On the reversible and fully repeatable increase in coercive field of sintered Nd-Fe-B magnets following post sinter annealing

摘要

The effect of different heat treatments on coercivity (H_c) has been investigated in three different Nd-Fe-B magnets. These were: commercially available Dy-free and Dy-containing Nd-Fe-B sintered magnets, both with minor additions of Al and Cu, and a third Nd-Fe-B sintered magnet which contained no measurable Cu but a similar amount of Al to the other materials. Heat treating the materials in the as-sintered state at 500 ℃ for 1 h resulted in an increase in H_c of ≈ 30% in all cases. Taking these annealed materials and heat treating them at 1050 ℃ for 1 h resulted in a reduction in H_c of a similar magnitude. This process was repeated several times and H_c changed by ≈30% each time. The H_c increase was therefore not only reversible but also fully repeatable. The remanence and oxygen content were stable throughout. Experiments with annealing duration showed that the beneficial effects to H_c were complete after ＜ 5 min at 500 ℃. A simple mechanism to explain the results was proposed, which implied that differences in the distribution of the Nd-rich phases between the low and high H_c states should be present. Scanning and transmission electron microscopy confirmed the presence of such differences. In the Cu doped materials, the increase in H_c was linked to the melting of a Nd-Cu eutectic. This reaction was not observed in the low-Cu material, indicating that it was not essential for the increase in H_c.