Nanocrystalline Soft Magnetic Fe–M–B (M=Zr, Hf, Nb) Alloys Produced by Crystallization of Amorphous Phase (<I>Overview</I>)

TLDR

This review presents the development of Fe–M–B nanocrystalline alloys that combine high saturation magnetic flux density with excellent soft magnetic properties. The alloys are produced by crystallizing melt‑spun or sputtered amorphous Fe–M–B precursors to form a mostly single bcc phase with 10–20 nm grains and thin intergranular amorphous layers that provide strong magnetic coupling, low anisotropy, and high Curie temperature. The resulting Fe–M–B alloys exhibit saturation flux densities above 1.5 T, effective permeabilities exceeding 20 000 at 1 kHz, very low core losses, good thermal stability, and low stress sensitivity, making them suitable for transformers, inductors, and related devices.

Abstract

This paper reviews our results on the development of a new type of soft magnetic material with high saturation magnetic flux density (Bs) combined with excellent soft magnetic properties. A mostly single bcc structure composed of bcc grains with about 10–20 nm in size surrounded by a small amount of intergranular amorphous layers was obtained by crystallization of amorphous phases prepared by melt-spinning and sputtering technique in Fe-rich regions of Fe–M–B (M=Zr, Nb, Hf) ternary systems. The typical nanocrystalline bcc Fe90Zr7B3, Fe89Hf7B4 and Fe84Nb7B9 alloys subjected to the optimum annealing exhibit high Bs above 1.5 T as well as high effective permeability (μe) at 1 kHz above 20000.The high Bs for the Fe–M–B alloys is resulting from the high Fe concentrations owing to high glass-forming ability of M(Zr, Hf, Nb) and B. The origin of the good soft magnetic properties for the alloys are listed as follows. (1) The apparent anisotropy is decreased by the combined effects of the formation of the nanoscale bcc structure and the achievement of rather strong magnetic coupling between the bcc grains through the intergranular ferromagnetic amorphous phase. (2) The small saturation magnetostriction (λs) results from the nonequilibrium bcc phase. The solute-rich interglanular amorphous phase with high Curie temperature (Tc) and high thermal stability has an important role in the achievement of the good soft magnetic properties through the formation of the nanoscale bcc structure and the attainment of the rather strong magnetic coupling between the bcc grains.The soft magnetic properties of the nanocrystalline Fe–M–B alloys were improved through the decrease in the bcc grain size and the increase in Tc of the intergranular amorphous phase by optimizing heating rate in the crystallization process and adding small amounts of elements. For example, the improved Fe84Zr3.5Nb3.5B8Cu1 alloy shows the high μe of 100000 combined with the high Bs of 1.53 T. This excellent μe is comparable to those of nanocrystalline Fe73.5Si13.5B9Nb3Cu1 and the zero-magnetostrictive Co based amorphous alloys, and the high Bs is comparable to those of the Fe based amorphous alloys with good soft magnetic properties. The Fe–M–B based alloys also have very low core losses, the sufficient thermal stability and the low stress-sensibility of the soft magnetic properties. Therefore, the nanocrystalline Fe–M–B alloys are expected as practical magnetic materials for magnetic transformers, inductors, and other devices and parts.