Phase, microstructure and magnetic properties of 45.5Fe-28Cr-20Co-3Mo-1.5Ti-2Nb permanent magnet

Abstract

The 47.5-xFe-28Cr-20Co-3Mo-1.5Ti-xM; M = Nb; x = 0.5, 1, 1.5, 2.0, 2.5, 3.0 (wt%) permanent magnets have been investigated after solution annealing, thermomagnetic annealing and step-aging process. Alloys were prepared by vacuum melting and casting technique. Magnets with improved properties were developed through a modified thermal treatment cycle. Solution annealed specimens depict equiaxed α phase grain size of 250 µm while optimally thermomagnetic treated and step aged magnets yield ultra-fine Fe-Co nanorods (10 nm) precipitates in Cr-Ti-Nb matrix. The Nb addition to 47.5Fe-28Cr-20Co-3Mo-1.5Ti alloy expands the α phase structure by suppressing the nonmagnetic phases at annealed state and promotes shape anisotropy for spinodal phases in final treated samples leading to the improvements of magnetic properties. Spinodal phases preferentially aligned and elongated close to applied field direction and retained strong Goss {1 1 0}〈0 0 1〉 orientations. The 45.5Fe-28Cr-20Co-3Mo-1.5Ti-2Nb magnet reveals Hc of 86.9 kA/m, Br of 1.24 T and (BH)max of 57 kJ/m3. High coercivity and remanence in the magnets are attributed to the ideal shape anisotropy of Fe-Co rich nanorods or nanowires.