B.M. Ma

Recent development in bonded NdFeB magnets

The magnetic properties of isotropic NdFeB powders used for bonded magnets are compared. The characteristics of two newly developed powders, namely MQP-13-9 and MQP-S-9-8 were compared with respect to existing powders. The advantages of spherical powders for injection molding were highlighted. Development targets for the 180°C application and NdFeB nanocomposites were also discussed.

Comparison of the corrosion behavior of die-upset and sintered NdFeB magnets

Corrosion rates for sintered NdFeB magnets are compared with NdFeB magnets prepared from melt spun material (hot pressed and die upset). Corrosion rates were determined by standard electrochemical methods in 0.9 M Na/sub 2/SO/sub 4/, at pH 2.4 and 4.4. Autoclave tests correlated quite well with corrosion rates measured electrochemically in the more aggressive pH electrolyte. The more aggressive pH 2.4 electrolyte is more representative of the conditions occurring in the autoclave tests. The hot pressed and die upset magnets exhibit significantly lower corrosion rates than sintered magnets. The reduction in corrosion rate is probably due to smaller grain size and diminished amounts of Nd-rich material.

The effects of refractory metals on the magnetic properties of /spl alpha/-Fe/R/sub 2/Fe/sub 14/B-type nanocomposites

The phase transformations and magnetic properties of rare earth lean and boron rich (Nd/sub 0.95/La/sub 0.05/)/sub x/Fe/sub bal./M/sub 2/B/sub 10.5/, where x=9.5 or 11 and M=Cr, Ti, Nb, V, Mo, Zr, Nf, Ta, Mn or W, melt spun ribbons have been investigated. Two magnetic phases, namely /spl alpha/-Fe and R/sub 2/Fe/sub 14/B, were found in ribbons studied. A third magnetic phase, the R/sub 2/Fe/sub 23/B/sub 3/, was also detected in (Nd/sub 0.95/La/sub 0.05/)/sub 9.5/Fe/sub 78/M/sub 2/B/sub 10.5/ (M=Mo and Mn). Remanence (B/sub r/) and coercivity (/sub i/H/sub c/) values in the range of 8.0 to 9.1 kG and 9.5 to 18.9 kOe, respectively, have been achieved in nanocomposites with merely two magnetic phases. Among compositions studied, the Ti and W-substitutions were found to be most effective in increasing the B/sub r/ and /sub i/H/sub c/, respectively. For a fixed refractory metal substitution, namely, M=Cr, Ti or Nb, an increase in the total rare earth concentration resulted in nanocomposites of small grain sizes and a high volume fraction of the R/sub 2/Fe/sub 14/B phase. The combination of fine grain size and high volume fraction of R/sub 2/Fe/sub 14/B phase led to an increase in the /sub i/H/sub c/ and maximum energy product, (BH)/sub max/ of nanocomposites studied. A B/sub r/ of 9.1 kG, /sub i/H/sub c/ of 16.7 kOe and (BH)/sub max/ of 16.8 MGOe have been obtained on (Nd/sub 0.95/La/sub 0.05/)/sub 11/Fe/sub 76.5/Ti/sub 2/B/sub 10.5/.

The dependence of magnetic properties and hot workability of rare earth-iron-boride magnets upon composition

Hot deformed magnets can be commercially produced from hyper-stoichiometric nanocrystalline Nd-Fe-B compositions. This paper summarizes the influence of the rare-earth component (Nd, Pr, Dy) and gallium to magnetic properties and hot workability of die-upset magnets based upon hyper-stoichiometric compositions. Increasing the Nd from 13.3 to 14.4 at.% raises the H/sub ci/ from 770 to 930 kAm/sup -1/. B/sub r/ values are highest (1.31 T) at lower levels of RE (/spl sim/13 at.%), although such compositions exhibit a decrease in hot workability. The substitution of Dy and Pr for Nd promotes higher H/sub ci/ values at some expense in B/sub r/. Gallium additions up to 0.5 at.% significantly increase both B/sub r/ and H/sub ci/.

Magnetic properties enhancement of nanocomposite Nd9.5Fe81.5B9 melt–spun ribbons by La and Cr substitutions

The phase transformation and magnetic properties of nanocomposite (Nd0.95La0.05)9.5(Fe1−xCrx)81.5B9, where x=0, 0.033, 0.067, and 0.1, melt–spun ribbons have been studied. For Cr-free ribbons (i.e, x=0,) a slight substitution of La for Nd decreases the crystallization temperature of the over-quenched ribbons. Moreover, the (Nd, La)3Fe62B14 and (Nd, La)2Fe23B3 phases were found to coexist with (Nd, La)2Fe14B/Fe3B/α-Fe phases in the crystallized (Nd0.95La0.05)9.5Fe81.5B9 ribbons. The crystallization temperatures of (Nd0.95La0.05)9.5Fe1−xCrx)81.5B9 were found to increase with Cr concentration. The intrinsic coercivity of (Nd0.95La0.05)9.5(Fe1−xCrx)81.5B9 increased from 6.2 kOe for x=0 to 10.7 kOe for x=0.1: Transmission electron microscopy-energy dispersion analytical x-ray analyses indicated that the increase in intrinsic coercivity may be associated with the increase in Cr concentration around grain boundaries. A sample with Br=8.3 kG, iH0=9.1 kOe, and (BH)max=12.5 MGOe was obtained on (Nd0.95La0.05)9.5(Fe...

