Thermal and magnetic properties of Fe₅₆Co₇Ni₇Zr_10-xNb_xB ₂₀ amorphous alloys with wide supercooled liquid range

Amorphous alloys with a wide supercooled liquid region of 45 to 85 K were found to be formed in Fe₅₆Co₇Ni₇Zr_10-xNb_xB ₂₀ (x=0 to 10 at%) alloys by melt spinning. The glass transition temperature (T_g) and the crystallization temperature (T_x) increase by the dissolution of 2 to 4%Nb. The degree of increase is larger for T_x than T_g, leading to the maximum ΔT_x(=T_x-T_g) of 85 K for the 2%Nb alloy. The ΔT_x value is about 20 K larger than the largest value for the newly developed Fe-(Al, Ga)-(P, C, B, Si) amorphous alloys. The crystallization occurs through a single stage, amorphous→α-Fe+γ-Fe+Fe₂Zr+Fe₇₆Nb ₆B₁₈, for the alloys containing less than about 6%Nb and through two stages, amorphous→amorphous+γ-Fe→γ-Fe+Co₃Nb ₂B₅+Ni₈Nb, for the alloys containing more than 8%Nb. The change in the crystallization process for the Nb-rich alloys probably reflects the disappearance of Fe₂(Nb, Zr) precipitates because of the weaker bonding of Fe-Nb pair as compared with Fe-Zr one. As the Nb content increases, the saturation magnetization (I_s) and permeability (μ_e) of the annealed alloys decrease while the coercive force (H_c) remains almost unchanged. The good soft magnetic properties are obtained for the alloys containing less than 2%Nb subjected to annealing for 300 s at 800 K and the I_s, H_c and μ_e at 1 kHz are 0.96 T, 2.0 A/m and 19100, respectively, for the 0%Nb alloy and 0.75 T, 1.1 A/m and 25000, respectively, for the 2%Nb alloy. The success of synthesizing the new amorphous alloys with a wide supercooled liquid region more than 80 K and with good soft magnetic properties is promising for future development as soft magnetic bulk amorphous alloys.