Magnetic Properties of Extruded Nanocrystalline Fe-Nb-B Alloys.

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Abstract
A bulk Fe84Nb7B9 alloy was produced by extruding amorphous powder at temperatures (Te) between 653 and 723 K, at pressures (Pe) between 824 and 1208 MPa, and at a speed (Ve) of 5 mm/s. The subsequent annealing of the extruded bulk alloy for 3.6 ks at temperatures between 873 and 973 K allowed the formation of a mostly single bcc phase with a grain size of about 10 nm. The bulk alloy extruded at Pe above 1000 MPa has a high relative density of 99%. The soft magnetic properties are better for bulks extruded at lower pressures, and the bulk alloy extruded at Te=698 K and Pe=870 MPa exhibits a magnetization (B800) of 1.40 T, a permeability of 1123, and a coercive force (Hc) of 47 A/m, which are inferior to those (1.49 T, 22000, and 8 A/m) for the bee ribbon alloy. The difference between the soft magnetic properties of the bulk and the ribbon forms is presumably due to the inhomogeneity of the microstructure, the oxidation of the powder, and the residual existence of higher internal stress in the bulk alloy.