Preparation of ternary Mg-Li-Sn alloys from molten salt by electrolysis

Transcription

1 Available online at Acta Metall. Sin.(Engl. Lett.)Vol.25 No.4 pp August 2012 Preparation of ternary Mg-Li-Sn alloys from molten salt by electrolysis Peng CAO, Milin ZHANG, Wei HAN, Yongde YAN and Lijun CHEN Key Laboratory of Superlight Materials and Surface Technology, Ministry of Education, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin , China Manuscript received 4 December 2011; in revised form 6 February 2012 Electrochemical behavior of Mg, Li and Sn on tungsten electrodes in LiCl-KCl-MgCl 2- SnCl 2 melts at 873 K was investigated. Cyclic voltammograms (CVs) showed that the underpotential deposition (UPD) of magnesium on pre deposited tin leads to the formation of a Mg-Sn alloy, and the succeeding underpotential deposition of lithium on pre-deposited Mg-Sn alloy leads to the formation of a Mg-Li-Sn alloy. Chronopotentiometric measurements indicated that the codepositon of Mg, Li and Sn occurs at current densities more negative than 1.16 A cm 2. X-ray diffraction (XRD) indicated that Mg 2Sn phase is formed via galvanostatic electrolysis. The element Mg distributes homogeneously and Sn locates mainly on the grain boundaries in the Mg- Li-Sn alloy. KEY WORDS Mg-Li-Sn alloy; Molten salt; Cyclic voltammetry; Electrolysis 1 Introduction Magnesium-lithium alloys exhibit comparable properties to other light metal alloys based on aluminum, titanium and beryllium, and receives increasing attention in industrial application, such as in the military, electronic and aerospace industries [1]. However, low strength, poor creep resistance and thermal stability limit their wide application. As a result, a renewed interest in the research and development of magnesium-lithium alloys has focused on improving specific mechanical properties at elevated temperatures. Recent studies by different investigators on Mg-Li-RE (RE = rare earth elements) casting alloys have shown some strengthening effect on magnesium alloys [2]. However, Mg-Li-RE alloys offer only limited application because of the high cost of rare earth elements. It has also been reported that AZ91, as a commercial alloy, improves mechanical properties of magnesium alloys, but due to the formation of Mg 17 Al 12 phase at the grain boundaries, this alloy exhibits poor creep resistance at an elevated temperature [3 5]. According to the Mg-Sn binary phase diagram [6], the main intermetallic phase is Mg 2 Sn with a higher melting point (1043 K) than that of Mg 17 Al 12 phase (735 K) in the Mg-Al system. Mg-Li- Sn based alloys are therefore likely to have potential application at elevated temperatures than Mg-Al alloys. Liu et al [7] have reported that the tensile and creep behaviors of Mg alloys have been improved by the formation of Mg 2 Sn. However, the traditional method Corresponding author. Professor, PhD; Tel: ; Fax: zhangmilin@hrbeu.edu.cn (Milin ZHANG)

