Thermal Stability and Soft Magnetic Properties of Co–Fe–M–B (M=Nb, Zr) Amorphous Alloys with Large Supercooled Liquid Region

Abstract

A ferromagnetic amorphous phase with a large supercooled liquid region prior to crystallization and good soft magnetic properties was formed in melt-spun Co₄₀Fe₂₂Nb_8−xZr_xB₃₀ (x=0, 2, 4, 6, and 8 at%) alloys. The glass transition temperature (T_g) increases monotonously with increasing Zr content. The crystallization temperature (T_x) also increases by the dissolution of Zr up to 2% and then decreases with further increasing Zr content. The increases is more significant for T_x, resulting in a maximum ΔT_x(=T_x−T_g) of 98 K at 2% Zr. The Co-based amorphous alloys exhibit good soft magnetic properties, i.e., saturation magnetization of 0.41 T–0.42 T, low saturated magnetostriction of 2.4×10⁻⁶–2.9×10⁻⁶, low coercivity of 1.2–2.0 A/m, and high permeability of 29000–33000 at 1 kHz over the whole Zr content range. In addition, the cylindrical Co₄₀Fe₂₂Nb₆Zr₂B₃₀ glassy alloy with a diameter of 1.0 mm was prepared by the copper mold casting method. With increasing the cylindrical diameter up to 1.5 mm, the structure changes to coexistent amorphous, Co₂₁Nb₂B₆, Co₃ZrB₂ and Co₂B phases. It is noticed that the maximum thickness for glass formation is about 6 times larger than that for Co-based amorphous alloys reported up to date. The success of forming Co-based bulk amorphous alloys with high stability of supercooled liquid and good soft magnetic properties is encouraging further development of bulk amorphous soft magnetic materials.