Microstructure of highremanence NdFeB alloys with lowrareearth contentRaja K. Mishra and V. Panchanathan Citation: Journal of Applied Physics 75, 6652 (1994); doi: 10.1063/1.356884 View online: http://dx.doi.org/10.1063/1.356884 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/75/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Low temperature diffusion process using rare earth-Cu eutectic alloys for hot-deformed Nd-Fe-B bulkmagnets J. Appl. Phys. 115, 17A766 (2014); 10.1063/1.4869062 Microstructure and magnetic properties of sintered Nd–Fe–B magnets with high hydrogen content J. Appl. Phys. 109, 07A734 (2011); 10.1063/1.3563082 Effect of rare earth content on microstructure and magnetic properties of SmCo and NdFeB thin films J. Appl. Phys. 91, 8180 (2002); 10.1063/1.1453940 Thermomagnetic analysis of NdFeB meltspun alloys with low Nd content J. Appl. Phys. 77, 4133 (1995); 10.1063/1.359501 Highremanence rapidly solidified NdFeB: Dieupset magnets (invited) J. Appl. Phys. 73, 5751 (1993); 10.1063/1.353563

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Microstructure of high-remanence N&Fe-B alloys with low-rare-earth content

Raja K. Mishra Physics Department, GM Research and Developmerrt Cerztel; Warren, Michigan 4SO90-905.5

V. Panchanathan Magnequench-Delco Remy Division, General Motors Corporation, Anderson, Indiana 4601.3

The microstructure and magnetic properties of bonded and fully dense magnets produced from melt-spun ribbons of the composition RE3.sTM7~GalB1s.5 have been investigated, where RE refers to a mixture of Nd and Dy, and TM refers to a mixture of Fe and Co. Results show that annealing overquenched ribbons with about 3 wt. % Dy and 3 wt. % Co at 700 “C can produce materials with H,=4 kOe, B,-11.5 kG, and (BH),,,- -14.5 MGOe, the latter being comparable to those of commercial ribbons containing three times as much Nd. The microstructure of annea1e.d ribbons consists of about 30% Nd,Fet4B grains, 65% FesB grains, and 5% cr-Fe grains. The grains of all the phases are nearly spherical. The grain diameters are in the 30-50 nm range. Hot-pressed magnets made from overquenched ribbons are fully isotropic, with magnetic and mic.rostructural characteristics similar to those of the annealed ribbons. The high remanence and low coercivity correlate well with the microstructure consisting of a uniform mixture of the hard NdzFerJB phase and soft cr-Fe and Fe,B phases.

I. INTRODUCTION

Improvement of hard magnetic properties, simplification of processing, and reduction of cost, besides others, have been the principal motivating factors for much of the re- search on Nd-Fe-B permanent magnet materials”” since they were discovered a decade ago. Many studies of modification of chemical composition have helped improve the hard mag- netic properties, Curie temperature, thermal and environmen- tal stability, etc. Other research and studies have contributed toward streamlining preparation procedures which facilitate commercial production. There is now a sufficiently broad base of information on the NdzFe,,B magnets to help design compositions and microstructure suited for diverse applica- tions. This articIe discusses the results of a study of a Nd-Fe-B magnet which is a mixture of two different ferro- magnetic phases with the aim of enhancing magnetic prop- erties while reducing overall Nd content compared to the standard Nd,Fe,,B magnet. Since Nd still accounts for a sub- stantial fraction of the cost of the Nd,Fet4B alloys and lower Nd content generally improves the aging behavior of the Nd2Fe,,B magnets, preparing viable magnets through this route is of both commercial and technological interest.

By altering the amount of Nd in the Nd-Fe-B alloy one can enter into a phase field in the Nd-Fe-B phase diagram where Fe,B phase is one of the equilibrium phases3 Fe3B has a room-temperature saturation magnetization of I4 kG, comparable to that of the Nd,Fet4B phase.4 It has planar anisotropy and it behaves as a soft magnet.5 The Curie tem- perature of FesB is 513 “C, nearly 200 “C more than thrtt of

3 Nd,Fe,4B. European and Japanese ’ researchers have re- ported that in a Nd4Fe7sB1s alloy containing over 50% Fe,B, B, of 12 kG, H,i of 3.8 kOe, and (BH),,, of 12 MGOe can be achieved, Alloying additions such as Dy, Co, and Ga are known to alter anisotropy, coercivity, and Curie temperature of Nd,Fe,,B magnets.’ One of the aims in this study is to prepare and characterize an alloy utilizing the beneficial ef-

fects of both the mixing of two ferromagnetic phases as well as selective alloying modifications to improve coercivity and other characteristics. We. also report here the results of mi- crostructuml characterizations of the new alloy and correlate them with the observed properties.

