Phase evolution, microstructure and magnetic properties of bulk α-Fe/Nd2Fe14B nanocomposite magnets prepared by severe plastic deformation and thermal annealing

In this study, the phase evolution, microstructure and magnetic properties of bulk alpha-Fe/Nd2Fe14B nanocomposite magnets prepared by a combination of severe plastic deformation (SPD) and thermal annealing have been investigated. Experimental results demonstrate that SPD affects the phase evolution of (Nd,Pr)-Fe-Co-Nb-B alloy upon annealing significantly. The formation of metastable intermediate phases, such as Nd2Fe23B3 and Fe3B, in the alloy has been effectively inhibited as the strain epsilon is increased to epsilon = 6.1, yielding a high volume fraction of alpha-Fe phase (V-c similar to 37%) in the bulk alpha-Fe/Nd2Fe14B magnets. The grain size of alpha-Fe and Nd2Fe14B phases in the magnets decreases with increasing strain. A smaller grain size d = 13.3 nm for alpha-Fe and 26.5 nm for Nd2Fe14B is achieved at epsilon = 6.1, and the magnets show the magnetic properties, (BH)(max) = 14.9 MGOe and H-c = 6.4 kOe, larger than those in the alpha-Fe/Nd2Fe14B magnets made by directly annealing amorphous ribbons, (BH)(max) = 10.3 MGOe and H-c = 4.6 kOe. The enhanced coercivity in the SPD magnets results from an increase in domain-wall-pinning strength. (C) 2015 Elsevier B.V. All rights reserved.