Effect of Cu and Nb on crystallization and magnetic properties of amorphous Fe77.5Si15.5B7 alloys

MATERIALS SCIENCE & ENGINEERING ELSEVIER

Materials Science and Engineering A194 (1995) 77-85

A

Effect of Cu and Nb on crystallization and magnetic properties of

amorphous Fe77.5Si15.5B7alloys N. Mattern a, A. D a n z i g a, M. Mi.iller b "Institut fffr Festkfrper- und Werkstofforschung Dresden eV PosOeach, D-Ol171 Dresden, Germany b Technische Universita't Dresden, Institutfiir Werkstoffwissenschaft, Postfach, D-01062 Dresden, Germany Received 7 March 1994

Abstract The crystallization of amorphous Ee77.sSils.sB7 with the separate and combined addition of Cu and Nb was investigated. The addition of Cu increases the (metastable) temperature and time region of coexistence of Fe3Si and the amorphous matrix phase. The Cu does not affect the microstructure. The addition of Nb leads to the formation of nanocrystals but, for higher contents, boride phases simultaneously grow with the Fe3Si phase. The combined addition of Cu and Nb enhances the stability of amorphous Fe77 5Sil5.5B 7 against crystallization, shifts the crystallization temperature to higher values and increases the region of coexistence of nanocrystalline Fe~Si and the amorphous matrix phase. Nb is responsible for the small grain size of the Fe3Si crystals, and Cu for the phase composition. The magnetic properties are strongly correlated with the microstructure. Low coercivity and zero magnetostriction can be obtained only with the combined addition of Cu and Nb.

Keywords: Copper; Niobium; Iron; Silicon; Boron; Alloys

1. Introduction After partial crystallization, amorphous FeSiBCuNb alloys show excellent soft magnetic properties, as a result of their special microstructure which consists of b.c.c. Fe-Si precipitates with a grain size on the n a n o m e t e r scale, e m b e d d e d in an amorphous matrix phase [1-3]. T h e formation of nanocrystalline Fe-Si precipitates during annealing of amorphous FeSiBCuNb alloys correlates with the addition of Cu and Nb. Cu is thought to enhance the nucleus density, while Nb lowers the growth rate [l,2], but detailed experimental results confirming this picture in a quantitative manner have yet to be obtained. Cu-rich regions just before or during crystallization were observed by means of analytical field ion microscopy [4], and by extended X-ray absorption fine structure (EXAFS) measurements [5,6]. T h e Cu clusters were thought to modulate the Fe3Si formation, as a result of the chemical inhomogeneities, and should increase the n u m b e r of nuclei. However, nanocrystalline b.c.c. Fe precipitations are also found in annealed

0921-5093/95/$9.50 © 1995 - Elsevier Science S.A. All rights reserved SSD1 0921-5093(94)09666-X

amorphous alloys without Cu, such as in amorphous Fe91Zr7B2 [7] and Fe87.gHfg.sB2.s [8]. T h e aim of this work was to clarify the effects of Cu and Nb on the crystallization of amorphous FeSiBCuNb alloys. Therefore, amorphous Fe77.sSils.sB 7 with separate and combined additions of Cu and Nb was studied to distinguish the different effects of the elements.

2. Experimental details A m o r p h o u s ribbons 20 ktm thick and 10 mm wide were prepared by a rapid quenching technique from the melt. Pieces of ribbon were isochronously (1 h) annealed under a hydrogen atmosphere at different temperatures (TA) between 300 and 700 °C. All the measurements were performed at r o o m temperature. X-ray diffraction (XRD) patterns of the samples were recorded by means of a Philips X P E R T 3020 diffractometer using Co K a radiation, and a diffracted beam graphite monochromator.

