Sintered Nd-Fe-B magnets from gas-atomized powders and the effect of lubricants

Abstract

The production of sintered Nd-Fe-B magnets currently relies on alloys solidified via strip casting. However, the spacing of Nd2Fe14B lamellae in the strip-cast alloys is not optimal for obtaining monocrystalline powders with a mean particle size significantly smaller than 3 μm. Magnets made of finer monocrystalline powder are expected to have smaller grains and a higher coercivity, which would reduce current reliance on scarce and expensive heavy-rare-earth elements. Because of finer Nd2Fe14B grains, gas atomized Nd-Fe-B alloys generally yield more homogeneous fine powders than the strip-cast alloys. In this study, sintered Nd-Fe-B magnets were prepared from gas-atomized alloy and their properties were compared with those of magnets made from the strip-cast alloy of the same chemical composition. The gas-atomized alloy was found to contain an additional metastable phase of the TbCu7-type structure. As compared to the strip-cast alloy, the gas-atomized alloy yielded a jet-milled powder with smaller mean particle diameter and sintered magnets with finer grains and a higher coercivity. The metastable phase specific to the gas-atomized alloys was eventually eliminated during the sintering process without damaging the magnetic properties. Treatment of the new ultrafine powders prior to their compaction in a magnetic field with lubricants ensured a higher degree of the crystallographic alignment and lead to a higher coercivity while still maintaining high remanence. In particular, a magnet prepared from the gas-atomized alloy treated with methyl octanoate exhibited a remanence of 1.38 T, a coercivity of 1278 kA/m, and a maximum energy product of 359 kJ/m3. The results indicate that a simultaneous utilization of the gas-atomized alloys and appropriate lubricants may reduce the need for the heavy rare earths, making the magnets more sustainable.