FIM-atom probe investigations of melt-spun FeNdB-ribbons

Scripta METALLURGICA

Vol. 21, pp. 407-410, 1987 Printed in the U.S.A.

Pergamon Journals, Ltd. All rights reserved

FIM-ATOM PROBE INVESTIGATIONS OF MELT-SPUN FeNdB-RIBBONS A. HOtten and P. Haasen Institut for Metallphysik, UniversitBt GOttingen and SFB 126, FR Germany (Received December

29, 1986)

Introduction In recent years high energy permanent magnets based on the Fe-Nd-B system were developed either by powder metallurgy / I / or by rapid solidification /2-4/. Now, both methods are able to produce f u l l y dense magnets with comparable energy products. In both materials the tetragonal FeI4Nd2B phase /5/ is responsible for the Curie temperature, the saturation magnetization and the uniaxial magnetocrystalline anisotropy. On the other hand the efficiency to which these quantities lead to good permanent magnetic properties as coercivity, remanence and maximum energy product depends on the microstructure of the particular material. Therefore i t is necessary to obtain information about the morphology and the chemical compositions of the different phases in the magnet a11oys. The f i e l d ion microscope (FIM) in combination with a time of f l i g h t mass spectrometer called atom probe (AP) with a lateral resolution of about 2 nm and an atomic depth resolution provides a technique for the quantitative analysis of materials with extremelyfine grain sizes. So the objective of the present study was to determine the composition of the different phases in melt-spun FeNdB magnets in the magnetic optimum state using FIM and AP techniques. Experimental Melt-spun ribbons of FeNdB were provided by Thyssen AG, Dortmund (specimen T) and Siemens AG, Erlangen (specimen S). The nominal compositions of the analysed specimens are given in table I together with the substrate velocity used during melt-spinning, which is proportional to the cooling rate to a f i r s t approximation and hence determines the resulting grain sizes /2/. TABLE I Nominal compositions of the analysed specimens with the corresponding substrate velocities vs specimen

composition (at%)

Vs (m/s)

T

Fe74.6NdI5.3BIo.I

20

$I

Fe77NdI5AI2B6

18

$2

(Fe50Co50)77NdI5B8

18

For these substrate velocities the specimens were in the magnetic optimum state. FIM tips were made from ribbons by mechanically grinding in order to obtain a quadratic crossreaction. Then the specimens were electropolished in perchloric acid and ethyl alcohol in the proportion 1:30.

407 0036-9748/87 $3.00 ÷ .00 Copyright (c) 1987 Pergamon Journals Ltd.

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The FIM and AP studies were performed on the GOttingen instrument designed by P i l l e r /6/ and Wagner /7/ for the investigation of phase separation in alloys. In order to increase the resolution of the FIM images the tips were cooled to about 80 K and imaged by neon at a pressure of 10-4 Pa. B e s t FIM imaging conditions were obtained, however, by using a gas mixture of Ne with about 10% Ar, whereas Ne only was used during AP-analysis. For the AP studies a high voltage pulse ratio larger than 15% was used to prevent preferential evaporation of one component. I t should be pointed out, that the. FIM micrographsgiven in F i g . l , Fig.3 and Fig.4 show no well developed crystalline ring structures. This is probably due to the great difference in atomic sizes of the components of the a11oys. In contrast to overquenched specimens previously investigated /8/ thoses analysed in the present study showed a v i s i b l e FIM contrast between different phases. This phase contrast allowed the determination of the chemical composition of the relevant phases by selected area analysis, while the t i p was manipulated so that the projection of the probe-hole (dark round area in the FIM-images) covered the area, which we wanted to study. Results on the Cobalt-free specimens Figure I shows the microstructure of a Thyssen specimen in the optimum magnetic state. Selected area analysis was performed in area I, which we have separated from area 2 by a white dotted line, and in area 2, which is darkly imaging. Table 2 summarizeSthe results. Within the error bars:area I has the stoichiometric composition of the hard magnetic Fe14Nd2B phase, whereas area 2 which surrounds the f i r s t one is boron free but enriched in Nd and is described as NdBFe. A similar situation is seen in Fig.3. I t shows the microstructure of a Siemens specimen. Again area I which has a composition comparable to that of the hard magnetic phase is surrounded by a B-free phase with the composition o f - Nd3Fe. In order to investigate the phase boundaries between the relevant phases we performed a concentration depth analysis starting in the hard magnetic phase of a Thyssen specimen. Fig.2 shows the obtained concentration profiles of the components. After 80 desorbed atomic layers there was a transition between the hard magnetic phase to a B-enriched one. 20 layers later a transition back to the hard magnetic phase was observed. This new detected phase has a composition which can be obtained from the concentration profiles and given also in table 2. Our results are in contrast to Mishra /9/ who reported a two-phase microstructure consisting of the Fe14Nd2 B-phase and a Nd-enriched but B deficient grain boundary phase in the optimum magnetic state for ribbons with a starting composition of FeRINd13 5B5 5- We now find an additional third phase. Since this B-enriched phase was not ~ouna ~requently during FIM or AP-analysis i t must be irregularly distributed which is less important for the coercivity mechanism than the Nd-enriched phase which always surrounds the Fe|4Nd2B-phase. The mean diameters of area I (D(Fe14Nd2B)~(14,5~4)nm) and of area 2 (D(Nd3Fe)~ (4,6~2,3)nm) which are determined from Fig.3 are comparable to those obtained by Mishra, (FeI4Nd2B 20-30 nm, Nd-enriched ~ I-2 nm). TABLE 2 Compositions of the detected phases in Co-free ribbons (in at%)

