Synthesis, Structure and Properties of Nickel-Iron-Tungsten Alloy Electrodeposits PART II: Effect of Microstructure on Hardness, Electrical and Magnetic Properties
Nanostructured nickel-iron-tungsten alloys were produced by electrodeposition from an ammoniacal citrate bath. The tungsten content of the alloy ranged from 0.8 wt.% to 11 wt.%, and the crystal grain size of the FCC phase of the solid solution of iron and tungsten in nickel was between 14 nm and 3.3 nm. The amorphous phase content of the alloy increases with decreasing crystal grain size. As the amorphous phase content increases, the magnetization, electrical conductivity and hardness of the alloy decrease. Annealing the alloy to crystallization temperature results in structural relaxation during which the alloy undergoes short-range ordering in conjunction with decreases in the density of chaotically distributed dislocations and internal microstrain level, which increases the exchange integral value, the electronic density of states at the Fermi level, the mean free path of electrons, the ordering and the mean size of cluster in the sliding plane and results in more uniform orientation of dipole moments of certain nanoparticles. These changes: a) increase the mobility of magnetic domain walls, facilitate the orientation of domains in the external magnetic field and cause an increase in magnetization; b) cause a decrease in electrical resistance, and c) impede the sliding of grain boundaries and increase the hardness of the alloy. Annealing the alloys at temperatures above 400ºC results in amorphous phase crystallization and larger crystal grains of the FCC phase, along with a decrease in the density of chaotically distributed dislocations and a decrease in internal microstrain level. The formation of larger crystal grains reduces the hardness of the alloy, decreases its specific electrical resistance and impedes both the orientation of certain magnetic domains and the shift of walls of already oriented domains, thus inducing a decrease in magnetization. The heat released during the milling of Ni87.3Fe11.3W1.4 alloy with FCC-phase crystal grains 8.8 nm in average size causes amorphous phase crystallization, FCC crystal grain growth and an increase in magnetization. Alloys with relatively high tungsten content (11 wt. %) have an inhomogeneous composition, a high proportion of the amorphous phase and FCC crystal grains with an average size of 3.3 nm. This microstructure results in magnetic domains that have different and relatively low thermal stabilities and relatively low degrees of magnetization.
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