A study on the role of Nb in melt-spun nanocrystalline Nd-Fe-B magnets

The effect of Nb substitution on the thermal stability of melt-spun powders with near stoichiometric 2:14:1 composition of Nd₁₂Fe _82-xNb_xB₆ (x = 0, 1.5 and 3) has been investigated. It has been found that the thermal stability is significantly improved with 1.5 at% Nb substitution. As compared with the ternary Nd ₁₂Fe₈₂B₆, the Nb-substituted Nd ₁₂Fe_80.5Nb_1.5B₆ powder exhibits remarkably increased coercivity (H_ci) over a wide temperature range of 22-180°C. The temperature coefficient of coercivity (β) is reduced from -0.36%/°C at x = 0 to -0.32%/°C at x = 1.5. As a result of improved coercivity and its temperature dependence, the irreversible flux aging loss (δ_irr), measured on the epoxy bonded magnets after being exposed at 180°C for 100 h, is also significantly decreased from -13.7% at x = 0 to -5.0% with x = 1.5. Microstructure studies using X-ray diffraction and transmission electron microscopy have shown a significant microstructure refinement with Nb substitution. Nb substitution also increases the amount of α-Fe phase in the alloys. For x = 0, the average grain size of the magnetically soft phase (α-Fe) is 15 nm, while the hard phase (Nd ₂Fe₁₄B) has an average grain size of 30 nm. The soft and hard magnetically phases are reduced to <10 and 10-20 nm, respectively for x = 1.5 and 3. Enriched Nb along the grain boundaries is believed to be the main reason for the observed improved thermal stability in Nb-substituted powders.