Microstructure and magnetic properties of melt-spun Sm(Co0.58Fe0.31Cu0.04Zr0.05B0.02)z ribbons

We have investigated the microstructures of melt-spun Sm(Co0.58Fe0.31Cu0.04Zr0.05B0.02)z (z=7.5 and 12) ribbons by transmission electron microscopy (TEM) and three dimensional atom probe (3DAP) to correlate them with the hard magnetic properties. The ribbon with z=7.5 has a high coercivity (iHc∼12 kOe) while the ribbon with z=12 possesses a low coercivity (iHc∼4.9 kOe). The high iHc ribbon comprises of nanocrystals (∼10 nm) having 1:7H (TbCu7) type structure as main phase. However, in the low iHc ribbon, the microstructure consists of both 1:7H and 2:17H (Th2Ni17) phases with crystal size ranging from 50 to 80 nm. The low coercivity ribbon also contains (CoFeZr)23B6 (M23B6 type) soft magnetic phase. The low coercivity obtained in the melt-spun ribbon with z=12 is due to: (i) the formation of Th2Ni17 type low anisotropy phase, (ii) the presence of boron-rich soft magnetic phase, and (iii) larger crystal size. Realization of high coercivity in the ribbon with z=7.5 is due to the formation of 1:7H, a high an...

Phase evolution and magnetic properties of boron enriched (Nd0.7Pr0.25La0.05)xFebal.Co10Ti2By (x=4.5–10.7; y=10.9–18.6) nanocomposites

Abstract Alloy ribbons of (Nd 0.7 Pr 0.25 La 0.05 ) x Fe bal. Co 10 Ti 2 B y , where ( x , y )=(10.7, 10.9), (10.3, 11.3), (9.7, 12.1), (8.4, 13.7), (7.1, 15.3), (5.8, 16.9) and (4.5, 18.6), were prepared by melt spinning method to investigate the phase evolution and key factors controlling their magnetic properties. X-ray diffraction and thermal magnetic analysis (TMA) revealed that the phase mixtures of these alloy ribbons changed in the sequence of α-Fe/R 2 Fe 14 B→R 2 Fe 23 B 3 /R 2 Fe 14 B→R 2 Fe 23 B 3 →Fe 3 B/R 2 Fe 23 B 3 /α-Fe. The amount of R 2 Fe 14 B phase is very critical in determining the B r , i H c and ( BH ) max obtained. In general, the B r , i H c and ( BH ) max decrease with increasing boron content and reach local minimum when only R 2 Fe 23 B 3 was present in ribbons, for example sample with a nominal composition of ( x , y )=(5.8, 16.9). The presence of a large amount R 2 Fe 23 B 3 , with the diminution of R 2 Fe 14 B, is responsible for degradation of thermal stability of the ribbons. In the meantime, the average grain size of individual phases also showed a crucial influence on the B r and ( BH ) max of ribbons studied.

Microstructural control of Nb addition in nanocrystalline hard magnets with different Nd content

In this paper, nanocrystalline alloys with Nd content near 10 at % are investigated, and their magnetic properties and microstructure are examined to obtain better understanding of the effect of Nb addition on microstructure control in the nanocrystalline magnets.

Behavior of Nb atoms in Nb substituted Nd/sub 2/Fe/sub 14/B nanocrystalline alloys investigated by atom probe tomography

The behavior of Nb atoms in Nd/sub 12/Fe/sub 82-x/B/sub 6/Nb/sub x/ (x=0 to 3) nanocrystalline alloys has been investigated by using atom probe tomography (APT) technique on near-atomic scale in the present paper. Three-dimensional atom probe (3DAP) analyses on the Nb substituted alloys clearly reveal that Nb atoms are enriched at grain boundaries with a peak concentration about 4 times higher than the average for Nd/sub 2/Fe/sub 14/B grains. A grain boundary region with a chemical composition near the Nb : Fe : B stoichiometry of 3 : 3 : 5 is also measured for the x=3 alloy. The results provide direct evidence of microstructural refinement due to solute drag of Nb atoms during solidification, resulting in Nb enrichment at grain boundaries and possibly the formation of a Nb-rich interfacial phase which give rise to enhanced magnetic properties.

Neutron diffraction analysis of melt spun 2:14:1 type (NdPr)–Fe–B compounds with Ti and Zr additions

There has been continued technological and research interest in Nd 2 Fe 14 B -based compounds due to their excellent intrinsic magnetic properties [(BH) max * ∼64 MGOe ,H a ∼73 kOe ]. It is found that Ti and Zr substitution in Nd 2 Fe 14 B leaves its magnetic properties largely unaffected. We have carried out neutron diffraction studies on Zr and Ti substituted 2:14:1 compounds. Three specimens of melt spun alloys of compositions ( Nd 0.75 Pr 0.25 ) 12 Fe 80 B 8 , ( Nd 0.75 Pr 0.25 ) 8.4 Fe 79.7 Ti 4.7 B 7.2 and ( Nd 0.75 Pr 0.25 ) 9.2 Fe 79.2 Ti 2 Zr 2.2 B 7.4 were studied. Rietveld refinements reveal that the 4f site is equally occupied by Nd and Pr while the 4g site is occupied only by Nd. In the Ti-containing sample, the lattice expands and Ti is found on the j 1 , j 2 sites normally occupied by Fe, (4% and 8%, respectively). This accounts for about 30% of the added Ti. In the third sample, the lattice contracts, which could occur only if Zr replaces the larger rare-earth atoms. However, due to the possibility of four different species occupying the 4f site (i.e., Nd, Pr, Ti, and Zr), it is not possible to determine their relative concentrations from neutron diffraction data alone. The balance of Ti and Zr in this sample must be locked in the grain boundaries. Despite peak broadening associated with the fine grain structure, robust refinements were achieved.