Segregation in Aluminum-Zirconium Alloys, Electrodeposited from Ionic Liquids, and Its Modification By Pulse Plating

Abstract

It is quite typical for aluminum-zirconium alloys, obtained by electrodeposition in chloroaluminate ionic liquids, to have two or more distinct phases with different Zr concentrations. The Zr content in one phase can be several times higher than in the other. In the present work we have studied this segregation in [EMIM]Cl*AlCl3 ionic liquid. Factors like temperature, current density, agitation, and concentrations of both zirconium and aluminum in the electrolyte were investigated. A proposed mechanism for this phenomenon is based on a possibility of two different pathways of zirconium reduction. One pathway is a 4-electron process, taking Zr4+ directly to metal. Another way is a 2-electron reaction via intermediate Zr2+ ions formed in the solution. The difference between these pathways includes variations in chloride ions concentration in the vicinity of the electrode surface, as well as variations in charge and structure of the double layer. The above variations exist only near the cathode surface, and have a limited life time. One approach to eliminating the segregation in the alloy is to utilize pulsed electrodeposition methods, allowing re-mixing and equilibration of the electrolyte during the off periods. As expected, shorter pulses and longer off times lead to more compositionally uniform deposits. Figure 1. Backscatter SEM image of a cross section of Al-Zr deposit. Light areas are zirconium-rich. Figure 1