Nanocrystallization of Amorphous Fe–Zr–B–Cu Alloys and Low Frequency Soft Magnetic Properties

Abstract

In nanocrystalline alloys reported to date, Fe–Zr–B(–Cu) alloys shows the highest saturation magnetization (higher than Fe–B–Si amorphous alloys), they are thus expected to be applied to power transformers used at B_m=1.3∼1.4 T and f=50∼60 Hz. Usually they are expected to be used in high frequencies because of their excellent permeabilities. The low frequency (50 Hz) magnetic properties of Fe₈₆Zr_xB_13−xCu₁ nanocrystalline ribbons have been examined from the viewpoint of application to power transformers. Core loss W_13⁄50 (1.3 T, 50 Hz)=0.07 W/kg was observed in the annealed Fe₈₆Zr₅B₈Cu₁ alloy. Magnetic induction B_1k (in a field of 1 kA/m) is 1.54 T for the same sample. In annealing Fe₈₆Zr_xB_13−xCu₁ (x=5∼7) amorphous alloys, ultrafine bcc-Fe grains about 15 nm in size were obtained with remaining stable amorphous phases. The stable amorphous phase makes the grains very fine. The stability of the amorphous phase is related to the liquidus temperature T_L that was measured during cooling and solidifying of the molten alloy of each composition in a furnace. When the grains grew during annealing, the amorphous phase changed its composition to exhibit lower T_L and became more stable. The core loss W_13⁄50 shows the W∼D⁶ dependence in the range of D=15∼25 nm. However, when the grain size was D=8 nm, the core loss does not show the D⁶ dependence. The nanocrystalline Fe–Zr–B–Cu ribbons shows poor ductility, which should be improved if the materials are to be used in actual pole transformers.