A Novel Magnetic Hardening Mechanism for Nd‐Fe‐B Permanent Magnets Based on Solid‐State Phase Transformation

Permanent magnets based on neodymium-iron-boron (Nd-Fe-B) alloys provide the highest performance and energy density, finding usage in many high-tech applications. Their magnetic performance relies on the intrinsic properties of the hard-magnetic Nd${}_2$Fe${}_{14}$B phase combined with control over the microstructure during production. In this study, a novel magnetic hardening mechanism is described in such materials based on a solid-state phase transformation. Using modified Nd-Fe-B alloys of the type Nd${}_{16}$Fe${}_{bal-x-y-z}$Co${}_x$Mo${}_y$Cu${}_z$B${}_7$ for the first time it is revealed how the microstructural transformation from the metastable Nd${}_2$Fe${}_{17}$B${}_x$ phase to the hard-magnetic Nd${}_2$Fe${}_{14}$B phase can be thermally controlled, leading to an astonishing increase in coercivity from ≈200 kAm${}^{-1}$ to almost 700 kAm${}^{-1}$. Furthermore, after thermally treating a quenched sample of Nd${}_{16}$Fe${}_{56}$Co${}_{20}$Mo${}_2$Cu${}_2$B${}_7$, the presence of Mo leads to the formation of fine FeMo${}_2$B${}_2$ precipitates, in the range from micrometers down to a few nanometers. These precipitates are responsible for the refinement of the Nd${}_2$Fe${}_{14}$B grains and so for the high coercivity. ... mehrThis mechanism can be incorporated into existing manufacturing processes and can prove to be applicable to novel fabrication routes for Nd-Fe-B magnets, such as additive manufacturing