Misch-Metal and/or Aluminum Substitutions in ND-FE-B Permanent Magnets
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MISCH-METAL AND/OR ALUMINUM SUBSTITUTIONS IN ND-FE-B PERMANENT MAGNETS B. M. MA AND C. J. WILLZIAN Crucible Research Center, P. 0. Box 88, Pittsburgh, PA 15230 ABSTRACT The effects of misch-metal--and/or Al substitutions in the Nd-Fe-B alloy system have been investigated. Selected compositions were processed into magnets using the conventional powder metallurgy technique. As expected, misch-metal substitutions act to degrade the intrinsic magnetic characteristics and thus degrade the properties of the sintered magnet. It was observed that at a level of 25% of the total rare earth being comprised of misch-metal, energy products of the order of 25 MGOe were still able to be achieved. Processing of such material is difficult because higher sintering temperatures are required to fully densify the magnet, and the grain growth that occurs at these temperatures dramatically reduces the coercivity. The effects of misch-metal substitutions are quantified in this discussion. Al was found to be beneficial for improving the coercivity in both the Nd-Fe-B and misch-metal-Fe-B alloy systems. Unfortunately Al substitutions reduce both the remanence and the Curie temperature. If the Al content is kept to 1 wt.% or less, magnets exhibiting reasonable properties can be made. Al doping as opposed to doping with expensive heavy rare earth elements such as Th or Dy is a less costly method of improving Hc, in the Nd-Fe-B magnets. INTRODUCTION Energy products up to 52 MGOe have been reported in the conventional Nd-Fe-B alloy system.'. 2 Such an alloy is characterized by a saturation magnetization, MK, of 16 kG and a maximum theoretical energy product of 64 MGOe. The base alloy is characterized by a modest anisotropy field of about 65 to 70 kOe.Z Dy2Fe, 4 B and ThFe, 4B have been observed to have the largest anisotropy fields of the R2Fe, 4 B compounds. Partial replacement of Nd by these heavy rare earth elements, Dy or Th, leads to a large improvement in the intrinsic coercivity of sintered RE-Fe-B magnets. 3 Al substitutions into the Nd-Fe-B alloy system were investigated as a less costly means of increasing the anisotropy field in hopes of improving the intrinsic coercivity. While some investigators have observed an increase in anisotropy field at certain Al levels 4 , others have observed degradation of anisotropy field upon Al substitution. 5 However, most investigators agree 7 4 that Al is beneficial to intrinsic coercivity. ,6, One of the most attractive features of the Nd-Fe-B magnets is their comparatively low S/lb. and S/energy product cost when compared to SmCos type magnets.8 Complete replacement of Nd by misch-metal does not appear very promising for magnets produced by the powder metallurgy technique, however, partial replacement is feasible. 9 The intrinsic coercivity is far too low for most applications. A quantitative investigation on the effects of the least costly form of rare earths, misch-metal, on the properties of RE-Fe-B and RE-Fe-B-Al magnets was conducted.
Mat. Res. Soc. Symp. Proc. Vol. 96. '1987 Materials Rese
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