2 266 for preparing Mg-Li-Sn based alloys is mainly by mixing pure magnesium lithium and tin directly. This conventional method has many disadvantages, such as unhomogeneous alloy composition, a complicated production process, serious metal oxidation and highenergy consumption [8 10]. To overcome these problems, some electrochemical preparation methods for this kind of alloys have been investigated [11 13]. These methods have the following merits: the phases and component of the alloy could be controlled; the alloys could be prepared on a certain electrode; the deposition speed is fast. This work is concerned with the electrochemical deposition of Mg, Li, and Sn directly from LiCl-KCl-MgCl 2 -SnCl 2 melts. 2 Experimental A mixture of LiCl-KCl (50:50 wt pct, analytical grade) was melted in an alumina crucible placed in a quartz cell inside an electric furnace. The working temperature was measured with a thermocouple protected by an alumina tube inserted into the melt. Magnesium and tin ions were introduced into the bath in the form of anhydrous MgCl 2 and SnCl 2 powder. All experiments were carried out under a carefully purified and dehydrated argon atmosphere. The electrochemical techniques were performed using Im6eX electrochemical workstation (Zahner Co., Ltd.). The working electrodes were 1 mm tungsten (d=1 mm). The active electrode surface was determined after each experiment by measuring the immersion depth of the electrode in the molten salt. The reference electrode used was a solution of LiCl-KCl-AgCl (1 mass pct) prepared in a Pyrex tube. All potentials were referred to this Ag + /Ag couple. A graphite rod (d=6 mm) served as a counter electrode. The deposits prepared by galvanostatic electrolysis were analyzed by XRD (X Pert Pro; Philips Co., Ltd.), OM (DFC320; Leica Microsystems) and SEM/EDS (JSM 6480A; JEOL Co., Ltd.). 3 Results and Discussion The cyclic voltammograms for a tungsten electrode in LiCl KCl melts, as the control, show no significant peaks within the electrochemical window, except a couple of cathodic/anodic peaks (B/B ) corresponding to the reduction and oxidation of Li, respectively (curve (1) in Fig. 1). After the addition of 0.35 wt pct SnCl 2, peak A found at a more positive potentials ( 0.29 V vs Ag + /Ag) is attributed to the reduction of Sn (II) to Sn (0). Peak A shows the reoxidation of the deposited Sn metal on the tungsten electrode. We also observed a number of cathodic and anodic peaks corresponding to the formation and dissolution of different intermetallic compounds, due to the different formation energies of these phases rendering different dissolution potentials. From curve (2), when the potential sweep is made progressively more positive, additional oxidation peaks more positive than Li start to form. As the potential changes, Sn-Li alloys and their intermetallic compounds changes in composition. Figure 2 shows a representative example of cyclic voltammograms obtained at a tungsten electrode after the addition of 6.0 wt pct MgCl 2 and 0.35 wt pct SnCl 2 in LiCl-KCl melts. In addition to the peaks (labeled A and A ) corresponding to the reduction of Sn (II) and the oxidation of Sn, respectively, a clear cathodic peak (labeled D) at a potential of 1.60 V is observed, which is thought to correspond to the UPD of Mg on the liquid Sn

3 267 j / A cm -2 B' (1) B A' (2) A A' A E / V vs Ag + /Ag j / A cm C' B' A' D A C B E / V vs Ag + /Ag Fig. 1 Cyclic voltammograms for a tungsten electrode in LiCl-KCl melts before (curve 1) and after the addition of 0.35 wt pct SnCl 2 (curve 2) (scan rate 0.1 V s 1 ) Fig. 2 Cyclic voltammograms for a tungsten electrode in the LiCl-KCl melts containing 0.35 wt pct SnCl 2 and 6.0 wt pct MgCl 2 already deposited on the tungsten electrode, probably the due to the formation of a Mg-Sn deposit. After this peak, the typical of a reduction of the metal cation to the corresponding metal at the potential of 1.75 V (peak C) on the forward scan, and followed by the dissolution of the deposited metal (peak C ) on the reverse scan are observed. This pair of peaks is related to the reduction of Mg (II) and the oxidation of deposited Mg, respectively. Another couple of cathodic/anodic signals, which is ascribed to the formation of liquid lithium electrodeposition/oxidation (labeled B and B ), is observed. Figure 3 presents chronopotentiograms measured on the tungsten electrode in LiCl-KCl melts containing 6.0 wt pct MgCl 2 and 0.35 wt pct SnCl 2 at different current intensities. Four potential plateaus are readily observed in the potential range 2.50 to 0 V. Plateau 1 corresponds to the reduction of tin. At the cathodic current lower than 100 ma (current density 0.31 A cm 2 ), the curves clearly exhibit another two potential plateaus (plateaus 2 and 3), which are associated with the reduction of magnesium ions to the corresponding metals or form Mg-Sn phases on Sn-coated electrode. When the current reaches 375 ma (current density 1.1 A cm 2 ), fourth plateau appears which is caused by a reduction of lithium ions. At this current intensity, codeposition of Mg, Li and Sn occurs. It is evident that the potential ranges for the deposition of Mg, Li and Sn are the same as those observed in the cyclic voltammograms. Based on the results obtained by cyclic voltammetrys and chronopotentiometrys, codeposited samples were prepared E / V vs Ag/AgCl ma -175 ma ma -50 ma -100 ma -225 ma -275 ma ma -400 ma t / s Fig. 3 Chronopotentiograms for a tungsten electrode in the LiCl-KCl melts containing 0.35 wt pct SnCl 2 and 6.0 wt pct MgCl 2 3

