Effect of Zr substitution for rare earth on the thermal stability of melt-spun (Nd,Pr)-Fe-B powder and magnets

The effect of Zr substitution for rare earth on the thermal stability of melt-spun near-stoichiometric 2: 14: 1 composition of (Nd_0.75Pr_0.25)_12.5-xZr_xFe₈₂B_5.5 (x = 0 to 3) has been investigated from 20°C up to 125°C. It has been found that the temperature coefficient of remanence (a) of the melt-spun powder increases monotonically with increasing Zr content, from -0.141%/°C at x = 0 to -0.179%/°C with x = 3, whereas the temperature coefficient of coercivity (β) remains nearly unchanged at -0.392 ∼ -0.394%/°C regardless of the Zr content. The irreversible flux aging loss (δ_irr), measured on the epoxy bonded magnets (with pc = 2) after being exposed at 125°C for 100 hr, is found to increase monotonically with Zr content, from -2.00% at x = 0 to -3.98% with x = 3. The increase in a is believed to arise from the lower Curie temperature whereas the increased δ_irr is attributed to the combination of both the lower Curie temperature and the lower coercivity caused by the Zr substitution. The results indicated that Zr substitution for rare earth is detrimental to the thermal stability of the melt-spun R₂Fe₁₄B-type material.