Production of Nd-Fe-B alloy powders using high-pressure gas atomization and their hard magnetic properties
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I.
INTRODUCTION
Nd,Fe-B permanent magnets have attracted much attention from both the scientific and engineering points of view because they possess the superior hard magnetic characteristics t1~ and active research presently being made with the aim of improving the coercive force, remanence, and energy product. The production methods of the Nd-Fe-B magnets can be roughly classified into two groups consisting of sintering t2j and rapid solidificationt3]processes. The former process produces an anisotropic permanent magnet which exhibits good hard magnetic properties in a bulk form. However, the anisotropic magnet produced by the powder metallurgical method has the disadvantage that the hard magnetic characteristics are lost in the powder form[4] and hence the magnet is unable to be used as a raw material for bonded powder magnets consisting of powder and polymer. On the other hand, the magnet produced by the latter (rapid solidification) process has isotropic characteristics which remain almost unchanged even after comminution into powders, ~3Jeven though the hard magnetic characteristics are inferior to those of the anisotropic magnets. The advantage enables us to use the Nd,Fe-B powders produced by comminution of the rapidly solidified ribbons as bonded powder magnets and hence all the commercial bonded powder magnets in the Nd'Fe-B system have been produced by the combined treatment of rapid solidification and comminution.t31 In addition to the above2described process of producing powders by comminution of rapidly solidified Nd-Fe-B ribbons, a high-pressure gas atomization technique has also been known ~'6] as a process of producing, directly from melt, fine spherical powders having clean and smooth surfaces. Application of the gas atomization technique to Nd-Fe-B alloys is expected to produce fine powders with MASAMI YAMAMOTO, Visiting Researcher, AKIHISA INOUE, Associate Professor, and TSUYOSHI MASUMOTO, Professor, are with Institute for Materials Research, Tohoku University, Sendai 980, Japan. Manuscript submitted April 6, 1988. METALLURGICAL TRANSACTIONS A
isotropic permanent magnetic properties. This paper attempts to clarify the shape, morphology, microstructure, and magnetic properties of Nd-Fe-B powders produced by high-pressure gas atomization and to investigate whether or not powder magnets with good permanent magnetic properties comparable to the commercial bonded Nd-Fe-B powder magnet can be produced by the present simple process. II.
EXPERIMENTAL DETAILS
The specimens used in the present work were Nd-Fe-B ternary alloys ranging from 14 to 18 at. pct Nd and 7.5 to 12 at. pct B. A mixture of pure Nd (96.4 wt pct + 2.3 wt pct Pr) and Fe (99.0 wt pct) metals and Fe-19.56 wt pct B alloy was melted in an argon atmosphere in an induction furnace and chill-cast in a copper mold to prepare the master alloys. The compositions were specified as chemically analyzed atomic percent. About 400 g of Nd-Fe-B alloy was charged into the atomization alumina crucible and remelted in an argon atmosphere. The details
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