Soft magnetic properties of bulk nanocrystalline Fe-(Nb,Zr,Hf)-B alloys produced by extruding amorphous powders

Utilizing the softening phenomenon of amorphous alloys near the crystallization temperature, we have prepared bulk Fe84Nb7B9, Fe90Zr7B3 and Fe89Hf7B4 alloys by extruding amorphous powders. The extruded bulk alloys are composed of an amorphous phase containing a small amount of bcc phase in the as-extruded state, and the structure changes to a mostly single bce phase with grain sizes of 10-25 nm by annealing at 923 K. The soft magnetic properties of the bulk nanocrystalline alloys are better for the alloys containing smaller volume fractions of bce phase in the as-extruded state. This is presumably because the pre-existing bcc grains in the as-extruded state cause an inhomogeneous nanostructure after annealing. Of the three alloys, Fe90Zr7B3 shows the best tendency to consolidate to the bulk form, with a high relative density above 99%, even by extrusion at a relatively low pressure P-e = 860 MPa. This alloy exhibits the highest effective permeability mu(e) = 2350 at 300 Hz and flux density B-800, = 1.59 T, and the lowest H-c = 26 A/m.