Study On Electrochemical Codeposition And Mechanism Of (Al,Mg)-Li-Er(Tm)Alloys

Al-Li and Mg-Li based alloys,as the important light structural materials,have been widely used in the fields of aerospace,and automobile,etc.Most Al-Li and Mg-Li based alloys are prepared by directly melting and mixing the metallic elements.In this paper,Al-Li and Mg-Li based alloys are prepared by electrochemical codeposition.The electrochemical behaviour and codepositon mechanism of Al(III),Li(I),Er(III),Mg(II),and Tm(III)were investigated by cyclic voltammograms,square wave voltammetry,and open circuit chronopotentiogram in LiCl-KCl melts.The alloys of Al-Li,Al-Er,Al-Li-Er,Mg-Li-Al-Er,and Al-Li-Er-Tm with different phases have been prepared by electrochemical codeposition,and characterized by ICP-AES,XRD,SEM,and EDS.Five main contents in this paper are as following:1.The electrochemical behaviour of Al(?)ions was studied in molten LiCl-KCl melts and Al-Li alloys were prepared by electrochemical codeposition.Cyclic voltammetry indicated that cathodic peak potential value of Al(?)was-1.25 V(vs.Ag/AgCl),and the reduction process was found to be reversible at low scan rates that was controlled by the rate of the mass transfer.The diffusion coefficient and actication energy for diffusion of Al(?)ions in LiCl-KCl-(1.0 wt.%)AlF3 melts were determined.The under potential deposition(UPD)of lithium on pre-deposited aluminium led to the formation of liquid Al-Li alloys at-2.25 V(vs.Ag/AgCl).Chronopotentiograms and chronoamperograms demonstrated that codeposition of Al(?)and Li(?)ions formed Al-Li alloys at cathodic current densities higher than 0.188 A cm-2 A cm-2 or cathodic potentials more negative than-2.30 V.Al-Li alloys with AlLi,Al2Li3,and Al4Lig phases were prepared by galvanostatic electrolysis.2.The electrochemical behaviour of Er(?)ions was studied in LiCl-KCl-ErCl3 melts,and Al-Er alloys were prepared.Cyclic voltammograms and square wave voltammetry were used to study the reduction mechanism of Er(?)on inert tungsten electrodes.The results showed that the cathodic peak potential value of Er(?)was-1.93 V(vs,Ag/AgCl),the reduction proceeded according to a one-step mechanism,and found to be reversible at low scan rates being controlled by the rate of the mass transfer.The diffusion coefficient of Er(?)ions at 793 K in LiCl-KCl-(1.0 wt.%)ErCl3 melts was determined.Cyclic voltammograms and open circuit chronopotentiogram showed that the underpotential deposition of erbium on A1 electrodes made the potential more positive and formed three Al-Er intermetallic compounds at-1.48 V,-1.61 V,and-1.71 V(vs.Ag/AgCl),respectively.Al3Er,Al2Er,Al2Er3 and AlLi phase of Al-Er and Al-Li-Er alloys were prepared by potentiostatic electrolysis performed at different constant potentials.The gibbs energy of formation of the Al3Er,Al2Er and Al2Er3 intermetallic compounds was calculated.The thermal effects of Er(?)and electrodes for the alloying reaction were found by Gibbs-Helmnoltz equation.3.The electrochemical behaviour of Al(?),Li(?),and Er(?)were investigated in LiCl-KCl-AlCl3-Er2O3 melts,and Al-Li-Er alloys were prepared by electrochemical codeposition.The results indicated that potential of Al and Er metal deposition were-1.02 V and-1.91V(v,s.Ag/AgCl),respectively.The under potential deposition(UPD)of Er(?)on pre-deposited aluminium led to the formation of two Al-Er intermetallic compounds at-1.39 V and-1.49 V(vs.Ag/AgCl).The electrochemical codepositon of Al,Li,and Er occurred at cathodic current densities higher than-0.32 A cm-2 or cathodic potentials more negative than-2.30 V(vs.Ag/AgCl).Cyclic voltammetry XRD analyses showed that the chlorination of Er2O3 by AICl3 led to the formation of ErCl3.Al-Li-Er alloys with phase of Al2Er,Al2Er3,and Al4Li9 were successfully prepared via galvanostatic electrolysis.SEM and EDS showed that Er mainly distributed at the grain boundary in the alloys4.The electrochemical behaviour of Al(?),Li(?),Er(?)and Tm(?)were investigated on tungsten electrodes in LiCl-KCl-AlF3-ErCl3-TmCl3 melts,and Al-Li-Er-Tm alloys were prepared by electrochemical codeposition.The results indicated that Tm(?)ions are reduced to metallic thulium through two consecutive steps at-1.56 V,and-2.12 V(vs.Ag/AgCl),respectively.The under potential deposition(UPD)of Er(?)and Tm(?)on pre-deposited aluminium led to the formation of liquid Al-RE(RE=Er,and Tm)alloys.The electrochemical codepositon of Al,Li,Er,and Tm occurred at cathodic current densities higher than-0.469 A cm-2 or cathodic potentials more negative than-2.32 V(vs.Ag/AgCl).Al-Li-Er-Tm alloys with strengthening phase Al3Er and Al3Tm were successfully prepared via galvanostatic electrolysis in LiCl-KCl-(8.0 wt.%-11.5 wt.%)AlF3-(1.0 wt.%)ErCl3-(1.0 wt.%)TmCl3 melts.SEM and EDS showed that Er and Tm elements mainly distributed at the grain boundary in the alloys and have remarkably effects on grain refinement.5.The electrochemical behaviour of Mg(?),Li(?),Al(?)and Er(?)were investigated by electrochemical techniques in LiCl-KCl-MgCl2-AlCl3-ErCl3 melts,and prepared Mg-Li-Al-Er alloys at 823 K.The results indicated that potential of Al,Mg,Er and Li metal deposition were-1.03 V,-1.86 V,-1.99 V,and-2.25 V(vs.Ag/AgCl),respectively.The under potential deposition(UPD)of Er(III)on pre-deposited aluminium led to the formation of two Al-Er intermetallic compounds at-1.42 V and-1.50 V(vs.Ag/AgCl).The codepositon of Mg,Li,Al and Er occurs at current densities more negative than-0.89 A cm-2 or cathodic potentials more negative than-2.40 V(vs.Ag/AgCl).XRD,SEM and EDS indicated that Mg17Al12,Al2Er,Al2Er3 and Al4Li9 phases were prepared by galvanostatic electrolysis.Er and Tm elements mainly distributed at the grain boundary in the alloys.