The mechanism of magnetic properties improvement and microstructure refinement of Zr in Nd ₂Fe ₁₄B

In order to understand the role of Zr addition in Nd2 Fe14 B (2-14-1), a systematic study of Zr in Nd2 Fe14 B was done on a series of stoichiometric samples (Nd1-x Zrx) 2 Fe14 B with 0≤x≤0.16. As-cast ingots formed significant amounts of Nd2 Fe17 starting at x=0.06, while in the melt-spun ribbons, no Nd2 Fe17 was observed for the range studied. A monotonic decrease in lattice parameters for the 2-14-1 phase indicate Zr solubility in the melt-spun ribbons, but this is not observed in the ingots. Magnetic and thermal measurements on the ribbons showed that the saturated magnetization Ms (around 16 kG), the spin reorientation temperature (125 K), and the Curie temperature (583 K) do not depend on the Zr content, while the anisotropy field Ha slightly decreased as the Zr content increased. The energy product (B Hmax) and the coercive force Hc are highly dependent on the solidification and annealing process. Scanning electron microscopy and transmission electron microscopy on the as-melt spun alloys are consistent with x-ray diffraction showing Zr solubility in the 2-14-1 grains. After annealing, Zr diffuses to the grain boundaries to form Zr enriched compounds. Thus, while Zr may exhibit a metastable solubility in Nd2 Fe14 B, the enhanced magnetic behaviors observed in melt-spun ribbons are due to the refinement of the microstructure and enhanced grain-boundary pinning.