Wide supercooled liquid region and soft magnetic properties of Fe56Co7Ni7Zr0–10Nb (or Ta)0–10B20 amorphous alloys

An amorphous phase with a wide supercooled liquid region before crystallization was formed in ${\mathrm{Fe}}_{\mathrm{56}}{\mathrm{Co}}_{\mathrm{7}}{\mathrm{Ni}}_{\mathrm{7}}{\mathrm{Zr}}_{\text{10−x}}{\mathrm{M}}_x{\mathrm{B}}_{\mathrm{20}}$ (M=Nb or Ta, $\text{x=0–10\hspace{0.17em}}\mathrm{at.}\mathrm{\hspace{0.17em}\%}$ ) alloys by melt spinning. The glass transition temperature ${\mathrm{(T}}_g)$ and crystallization temperature ${\mathrm{(T}}_x)$ increase by the dissolution of 2% M and the degree of the increase is larger for $T_x,$ leading to maximum ${\mathrm{\Delta T}}_x{\mathrm{(=T}}_x{\mathrm{-T}}_g)$ of 85 K at 2% Nb and 87 K at 2% Ta which are larger by about 20 K than the largest value for newly developed Fe–(Al, Ga)–(P,C,B,Si) amorphous alloys. The crystallization of the Nb-containing alloys occurs through two stages of amorphous ${\mathrm{(Am)\to Am}}^{\mathrm{\prime }}{\mathrm{+\alpha -Fe+\gamma -Fe+Fe}}_{\mathrm{76}}{\mathrm{Nb}}_{\mathrm{6}}{\mathrm{B}}_{\mathrm{18}}$ ${\mathrm{\to \alpha -Fe+\gamma -Fe+Fe}}_{\mathrm{76}}{\mathrm{Nb}}_{\mathrm{6}}{\mathrm{B}}_{\mathrm{18}}{\mathrm{+Fe}}_{\mathrm{2}}\mathrm{Zr}$ in the range less than about 6% Nb and ${\mathrm{Am\to Am}}^{\prime }\mathrm{+\gamma -}\mathrm{Fe\to \gamma -}{\mathrm{Fe+Co}}_{\mathrm{3}}{\mathrm{Nb}}_{\mathrm{2}}{\mathrm{B}}_{\mathrm{5}}{\mathrm{+Ni}}_{\mathrm{8}}\mathrm{Nb}$ in the range above 8% Nb. The change in the crystallization process with Nb content seems to reflect the easy precipitation of γ-Fe by the increase in the number of Fe–Nb pairs with weaker bonding nature as compared with the Fe–Zr pairs. The best soft magnetic properties were obtained at 2% Nb or 8% Ta. The saturation magnetization, coercive force, effective permeability at 1 kHz, and saturated magnetostriction in the annealed state for 300 s at 800 K are, respectively, 0.96 T, 2.0 A/m, 19 100, and ${\text{10×10}}^{\text{−6}}$ for the 10% Zr alloy, 0.75 T, 1.1 A/m, 25 000, and ${\text{12×10}}^{\text{−6}}$ for the 2% Nb alloy, and 0.85 T, 1.5 A/m 17 400, and ${\text{14×10}}^{\text{−6}}$ for the 8% Ta alloy. The Curie temperature is 531 K for the 2% Nb alloy and 538 K for the 8% Ta alloy. The success in synthesizing the new amorphous alloys with the wide supercooled liquid region and good soft magnetic properties is promising for future development as soft magnetic bulk amorphous alloys.