4 268 by galvanostatic electrolysis for 2 h using tungsten electrodes in a molten LiCl-KCl- MgCl 2 -SnCl 2 system. As seen from the XRD patterns (Fig. 4), Mg-Li-Sn alloys are composed of α+β+mg 2 Sn and α+mg 2 Sn phases, which proves that the first formed Mg-Sn alloy is Mg 2 Sn phase. Moreover, the lithium contents of Mg-Li-Sn alloys increase (from α+β to β phase) with the decrease of MgCl 2 concentrations in the LiCl-KCl-SnCl 2 (3.4 wt pct) melts at constant current intensity. According to the fundamentals of solidification and the Mg-Sn alloy binary phase diagram, the solubility of Sn in the α solid solution drops sharply with decreasing temperature. The phase diagram of the Mg-Sn alloy system also shows that the equilibrium phase is Mg 2 Sn, which has a high melting temperature of about 1043 K. Thus, stable Mg 2 Sn particles can be formed during solidification and at the same time the grain growth is further inhibited. Observations by optical microscopy and SEM show that Mg-Li-Sn phases were clearly produced in the alloy. From optical and SEM images of Mg-Li-Sn Intensity / a.u. Intensity / a.u (a) / deg (b) / deg -Mg -Li Mg 2 Sn -Mg Mg 2 Sn Fig. 4 XRD patterns of deposits obtained by galvanostatic electrolysis on W electrodes in the LiCl-KCl melts: (a) 9 wt pct MgCl 2 and 3.4 wt pct SnCl 2; (b) 10 wt pct MgCl 2 and 3.4 wt pct SnCl 2 alloy, the alloy exhibits a continuous or semicontinuous microstructure (Figs. 5 and 6). To examine the distribution of elements of Mg and Sn in the microstructure of the Mg-Li-Sn alloy, a SEM equipped with EDS quantitative analysis was applied, the results of which are presented in Figs. 7 and 8. The result of Fig. 7 confirms that the element Mg distributes homogeneously and Sn locates mainly on grain boundaries in the Mg-Li-Sn alloy. The EDS results of the points labeled A and B taken from grain boundary and inner grain Fig. 5 Optical micrographs of the Mg-Li-Sn alloys by codeposition from LiCl-KCl-MgCl 2 (10 wt pct) melts containing 3.4 wt pct SnCl 2 at 873 K

5 269 Fig. 6 SEM images of the Mg-Li-Sn alloys by codeposition from LiCl-KCl-MgCl 2 (10 wt pct) melts containing 3.4 wt pct SnCl 2 at 873 K Fig. 7 SEM morphology (a) and EDS mapping analysis (b, c) of the Mg-Li-Sn alloy by codeposition from LiCl-KCl-MgCl 2 (9 wt pct) melts containing 3.4 wt pct SnCl 2 Fig. 8 Chemical constitutions of point A (a) and point B (b) in Fig. 7 obtained by EDS analysis indicate that the deposits are composed of elements of Mg, Sn, O and C, as shown in Fig. 8. Because of the existence of α phase in the eutectic phase, the amount of Mg atom was relatively high in the EDS analysis. The result also confirms that a greater amount of Sn concentrates at the grain boundaries is due to the formation of the intermetallic phase and the reduction of atoms diffusion rates during the solidification process. As windows in front of the Si (Li) detector can absorb low energy X-rays, the presence of elements with atomic number less than 4 can not be detected by EDS detec-

