Magnetic properties and core losses of nanocrystalline FeMB (M Zr, Hf or Nb) alloys

A mostly single b.c.c. phase with nanoscale grain sizes of 10-20 nm was found to form by annealing amorphous FeZrB, FeHfB and FeNbB alloys prepared by the melt-spinning method for 3.6 ks in the temperature range 773-923 K. The highest μ_c and B_s values reach 22 000 and 1.63 T for Fe₉₀Zr₇B₃, 32 000 and 1.59 T for Fe₈₉Hf₇B₄ and 22 000 and 1.49 T for Fe₈₄Nb₇B₉ alloys. The low core loss of 0.066 W kg^-1 at 1 T and 50 Hz was confirmed for the b.c.c. Fe₈₉Hf₇B₄ alloy and is considerably smaller than that of a commercial Fe-based amorphous alloy being used in practical uses as core materials in transformer. The magnetostriction λ_s of FeMB alloys shows a tendency to decrease with increasing annealing temperature and is as small as -1 × 10^-6 for the b.c.c. Fe₈₉Hf₇B₄ alloy. The small λ_s and the small grain size are concluded to be the reasons for the good soft magnetic properties. The residual existence of the amorphous phase with a large amount of solute elements in the grain boundaries of the b.c.c. phase was observed by high resolution transmission electron microscopy observations on the Fe₈₄Nb₇B₉ alloy annealed for 3.6 ks at 923 K and is presumed to suppress the grain growth of the nanocrystalline b.c.c. phase.