Study On Preparation And Mechanism Of Mg-Li Based Alloy By Electrolysis In Molten Salt

Mg-Li alloys, is called superlight alloys. These alloys have great potential in the fields of industry for light materials. In general, Mg-Li alloys are conventionally prepared by directly mixing and fusing the metallic elements. In this thesis, Mg-Li based alloys are prepared by electrolysis in molten salts. Moreover, the electrochemical process, the electrochemical mechanism, phase structures of Mg-Li based alloys and so on were deeply investigated. In the aspects of the process of metallic deposition, the alloying procee, phase control of alloys and composition control of alloys, we obtained important achievements.In the first section of this thesis, Mg-Li alloys were prepared by cathodic alloying method on solid magnesium electrode in a molten LiCl-KCl (50:50 wt.%) system at 480℃. The phase structures of Mg-Li alloys could be controlled by parameters of electrolysis in molten salt system at low temperature, a, a+P andβphases Mg-Li alloys with the thickness of 182,365 and 2140μm were obtained by potentiostatic electrolysis at-2.26,-2.30 and-2.39 V (vs. Ag/AgCl) for 30 min, respectively.In the secnod section of this thesis, Mg-Li alloys were directly prepared by co-electrodeposition method on an inert electrode in a molten LiCl-KCl (50:50 wt.%) system at 670℃. The diffusion coefficient of magnesium ions in the melts was determined by cyclic voltammtery, chronopotentiometry and chronoampertry. The condition of co-electrodeposition of Mg and Li was investigated. Three kinds of phases Mg-Li alloys were prepared via galvanostatic electrolysis. The codepositon of Mg and Li happens when the current indensity exceed about 0.35 A-cm-2 (value of limited current density of MgCl2) in KCl-LiCl melts containing 5 wt.% MgCl2. Under the electrolytic condition of 6.21 A·cm-2 for 2 h, only MgCl2 concentration is lower than 10 wt.%, Mg-Li alloys can be obtained. The method of co-electrodeposition have some advantages, whish are short process time and low energy consumption.In the third section of this thesis, this work presents an electrochemical study on the codeposition of Mg, Li and X (X=Al, Zn and Y) on a molybdenum electrode in LiCl-KCl-MgCl2-XCln (X=Al, Zn and Y, n=2 or 3) melts at 670℃to form Mg-Li-X alloys. The electrochemical reduction process and co-electrodeposition conditions of Mg(Ⅱ), Li (Ⅰ) and the third alloying elements were investigated by different electrochemical techniques. The chronopotentiometry and chronoampertry were first applied to investigate deposition order of different metals and alloy formation process. Consequently, Mg-Li and Mg-Li based alloys were prepared by a new method of electrolysis in molten salts. This method can start from origin and incorporate co-electrodeposition, alloying and uniformity into an entire. In LiCl-KCl-MgCl2-XCln system, the first formed alloy is Mg-X alloy. The succeeding underpotential deposition of lithium on pre-deposited Mg-X leads to the formation of a Mg-Li-X alloy. The codeposition conditions of three kinds of Mg-Li based alloys is:The codepositon of Mg, Li and Al occurs at current densities lower than-0.47 A-cm-2 or electrode potentials more negative than-2.100 V in LiCl-KCl-MgCl2 (5 wt.%) melts containing 1 wt.% AlC13. The codepositon of Mg, Li, and Zn occurs at current densities lower than-0.78 A cm-2 or electrode potentials more negative than-2.000 V in LiCl-KCl-MgCl2 (8 wt.%) melts containing 1 wt.% ZnCl2. The codepositon of Mg, Li, and Y occurs at current densities lower than-0.47 A-cm-2 in LiCl-KCl-MgCl2 (5 wt.%) melts containing 5 wt.% YCl3.Mg-Li based alloys with different lithium and X (X=Al, Zn and Y) contents were obtained via potentiostatic and galvanostatic electrolysis. The obtained alloys were characterized by XRD, optical microscope, scanning electron microscopy, energy dispersive spectrometry and inductively coupled plasma (ICP). And the lithium and X contents of Mg-Li-X alloys can be controlled by MgCl2 and XCln concentrations and the electrolytic parameters.In this thesis, co-electodeposition mechanism of ternary Mg-Li-X (X=Al, Zn and Y) alloys was first investigated. The advantages low temperature electrolysis, the depolarization effect of liquid cathode and the effect of alloying make electrolysis condition moderate and is easy to bring about industrialization. All of this supply a reliable technical routine for new alloy materials.