Study On Preparation And Mechanism Of Al-Sc And Mg-Li Based Alloys By Electrolysis In Molten Salt

Al based and Mg-Li based alloys are called light alloys. These alloys have great potential in the fields of industry for light materials. In general, Al based and Mg-Li based alloys are conventionally prepared by metal mixing method and metallic thermic reduction. In this thesis, Al based and Mg-Li based alloys are prepared by electrolysis in molten salts. Moreover, the electrochemical mechanism, the electrochemical process and microstructure of alloys were deeply investigated.In the first section of this thesis, the electrochemical reduction process and co-electrodeposition conditions of Al3+ and Sc3+ on a molybdenum electrode in LiCl-KCl-AlF3 and LiCl-KCl-AlF3-Sc2O3 melts at 700℃were investigated by different electrochemical techniques. The results showed that, the reduction processes of the Al3+ and Sc3+ ions were diffusion-controlled charge-transfer reversible processes. However, there were kind of irreversible in the processes. The codepositon of Al and Sc happened when the current indensity lower than -0.35 A·cm-2 or electrode potentials more negative than -1.7 V in KCl-LiCl melts containing 1.4 wt.% AlF3 and 1.1 wt.% Sc2O3. The codepositon of Al and Sc occured at current densities lower than -0.20 A-cm-2 or electrode potentials more negative than -1.65 V in LiCl-KCl-2.1 wt.% AlF3-2.1 wt.% Sc2O3 melts. Al-Sc alloys were obtained by galvanostatic electrolysis in LiCl-KCl-AlF3-Sc2O3 melts at a temperature range of 650℃-740℃. The highest current efficiency can reach to 93%. The main phases of the obtained alloys were a-Al and Al3Sc, and the distribution of AI3SC was even. There was no segregation in the obtained Al-Sc alloys.In the secnod section of this thesis, the Mg-Li-Ce-La quaternary alloys were obtained in LiCl-KCl-MgCl2-KF-RE2 (CP3)3 melts by galvanostatic electrolysis in the temperature range of 680℃-800℃. The electrolysis parameters and microstructure of alloys were also studied. The codeposition of Mg, Li, Ce and La occurred when the concentration of MgCl2 reached to 5 wt.%. The current efficiency can reach to 55% when cathodic current density was 15.9 A-cm-2, electrolytic temperature is 750℃and electrolytic time was 2 hours. XRD patterns indicated that the main phases were a-Mg, CeMg12 and La2Mg17 of low Li contents alloys, and when the contents of Li were high, the main phases wereβ-Li, CeMg3 and LaMg3. The surface of the cross-section presented a reticulate structure. The intermetallic compounds mainly distributed in the grain boundary.In the third section of this thesis, the electrochemical reduction process of Sc3+ on a molybdenum electrode was investigated in LiCl-KCl-ScCl3(0.66wt.%) melts at 600℃-750℃. The reduction process of Sc3+ ions was a diffusion-controlled reversible process. The diffusion coefficient of scandium ions in the melts was determined by cyclic voltammtery and chronopotentiometry. The activation energy EA was obtained about 54.52 kJ-mol-1 by Arrhenius's law. The electrochemical mechanism on the codeposition of Mg, Li and Sc on a molybdenum electrode was studied in LiCl-KCl-ScCl3(1.43 wt.%)-MgCl2(1.1 wt.%) melts at 650℃. The deposition potentials of Mg2+ and Sc3+ were very close, so the codeposition of Mg and Sc occured easily. Then, underpotential deposition of lithium occured in the Mg-Sc alloy pre-deposited on the molybdenum. The codepositon of Mg and Sc occured at current densities lower than-0.07 A·cm-2, and the codepositon of Mg, Li and Sc occured at current densities lower than-0.08 A·cm-2.