78

N. Mattern et al.

/

Materials Science and Engineering A194 (1995) 77-85 after isochronous annealing at different temperatures Ta. T h e crystallization takes place in two distinct steps. In the first stage, at 425 ° C < T~I < 4 5 0 °C, cubic Fe3Si (space group, Fm3m) is formed primarily in an amorphous matrix. Weak reflections indicate the existence of the superstructure of the b.c.c. Fe lattice resulting from ordering of Si atoms at Fe sites. T h e lattice constant a 0 of the F%Si phase depends on the chemical composition [10]. As shown in Fig. 2, the lattice constant and, therefore, the silicon content of the Fe3Si phase are changed during annealing. T h e composition at low annealing temperatures corre-

T h e coercivity H~ was measured using a F6rster Koerzimat. To determine the magnetostriction, the method of small rotation according to Narita [9] was applied.

3. Results and discussion 3.1. Crystallization and phase formation Fig. l(a) shows as an example the diffraction patterns of Fe77.sSils,sB7 in the as-quenched state and

~to 4

3.3~

,l

/

[cps]

I

I I /

I

I

illlll IIIIrll IIIIIii IIIIIII

i t

t .68 i

~j i

,I

i

/ I

7OO

0.0_C -, ; T (a)

30.0

- - - 50.0

aS3qu.

70.0

90.0

liO.O

TAt°C]

130.0

20 [°]

~10 4

I

[cps]

-'; -~~ o.oo (b) 30.0

~-~

'

-~ I

700 TA[°el

I

50.0

I

I

70.0

I

I

90.0

I

I

iiO.O

I

130.

0 ClS-qu.

20[ ° ] Fig. 1. XRD patterns of amorphous (a) FeTv.sSil5.5B7and (b) Fev3.sSi 5.sB7Cu Nb3 in dependence on annealing temperature TA.

N. Mattern et aL

/

Materials Science and Engineering A 194 (1995) 77-85

Fe77.5Si15.5B7

og

•D- Fe76.0Si 15.5B7Cu 1.5

5,69

Fe73.5Si 15.5B7CUl Nb 3

16,6 60 T"

aso C aS O

g

-d

5,68

19,0 8

5,67

21,4

5,66

23,8

400

i 425

450

475

500

525

i 550

575

600

i 625

650

Annealing temperature TA[°C]

Fig. 2. Lattice constant a0 and corresponding Si content of the Fe3Si phase vs. annealing temperature.

sponds to the mean value 15.5 at.% of the alloy. Annealing at temperatures higher than 450 °C mainly enhances the volume fraction, and the Si content of the Fe3Si phase reaches about 20 _+ 1 at.%. As a result of the precipitation of Fe and Si, the chemical composition of the remaining amorphous matrix is changed towards Fe2 B. In the second stage, at 475 °C < Tx2 < 500 °C, tetragonal FezB (space group, I 4 / m c m ) is observed in the diffraction patterns (Fig. l(a), Table 1). Annealing at higher temperatures up to TA----750 °C does not change the phase composition further. The effect of Cu on the crystallization temperature and the phase formation of amorphous Fe77.sSi15.sB7 is summarized in Fig. 3. As well as the results of isochronous annealing for 1 h, the crystallization temperatures T~ sc measured by differential scanning calorimetry (DSC) at a heating rate of 10 K min-~ are indicated by crosses connected by lines. The transition temperatures Tx°sc in the continuous heating DSC measurements are about 50 K higher than the Tx values of crystallization by isochronous annealing for 1 h. The addition of Cu leads to a decrease in the temperature TA= Txl where Fe3Si is formed from 450 °C (without Cu) to 425 °C (0.5-1.5 at.% Cu). This behavior was also reported in refs. [11] and [12]. 7xl and T~ sc are found to be independent of the Cu content, in agreement with the results for amorphous Fe74.5 _xSil3.sBgCuxNb3 given in ref. [11]. However, the Cu addition leads to surface crystallization before bulk crystallization, as indicated in Fig. 3. The XRD patterns presented in Fig. 4 of annealed Fe76Sils.sB7Cul. 5 show crystalline reflections already after annealing at TA= 350 °C. Only (110) reflections of Fe(Si) are visible in the Bragg-Brentano geometry. This means that the surface crystallites have a strong

