lithium contents of Mg-Li-Yb alloys increased with the increase of cathodic current density. And when the concentration of Yb2O3 increased, the Yb contents of Mg-Li-Yb alloys increase. Based on these results, it can be concluded that the contents of Li and Yb in Mg-Li-Yb alloys can be adjusted by changing the electrolysis conditions.

3 Conclusions Mg-Li-Yb alloys were successfully prepared by codeposition in the molten LiCl-KCl-MgCl2-KF-Yb2O3 system at different electrolysis conditions. Both the electrolytic temperature and cathodic current density had obvious influence on current efficiency. When the electrolytic temperature was 660 ºC, cathodic current density was 9.3 A/cm2, and the

CHEN Lijun et al., Electrochemical study on preparation of Mg-Li-Yb alloys in LiCl-KCl-KF-MgCl2-Yb2O3 melts

electrolytic time was 1 h, current efficiency could reach the highest. XRD patterns indicated that the main phases of the alloys were -Mg and Mg2Yb when the content of Li was low. When the content of Li was high, the main phases are -Li and Mg2Yb. SEM images showed that the surface of the cross-section presented a reticulate structure, and the intermetallic compounds of Mg-Yb mainly distributed in the grain boundary. Mg distributed homogeneously in the alloys. ICP analyses of the samples obtained by electrolysis showed that of the lithium content in Mg-Li-Yb alloys could be controlled by changing the electrochemical parameters.

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