Region

Fe

Nd

1

82.5±0.5

10.7±0.4

2

22.9±1.2

77.1±1.2

B 6.8±0.3

Phase "Fe14Nd2B" "Nd3Fe"

obtained from Fig.2: 41.8±5.7

9.3±3.4

48.9±5.8

"Fe4NdB4"

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RESULTS ON COBALT CONTAINING SPECIMENS The main disadvantage of FeNdB magnets is their poor temperature s t a b i l i t y due to their low Curie temperature. For sintered Fe77NdI5BI specimens Fe was replaced by Co in order to increase the Curie temperature /10/. We studied Co containing ribbons to answer the question how the Co atoms distribute among the different phases. Therefore we performed selected area analysis on a Siemens specimen shown in Fig.4. The micrograph shows a microstructure consisting of two phases. From the data listed in table 3 area I is identified as (Fe50Co50)14 Nd2B i f the error bars of the concentrations are taken into account. In this phase haIf of the Fe is replaced by Co. The other phase is enriched in Nd and B-free whereas the Fe content is less than that of the magnetically hard phase. The Co fraction of the Nd-enriched phase does not d i f f e r much from that of the hard magnetic phase. In contrast to the Co-free specimens we have not found a B-enriched phase in Co-containing material so far. TABLE 3 Compositions of the detected phases in Co-containing ribbons (in at%) Region

Fe

Co

Nd

B

5±1

1

42.7±1

41.4±1

10.8±1

2

31.8±1.1

46.4±1.2

21.8±1.0

Phase

"(Fe50C°50~4Nd2~-7

ACKNOWLEDGEMENT The authors wish to thank Dr. K. Kuntze (Thyssen AG, Dortmund), Dr. J. Wecker (Siemens AG, Erlangen) for providing the material and Dr. R. Wagner, L. V. Alvensleben, R. GrQne and M. Oehring for many f r u i t f u l discussions. REFERENCES I. 2. 3. 4. 5. 6. 7.

M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto and Y. Matsuura, J.AppI.Phys. 55, 2083 (1984) J. J. Croat, J. F. Herbst, R. W. Lee and F. E. Pinkerton, J.Appl. Phys. 55, 21~'[8 (1984) R. W. Lee, Appl.Phys. Lett. 46, 790 (1985) R. W. Lee, E. G. Brewer, N. ~ S c h a f f e l , IEEE Trans.Magn. 21, 1958 (1985) J. J. Herbst, R. W. Lee and F. E. Pinkerton, Ann.Rev.Mater~ci. 16, 467 (1986) J. P i l l e r , Diploma thesis, University of GOttingen, 1977 R. Wagner, Field Ion Microscopy in Material Science, Crystals, Vol.6, Springer Verlag, Berlin (1982) 8. A. HOtten and P. Haasen, submitted to J.App1.Phys., April 1987 9. R. K. Mishra, J.Magn.Magn.Mater. 54-57, 450 (1986) 10. M. Sagawa, S. Fujimura, H. Yamamoto, Y. Matsuura and K. Hiraga, IEEE Trans.Magn.MAG 20,

1584 (1984)

410

FeNdB-RIBBONS

Vol.

Thyssen ....6 Nd15 3, B

No.

Fe74.sNd~5.3B~o.1

opI~mum magnet~c s~ate

Fe z

21,

10.t

'°° F, "•

~ ~

/

.

t00

. . . . .

150

Number of Oesorbed Lsyers

..........

I0 n m

Fig. 2 Composition profiles of specimen T

Fig. I Field ion micrograph of specimen T

S[emens opt lmum magnet lc s'(ate

S Eemens op~ ~mummagne'(tcs}a~e

Fe77Nd 15A [ 2B G :

!Feso C050 )zzNd ~sBB : e

•"t'P.~. " •

•"

: ~

:; ¸¸:%;i¸ ~ . /

:~

l

.

.G."~

"

~

S'i,"

a

'"

~i;!:~ :~iTi~ii~;!~i{: ~:;~:!>!;~i ?:!%! f:i¸ 'if ~:i;¸ i ! i ~ i j ~ l

f,

10 n m

~ 10 n m

Fig. 3

Fig. 4

Field ion micrograph of specimen $I

Field ion micrograph of specimen $2

200

3