Microstructural properties of electrochemically prepared Ni-Fe-W powders

Abstract

A nanostructured Ni-Fe-W powder was obtained by electrodeposition from ammonium citrate electrolyte within the current density range of 500-1000 mA cm -2 at the electrolyte temperature of 50°C-70°C. XRD analysis shows that the powder contains an amorphous matrix having embedded nanocrystals of the FCC solid solution of iron and tungsten in nickel, with an average crystal grain size of 3.4 nm, a high internal microstrain value and a high density of chaotically distributed dislocations. EDS analysis exhibits that the chemical composition of the Ni-24%Fe-11%W powder does not depend upon current density and electrolyte temperature due to the diffusion control of the process of codeposition of nickel, iron and tungsten. SEM micrographs show that the electrodeposition results in the formation of two particle shapes: large cauliflower-like particles and small dendrite particles. The cauliflower-like particles contain deep cavities at hydrogen evolution sites. Cavity density increases with increasing deposition current density. Smaller powder particles are formed at higher temperatures and at higher current densities. During the first heating, relative magnetic permeability decreases reaching the Curie temperature at about 350°C and after cooling exhibits a 12% increase due to the performed relaxation process. Following the second heating to 500°C, the magnetic permeability of the powder is about 5% lower than that of the as-prepared powder due to crystallization of the amorphous phase of the powder and the crystal grain growth in FCC phase. © 2012 Elsevier B.V. All rights reserved.