II. EXPERIMENT

The starting material used in this investigation was pre- pared from an induction melted ingot (5 pound charge) of nominal composition ‘13.2 wt. % Nd, 83.6 wt. % Fe, and 3.2 wt. 5% B (Nd,,Fe,,B,,,j, melt spun at wheel speeds be- tween 4 and 22 m/s. The ribbons were annealed at tempera- tures between 630 and 720 “C for 5 min. Additional alloys were prepared by substituting 3 wt. % Co and 1 wt. % Ga for Fe and 3 wt. % Dy for Nd. Some alloys were made with B content up to 3.6 wt. % and Nd (Dyj content between 10.3 and 21.2 wt. %. Second quadrant demagnetization curves were measured from powder samples (demagnetization cor- rection factor of 0.33) as well as from resin bonded magnets. Hot-pressed samples were prepared from the 22 m/s ribbons by standard procedure at 750 “C. Temperature coefficients of B, and ff,i were calcu1ate.d from demagnetization data at different temperatures. Loss characteristics were measured for bonded magnets held at elevated temperatures7 for up to 1000 h.

Thin foils for electron microscopy were prepared from annealed and unamrealed ribbons by ion milling. The elec- tron transparent foils were examined in a Philips 430 micro- scope operating at 300 kV.

Ill. RESULTS

Since it is possible to prepare microcrystalline ribbons of Nd-Fe-B alloys either by directly quenching from the melt at an optimum cooling rate or by annealing ribbons which were first overquenched from the melt, we have investigated both of these paths for optimum properties. For the low Nd con-

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I / i ,I

/ / ,/ .‘;’

-4 -3 -2 -1 I +1

R (Me)

FIG. 1. Second quadrant demagnetization cwve of ~d”d,,T)~l~e,:~C~~.~(jBLBIF; 5 Ca:I powder overquenched at 22 tis and annealed at 700 “C with demagnetization correction factor 0.333; ih) hot-pressed magnet. The remanrncc to saturation magnetization ratio of 0.62 is the high- est reported for any fully dense isotropic magnet.

tent compositions studied in this article, the best properties are exhibited by ribbons that are melt spun at 22 m/s and then annealed at temperatures close to 700 “C. Transmission electron microscopy results show that the ribbons of all com- positions used in this study are microcrystalline for wheel speeds of 22 mis. Figures I(a) and 1 (h) show the second quadrant demagnetization curves for overquenched ribbons of Nd,,,Dy,Fc,Co,Cja,1318,5, annealed for 5 min at 700 “C and hot pressed to full density, respectively. Microstructure of annealed ribbon corresponding to Fig. l(a) is shown in Fig. 2ia~. The microstructure consists of three phases, namely Nd2Fe,,B, tctmgonal Fe,B, and &-Fe. The grain sizes of Fe$ and Nd2Fe1,B are nearly the same (average grain diameter in the 20-50 nm range) while o-Fe appears as iso- lated grains. The distribution of Nd,Fe,,B and Fe,B phases is homogeneous. There is no evidence of intergranular phase at the grain boundaries.s The microstructure of the hot-pressed sample is similar to that of the annealed ribbon in all re- spects. In contrast, the microstructure of the best directly quenched ribbon (wheel speed of 7 m,%j of the same compo- sition, shown in Fig. 2(h), is not homogeneous. The grain diameters of the FesB grains are much larger than those of the Nd,Fer.+B grains. There is a clearly defined intergranular phase at the Nd,Fer$ grain boundaries. &-Fe is found as isolated particles. EDX analysis failed to detect any Nd in the Fe&3 grains. Dy, Co, and Ga additives were all detected in the Nd:Fcr,B phase with Ga concentrations mostly near the grain boundary. Microstructural features in ribbons of different compositions are the same except for the relative amounts of the three phases.

Table 1 lists the temperature coefticients of B,. and H,,

FIG. 2. Transmission electron micrographs of Nd,,,Dy,Fe,lCo,(;a,B,X,s rib- bons (a) quenched at 22 m/s and annealed at 700 “C and ib) quenched at 7 m/s. Note the nonuniform grain sizes of different phases in (b). A=Fe,H, C=NdsFe,,B, and D=cr-Fe.

between room temperature and 175 “C, measured using bonded magnets of composition Nd4.iiFe77,gB,7,7 and Nd3.SDylFe73Co,GaIB1s.5. These values are essentially the same as those of commercial grade bonded Nd-Fe-B mag- nets. We also find that the structural and irreversible loss characteristics of these alloys are the same as commercial grade magnets. There is practically no loss in Hci after aging for 1000 h at 100 “C and alloys without Co have no struc- tural loss,

The initial magnetization and demagnetization curves of a bonded magnet premagnetized to two different tield strengths are shown in Fig. 3. It is observed that while the coercivity of the material is not very high, the field needed for saturation is at least 10 times higher than the coercive

TABLE I. Temperature coefficients of B, and ftI,i in bonded magnets. Com- . . posttton A: Nd,,,Fe,,,, B t7,-,; composition B: N~f,sDy,Fe7~Co3C’;llB18,5.

