Aluminum Substitutions in Nd-Fe-B Sintered Magnets
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ALUMINUM SUBSTITUTIONS IN ND-FE-B SINTERED MAGNETS J. K.CHEN* AND G. THOMAS** *Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720 "**Departmentof Materials Science and Mineral Engineering, and the National Center for Electron Microscopy, Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, University ot California, Berkeley, CA 94720.
ABSTRACT A microstructural and microanalytical study of aluminum substituted Nd-Fe-B sintered permanent magnets were carried out to determine the effect, ifany, of aluminum on structure, composition, and magnetic properties, particularly on the observed increase incoercivity. It was found that Al enrichment occurred in the Nd-rich phase at the grain boundaries. The possible role(s) of this enrichment on the observed coercivity increase are discussed.
INTRODUCTION Ever since its introduction about three years ago, the Nd-Fe-B ternary system has made a big impact on the permanent magnet industry. [1] Since then, many alloying substitutions have been carried out in hopes of increasing this system's magnetic properties. One of the most prominent substitutional elements for increasing the coercivity is aluminum. [2,3] In an effort to explain this effect, two Nd-Fe-B permanent magnet alloys were prepared in identical fashion, except that 2 at.% Al was substituted for B in the second alloy (see Table 1). Both alloys were also given the same, optimizing post-sintering heat treatments, after which the magnetic properties were measured. As clearly shown, the Al-substituted alloy has a 25% increase in coercivity, with only a 3% decrease in saturation magnetization, relative to the standard Nd-Fe-B alloy. This dramatic coercivity increase can be partially attributed to the increase in anisotropy field (HA) in Al-substituted Nd-Fe-B magnets, where Al substitutes for Fe inthe Nd2 Fe 1 4 B hard magnetic phase. [2] If the expected coercivity increase is scaled by the increase in anisotropy field
Mat. Res. Soc. Symp. Proc. Vol. 96. 1•987Materials Research Society
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Table I Processing Conditions and Magnetic Properties of filloys Studied Alloy Composition
Post-Sintering Heat Treatment 950°C/1 hr, slow cooling to 630°C/lhr, rapid cooling to RT
Nd14 Fe 78B 8 Nd Fe 14
78
B Al 6
it
2
B Sat [Tesla]
Coercivity [kA/cm]
1.28
6.9
1.24
8.6
alone, using values of HA from Young et. al., it is possible to determine to a first approximation the contribution of the anisotropy field to the coercivity increase. As shown in Fig. 1, these expected coercivities are far below the measured coercivity, leading to the conclusion that Al-induced structural or
9
2
I
4-) .,-I
Predicted Actual
7-1
0 U C
U0
0.00
0
0.02
i
i
0.04
0.06
0.08
X
Fig. 1.
Comparison of measured coercivity increase with predicted increase from variations in anisotropy field with x in Nd2 (Fe 1 _xAlx)1 4 B. HA values are based on measurements by Yang et. al. [2]
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compositional changes are the primary reason for the coercivity increase. In orde
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