Rapid coercivity increment of Nd-Fe-B sintered magnets by Dy ₆₉Ni₃₁ grain boundary restructuring

To enhance coercivity and reduce the rare-earth consumption of Nd-Fe-B sintered magnets simultaneously, low melting point Dy₆₉Ni ₃₁ (at%) powders are introduced into a slightly off-stoichiometric Nd₂Fe₁₄B (2:14:1 phase) (Pr, Nd)_12.36Fe _balB_6.09 (at%) starting magnet. Because of insufficient rare-earth element, the starting magnet possesses discontinuous intergranular grain boundaries (GBs) that cannot decouple the 2:14:1 phase grains well, hence leading to a low coercivity of only 9.53 kOe. The Dy₆₉Ni₃₁ additive provides extra rare-earth that improves the liquid-phase sintering with the formation of continuous intergranular GBs, accompanied by the diffusion of Dy towards surface region of the 2:14:1 phase grains. A significant increment in coercivity of 6.25 kOe is realized with a slight reduction in remanence by -3.0% per unit at% Dy. Satisfactory magnetic performance with (BH)_max=47.49 MGOe, B_r=13.92 kGs and H_cj=13.93 kOe, is obtained in the magnet containing only 12.92 at% rare-earth elements, in which 0.68 at% Dy is included. It combines advantages of the modification of intergranular GBs and the formation of a magnetically hardening shell surrounding 2:14:1 phase grains, which can be a promising approach to fabricate low rare-earth and high performance Nd-Fe-B magnets for mass production.