Temp. co&f. B, Temp. coeff. N,,

‘Temperature m7cl ocj (%/ “C)

( “Cl A B A H

50 -0.12 -0.43 75 -0.10 -0.08 -0.42 PO.39

100 -0.13 -0.1 -0.39 PO.39 125 -0.14 -0.1 ~- 0.38 -0.38 150 -0.14 -0.1 -0.38 -0.37 175 -0.11 -0.37

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FIG, 3. initial magnetization and second quadrant demagnetization curves for Nd,,,Dy,Fc73C03Ga,B,x.3 bonded magnet after magnetizing up to <a) 9 kOe and (b) 18 kOe. The kinks in (a) and (.b) are characteristic of the two-phase material.

field. The initial magnetization curve is linear up to a field at least twice the coercive field. The demagnetization curves for the material magnetized up to 9 and 18 kOe are nearly par- allel. The entire demagnetization curve shifts toward higher B, and H, if the magnet is premagnetized to 45 kOe.

IV. DISCUSSION

Microstructural measurement shows that the material consists of approximately 30% NdrFe14B, 65% Fe,B, and 5% U-Fe phase. These volume fractions are consistent with what one would expect from atomistic considerations if one assumes that all the Nd atoms are incorporated ito the Nd,Fe14B phase, the remaining B atoms are incorporated into the Fe,B phase and the remnant transition metal atoms are loft behind in elemental form. The saturation magnetiza- tion values of the three phases are roughly the same, close to 16 kG. Tetragonal Fe,B has planar anisotropy, and the m&xi- mum contribution to remanence from isotropic distribution of the Fe3B grains will be ~$4 or 0.79 of saturation value if the basal plane of the tetragonai Fe,B structure is assumed to contain no preferential direction for the magnetization vector.” Since there are at least two grains of Fe3B for every Nd2Fe14B grain, in spite of the strong uniaxial anisotropy of the Nd,Fe14B grains, the demagnetization characteristics of the material will be dominated by that of Fe3B phase, giving a remanence close to 0.79 of the saturation value. If the Nd2Fe14B phase were to determine the demagnetization be- havior, the remanence would be in the range of half of the saturation magnetization value. Clearly, the high remanence in the material observed here and by others suggest the latter not to be the case.

The coercivity of the material is dominated by the crys- tallite distribution. Grain size plays a role in changing the value of Hti . The moderate value of H,i must be a result of the influence of the Nd2Fe14B phase since the planar anisot- ropy of the Fe,B phase makes that phase a soft ferromagnet. Application of a reverse field in this system can easily nucle- ate reverse domains and these domain walls can move rela-

tively freely in the Fe;B grains while NdzFe14B grains will act as barriers to domain-wall motion. Approach to saturation in such a material will be determined by the flipping of the magnetization vector in individual Nd,Fe14B grains. The ap- proach to zero magnetization at the coercive field will be a result of a sufficient amount of FesB with reverse magneti- zation canceling out the contribution from the Nd2Fe14B phase. Such an explanation is consistent with the large field (compared to the. coercive force) needed for full saturation. This is also consistent with the observation of parallel de- magnetization curves (a) and (b) in Fig. 3.

The phase distribution is a result of both the chemistry and the kinetics of crystallite formation during quenching or annealing. The grain size distribution in the directly quenched alloy suggests that grain growth kinetics for Fe,B and NdzFe.,4B are different at elevated temperatures encoun- tered during melt spinning. The grain growth kinetics of the two phases are similar at temperature and time intervals en- countered during annealing of the overquenched alloy. It is also possible that nucleation of Nd2Fe14B phase takes place in pockets which are saturated with Nd after Fe3B grains have formed and grown in the directly quenched ribbons. Excess cw-Fe is le.ft behind in both cases at the end of soiidi- fication as a terminal phase. That no Nd atoms were detected in the Fe-,B phase contrary to the suggestion in Ref. 9 indi- cates that the Fe-B trigonal prism which forms the building block in Fe3B and Nd,Fe14B does not readily permit substi- tution of Fe by Nd. This is also consistent with the observa- tion that various substitutions to the starting alloy do not alter the. microstructure in any significant way. Most of the substituted atoms are incorporated ito the Nd,Fe,,B phase. Fe,B phase or boundaries between Fe,B grains does not show any segregation of Dy, Co, or Ga. We find Ga associ- ated with the Nd,Fe14B phase, mostly at the boundary be- tween Nd2Fe,4B and Fe,B grains. The changes in the tem- perature coefficie.nt of B, and H,i and the loss properties with various alloying additions are thus considered to be primarily due to changes to the Nd2Fe14B phase upon alloying. Finally, these results indicate that further research on optimization of hard and soft magnet mixtures can potentially yield im- proved permanent magnets.

ACKNOWLEDGMENTS

The authors wish to thank Jan Herbst, Fred Pinkerton, and John Croat for stimulating discussions and encourage- ment during this work.

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