Structure and magnetic properties of electrodeposited Ni87.3Fe11.3W1.4 alloy

Abstract

A dark gray nanostructured coating of an alloy composed of 87.3wt.% Ni, 11.3wt.% Fe and 1.4wt.% W (Ni87.3Fe11.3W1.4) was electrodeposited from an ammonia citrate bath on a titanium cathode at a current density of 500mAcm−2.A cathodic polarization curve was recorded and dependence of the current efficiency of alloy deposition on current density was determined. Partial polarization curves for alloy deposition and hydrogen evolution were also measured. Alloy deposition at current densities of up to 300mAcm−2 is an activation-controlled process which turns into a diffusion-controlled process at higher current densities. At potentials more positive than −960mV, hydrogen is evolved from NH4+ and (HCit)3− ions (where (HCit)3− denotes triply deprotonated citric acid, C6H5O73−). At potentials more negative than −960mV, hydrogen evolution from water is the dominating reaction.SEM images show that the surface of the deposit obtained at 500mAcm−2 has a globular structure containing a large number of craters, mostly located between the globules.XRD analysis revealed that the alloy contains an amorphous matrix with embedded nanocrystals of the FCC-structured solid solution of Fe and W in Ni with a mean particle size of 8.8nm. The deposit has a high internal microstrain value and a high density of chaotically distributed dislocations.Heating and milling the alloy cause structural changes involving changes in the magnetic properties of the alloy. Structural relaxation takes place in the temperature interval of 80°C tо 420°C. In this temperature range, magnetization of both as-deposited and milled alloy samples increases with increasing temperature, reaching maximum at a certain temperature, but decreases thereafter with further heating. During structural relaxation, short-term structural arrangement facilitates the expansion and orientation of magnetic domains, leading to increased magnetization of the alloy. The abrupt decline in magnetization at higher temperatures is the result of a heat-induced change in magnetic domain orientation.Annealing the alloy at temperatures above 420°C causes amorphous phase crystallization and growth of crystal grains of the FCC-structured solid solution of Fe and W in Ni, as well as a simultaneous decrease in internal microstrain and mean density of chaotically distributed dislocations. The same structural changes, somewhat lower in intensity, are also caused by alloy milling. The new state of the microstructure achieved through annealing and milling is best illustrated by the mean crystal size. The increase in mean crystal size results in a shift of the Curie temperature towards lower temperatures, whereas magnetization increases at first, reaching maximum at a certain mean crystal size, but decreases, thereafter, with a further increase in mean crystal size.