First Principles Thermodynamics of Zr B Segregation at Grain Boundaries in Recycled Nd2Fe14B

Abstract

Grain-boundary chemistry is central to the coercivity and thermal stability of Nd2Fe14B permanent magnets, particularly in recycled magnets where recovery of the hard-magnetic phase does not by itself restore the intergranular microstructure. Our recent experimental study showed that nanoscale ZrB2 precipitates can emerge at grain boundaries and triple junctions during recycling despite an overall Zr concentration of only about 0.1 at.%, where they are associated with boundary stabilization and suppression of grain coarsening. Here we use spin-polarized density-functional theory with a Hubbard correction (DFT+U) to determine the atomistic thermodynamics underlying this preferential localization. A systematic set of composition-matched bulk/grain-boundary DFT+U supercells provides a common correlated-electron description across boundary chemistries. We compare B, Zr, Dy, and ZrB2-like local configurations in bulk and grain-boundary environments of Nd2Fe14B. Excess B is strongly stabilized at the boundary, while Zr also shows an independent thermodynamic preference for the interfacial region. When Zr and B are combined in a ZrB2-like configuration, the boundary preference is retained, indicating that the interface remains favorable as Zr-B coordination develops. These results establish a thermodynamic pathway for the co-localization of Zr and B prior to ZrB2 formation. Magnetic-state analysis further shows that all compared structures remain within the same high-moment Fe-sublattice regime, and boundary-localized defects generally perturb the normalized magnetization less than their matrix counterparts. The calculations therefore provide a first-principles explanation for why Zr-B chemistry concentrates at intergranular regions and how such boundary-localized states can support the microstructural stability required for high-coercivity recycled Nd-Fe-B magnets.