Fundamental Study On Electrochemical Preparation Of Y-Ni-Al Alloys In Molten Chlorides

Aluminum alloy is one of the light alloys used for structural parts and has attracted wide attention due to its excellent mechanical,physical properties.With the rapid development of aerospace industry and military industry in China,the comprehensive performance of aluminum alloy needs to be further improved,and these properties can be improved by adding rare earth and iron family elements.Among these alloys,Y-Ni-Al alloys not only have low density,high temperature resistance,corrosion resistance,high tensile strength and other characteristics,but also can be applied to the preparation of hydrogen storage materials,and thus show great significance for aerospace industry,military industry and new energy materials of China.However,the conventional process of smelting for preparing Y-Ni-Al alloys may cause problems such as serious oxidation and burning of Y and Al,uneven composition of the alloys and so on.The molten salt electrolysis provides a great opportunity for producing active metals such as aluminum,magnesium,rare earth metals and their alloys.Therefore,molten salt electrolysis is adopted to prepare Y-Ni-Al alloys.The electrochemical behaviors of Y?III?and Al?III?in present work were systematically investigated by employing cyclic voltammetry,square wave voltammetry and open-circuit chronopotentiometry in NaCl-KCl molten salts at700�C.The reduction potential and reduction mechanism of these two ions on the inert electrode and Ni electrode,and the electrochemical formation process of Y-Al and Al-Ni alloys were analyzed.The structure and composition of the alloys were analyzed by XRD and SEM-EDS.In addition,the thermodynamic parameters of Y-Al and Al-Ni alloys were obtained by electromotive force method.Based on these works,Y-Ni-Al alloys were prepared by potentiostatic electrolysis on the cathode of nickel in NaCl-KCl-YCl₃-AlCl₃ molten salts and characterized by SEM-EDS.The research results are concluded as follows:?1?The results of cyclic voltammetry show that the initial reduction potential of Y?III?at W electrode is-2.1V?vs.Ag/AgCl?in NaCl-KCl molten salts at 700�C.The reduction of Y?III?to metallic Y is a single-step process with three lectrons exchanged and is considered to be a quasi-reversible process,which is controlled by diffusion at scanning rates of 0.1-0.5V/s.The diffusion coefficient of Y?III?ions is estimated to be about 2.8�10^-5cm²�s^-1.According to the analyses of cyclic voltammetry and open circuit chronopotentiometry in the NaCl-KCl-YCl₃-AlCl₃system,it can be deduced that Y?III?and Al?III?can be reduced and precipitated on the W electrode to form two intermetallic compounds YAl₃ and YAl₂.The Y-Al alloys were obtained by potentiostatic electrolysis at-2.2V in the NaCl-KCl-YCl₃-AlCl₃ molten salts for 10 h.The results of XRD and SEM-EDS analysis show that the obtained alloys were mainly composed of YAl₂,YAl₃ and YAl phases.?2?The results of Cyclic voltammetry and square wave voltammetry show that the reduction of Al?III?to metal Al on the W electrode is one step reduction reaction with three electron exchanged,and the initial reduction potential is-1.0 V?vs.Ag/AgCl?in NaCl-KCl molten salts at 700�C.Cyclic voltammetry,square wave voltammetry and open-circuit chronopotentiometry of NaCl-KCl-YCl₃-AlCl₃ system indicate that Al?III?can be reduced and precipitated on the Ni electrode and form Al₃Ni₂,AlNi and AlNi₃ intermetallic compounds.In the NaCl-KCl-1.5mol%AlCl₃molten salts,kinds of Al-Ni alloys can be produced by the potentiostatic electrolysis under different potentials.According to XRD and SEM-EDS analysis,the alloys are mainly composed of Al₃Ni₂ containing a small amount of AlNi,AlNi₃ and Al₃Ni by potentiostatic electrolysis at-1.2 V.The product mainly consists of AlNi and AlNi₃and a small amount of Al₃Ni intermetallic compounds by potentiostatic electrolysis at-1.1 and-1.0 V.?3?The results of electrochemical measurements represent that Y?III?could be reduced and precipitated on the Ni electrode and formed intermetallic compounds YNi₃,Y₂Ni₇ and YNi₅ when YCl₃ was added to the NaCl-KCl salts at 700�C.Y?III?and Al?III?can be reduced to form Y-Al alloys and Al-Ni alloys on the Ni electrode,and the Y-Ni phases disappeared after Al?III?adding to the NaCl-KCl-YCl₃ system.The alloys were obtained by potentiostatic electrolysis at-2.2 V on a Ni plate electrode for 5 h in NaCl-KCl-1.5mol%YCl₃-1.5mol%AlCl₃ molten salts at 700�C.The results of SEM-EDS analysis indicate that the product mainly consisted of YAl₃,Al,Al₃Ni₂ and AlNi,which are consistent well with the results of electrochemical measurements.However,Y-Ni-Al ternary alloys were prepared by step electrolysis,i.e.YCl₃ was added into NaCl-KCl molten salts with potentiostatic electrolysis at-2.2V for 6 h firstly and then AlCl₃ was added with potentiostatic electrolysis at-0.9 V for5h.The product consists of Al₃Ni₂,AlNi,Y₂Ni₇,Y₂Ni₇Al and YNi₇Al₂ phases.The results of this work show that it is feasible to prepare Y-Ni-Al alloys by molten salt electrolysis using Ni as a consumable cathode by co-deposition of Y and Al in chloride system,and provide theoretical knowledge and technical reference for the preparation of Y-Ni-Al alloys by molten salt electrolysis.