6 270 Table 1 ICP analyses of all samples obtained by galvanostatic electrolysis (2 A) on W electrodes from the LiCl-KCl melts containing different MgCl 2 and SnCl 2 concentrations for 2 h Sample MgCl 2 concentration SnCl 2 concentration Li content Sn content Mg content wt pct wt pct wt pct wt pct wt pct Bal Bal Bal Bal Bal Bal. tor, such as element Li. Therefore, we carried out inductively coupled plasma (ICP) analyses of all samples obtained by galvanostatic electrolysis (Table 1). The lower the MgCl 2 concentration is in the LiCl-KCl melts with equivalent SnCl 2 concentration at a constant current intensity, the higher the lithium content of the Mg-Li-Sn alloys is. The tin contents in Mg-Li-Sn alloys increase with the increase of SnCl 2 concentrations in LiCl- KCl-MgCl 2 melts. These results indicate that the lithium and tin contents in Mg-Li-Sn alloys are adjustable simply by changing the concentrations of MgCl 2 and SnCl 2. 4 Conclusion Electrochemical codeposition properties of Mg, Li and Sn were studied by various electrochemical techniques in LiCl-KCl melts at 943 K. For the tungsten electrode, the experiment results obtained by cyclic voltammetry and chronopotentiogram showed that Mg-Li-Sn alloys can be directly prepared via codeposition of Mg, Li and Sn from LiCl- KCl-MgCl 2 SnCl 2 melts. The results showed that the potential of Sn, Mg and Li metal deposition was 0.29, 1.75 and 2.00 V vs Ag/AgCl, respectively. In addition, the underpotential deposition of magnesium on pre deposited tin was 1.60 V. The electrochemical codeposition of Mg, Li and Sn metals occurred at current densities lower than 1.16 A cm 2. The Mg-Li-Sn alloys with α-mg and Mg 2 Sn, α-mg, β Li and Mg 2 Sn were obtained by galvanostatic electrolysis at 2 A for 2 h with different concentrations of MgCl 2 and SnCl 2, respectively. The analysis of the microstructures shows that the grain size of the Mg-Li-Sn alloys decreases with increasing of Sn content. With more addition of SnCl 2, enough Sn atoms concentrate at the grain boundaries and react with Mg to form compound-mg 2 Sn phase. Acknowledgements This work was financially supported by National High Technical Research and Development Programme of China (Nos. 2011AA03A409 and 2009AA050702), National Basic Research Program of China (No. 2007CB200906), the National Natural Science Foundation of China (Nos , and ) and the Fundamental Research Funds for the Central Universities. REFERENCES [1] H. Takuda, T. Enami, K. Kubota and N. Hatta, J Mater Process Technol 101 (2000) 281 [2] B. Liu, M.L. Zhang and R.Z. Wu, Mater Sci Eng A 487 (2008) 347

7 271 [3] Y. Wang, G. Liu and Z. Fan, Acta Mater 54 (2006) 689 [4] P. Bakke and H. Westengen, Adv Eng Mater 5 (2003) 879 [5] J. Zhang, Z.X. Guo, F.S Pan, Z.S. Li and X.D. Luo, Mater Sci Eng A 456 (2007) 43 [6] A. Kozlov, M. Ohno, R. Arroyave, Z.K. Liu and R. Schmid-Fetzer, Intermetallics 16 (2008) 299 [7] H.M. Liu, Y.G. Chen, Y.B. Tang, S.H. Wei and G. Niu, J Alloy Compd 440 (2007) 122 [8] C.H. Chiu, H.Y. Wu, J.Y. Wang and S. Lee, J Alloy Compd 460 (2008) 246 [9] Y.N. Lin, H.Y. Wu, G.Z. Zhou, C.H. Chiu and S. Lee, Mater Design 29 (2008) 2061 [10] H. Takuda, S. Kikuchi, T. Tsukada, K. Kubota and N. Hatt, Mater Sci Eng A 271 (1999) 251 [11] B.J. Piersma and D.M. Ryan, J Electrochem Soc 143 (1996) 908 [12] A.M. Martínez, B. Bφresen, G.M. Haarberg, Y. Castrillejo and R. Tunold, J Appl Electrochem 34 (2004) 1271 [13] A. Krishnan, X.G. Lu and U.B. Pal, Metall Mater Trans B 36 (2005) 463