Soft-Magnetic Properties of Nanocrystalline bcc Fe–(Nb, Zr)–B Bulk Alloys Consolidated by Warm Extrusion

Abstract

We obtained Fe₈₄Nb₇B₉ bulk alloys by extruding amorphous powders at temperatures (T_e) between 653 and 723 K and pressures (P_e) of 820 to 1210 MPa, and Fe₉₀Zr₇B₃ bulk alloys by extruding at T_e of 673 to 698 K and P_e of 920 to 940 MPa. Subsequent annealing of these bulk alloys at 923 K for 3.6 ks causes the formation of a nanocrystalline bcc phase with grain sizes of 9 to 11 nm for Fe₈₄Nb₇B₉ and 20 to 30 nm for Fe₉₀Zr₇B₃. The bcc Fe₈₄Nb₇B₉ bulk alloy exhibits a magnetization (B₈₀₀) of 1.40 T, a permeability (μ_e) of 1120 at 300 Hz, and a coercive force (H_c) of 48 A/m in the extrusion condition of T_e=698 K and P_e=870 MPa. Similarly, the B₈₀₀, μ_e and H_c of the bcc Fe₉₀Zr₇B₃ bulk alloy prepared at T_e=698 K and P_e=923 MPa are 1.57 T, 1880 and 29 A/m, respectively. The soft-magnetic properties of the bcc Fe₉₀Zr₇B₃ bulk alloy are superior to those of the bcc Fe₈₄Nb₇B₉ bulk alloy, presumably because of the formation of a more homogeneous bcc structure for the former alloy.