Production of Nd-Fe-B alloy powders using high-pressure gas atomization and their hard magnetic properties

Abstract

Fine spherical Nd−Fe−B powders with a tetragonal Nd₂Fe₁₄B phase have been produced by high pressure argon or helium atomization. The average size defined by 50 pct cumulative weight fraction is as small as about 25 μm. The Curie temperature of the powders is about 580 K and the intrinsic coercivity (iH _c ) of the bonded products made from the powders increases with decreasing particle size and reaches about 0.581 MA/m for the powder below 25 μm diameter. TheiH _c value increases with an increase in the cooling rate by helium atomization as well as with an increase in neodymium and boron content to 18 at. pct Nd and 12 at. pct B with the highestiH _c value reaching 0.716 MA/m. Annealing in the range of 773 to 1073 K gives rise to a further increase ofiH _c to 1.035 MA/m. The highiH _c value is promising for practical use as isotropic bonded powder magnets. The increase of magnetization for the bonded powder magnets takes place rapidly and the behavior is similar to that for sintered Nd−Fe−B magnets, in goods contrast to a sluggish increase of magnetization for the bonded Nd−Fe−B powder magnets made from the comminuted powders of melt-spun ribbon. From the magnetization behavior, it was presumed that the generation of the large intrinsic coercivity is due to the difficulty of the nucleation of reverse domain rather than the pinning of domain walls.