Phase relations and magnetic properties of new phases in the Fe-Nd-Al and Fe-Nd-C systems and their influence on magnets

The drastic increase in coercivity observed after adding Al in Fe-Nd-B-based magnet alloys seems to originate partially from the occurrence of new Al-stabilized phases in the grain boundary region and at the surface of the Fe₁₄Nd₂B grains. To describe these phases, they were synthesized and their properties were determined. One of these phases is antiferromagnetic with a high susceptibility of 0.05, while a second one is ferromagnetic with an anisotropy field higher than that of the Fe₁₄Nd₂B (Φ) phase. Magnetization is only half of the Φ phase because of a higher Nd content. The phase relations of these phases in the Fe-Nd-Al system are reported. It is pointed out that it is impossible so far to produce hard magnets from Fe-Nd-C by a sintering process due to phase relations and an inhibition of the formation of Fe₁₄Nd₂ C. The low nucleation and growth rate of Fe₁₄Nd₂ C can be accelerated by the addition of small amounts of a fourth element to the Fe-Nd-C material. The stable phase relations reveal the occurrence of new magnetically hard phases as well as very corrosive phases, which make this part of the system unsuitable for hard magnet production