79

texture with (110) direction perpendicular to the surface. The surface crystallites are found on both sides of the ribbon. The reflections obtained from the samples annealed below TA=425 °C disappear after removing the surface crystallites by polishing. Surface crystallization was found also in other amorphous alloys, such as Fe80B20 [13] and Fe78Si9B13 [14]. There is no unique cause for this behavior, but local changes in the surface composition by selective oxidation or segregation are assumed to have the strongest effect [15]. Analysis of the element distribution in the Fe77.5_xSi15.sB7Cux alloys by means of energy-dispersive measurements of the X-ray fluorescence radiation in a scanning electron microscope showed homogeneity of the Cu, Si and Fe contents over the whole ribbon cross-section, within the resolution limit of about 1 ,urn. From the XRD pattern shown in Fig. 5, there is no evidence for the formation of crystalline Cu before or during the first step of crystallization. Even these longtime measurements (250 s per step) in the angular range of the (111) and (200) reflections give no indication of the f.c.c. Cu phase. The intensity ratio iCu /lFe3Si (111)/~(2}0) i s 0 . 0 1 i n a phase mixture consisting o f about 99 vol.% Fe3Si and 1 vol.% Cu. The formation of 1 vol.% Cu before or during the first stage of the crystallization should be detectable in the diffraction diagram, even with a grain size of 5 nm. The intensity would be similar to that of the (311) superstructure reflection of the Fe3Si phase at 2 0 = 62.71 °. Because of the absence of any enhancement of the intensity at the Bragg angles 2 0 = 50.73 ° and 59.30 °, we conclude that f.c.c. Cu with a grain size of 5 nm or more is not formed in this stage. The formation of Cu-rich clusters just before or during the crystallization of amorphous Fe73.sSi~3.5B9CulNb 3 has been observed by atom probe field ion microscopy [4]. The particles were found to be a few nanometers in diameter, containing 20-60 at.% Cu. E X A F S measurements of the Cu and the Fe K edges in refs. [5] and [6] showed that the Cu in annealed Fe74Si13B9CulNb 3 is not incorporated into the b.c.c. lattice of the Fe3Si phase formed; this observation agrees with the negligible solubility of Cu in b.c.c. Fe-Si. The radial atomic distribution function obtained for the Cu environment is similar to that in f.c.c. Cu, but some differences exist between the Cu Fourier transform of the E X A F S oscillations of the annealed sample and that of f.c.c. Cu standard. With regard to the XRD results, this suggests that the f.c.c. Cu clusters are less than 5 nm in size, and that at least a fraction of the Cu atoms is still situated in the amorphous matrix phase [5]. The crystallization temperatures Tx2 and Tx~sc of the second stage are not affected by Cu additions. The

80

N. Mattern et al.

/

Materials" Science and Engineering A194 (]995) 77-85

Table 1 The d spacings (A) and normalized intensities (%, x = 100%) of amorphous Fe77.sSi 15.5B7, Fe768il 5.5BTCul 5, Fe74.sSi 15.sB7Nb3, Fe73.sSils.sBTCUlNb 3 annealed for 1 h at 750 °C, and reference patterns of Fe3Si(calculated), Fe2B, Cu and Fe23B~,[24] FeSiB

FeSiB + Cu1.5

FeSiB + Nb3

FeSiB + Cu 1Nb3

Fe3Si (calc.)

3.282/1

3.275/1 2.837/1 2.554/2

3.275/4 3.126/1 2.958/2 2.840/2

3.274/6

2.842/1 2.557/2

3.275/3 3.122/1 2.955/2 2.839/1 2.444/4 2.404/1 2.346/3 2.330/4

2.448/5 2.407/1 2.347/3 2.331/2

2.166/10

2.178/4 2.166/1

Fe2B 36-1332

Ctl 4-836

Fe23B6 34-991

2.835/3 2.55/30 2.42/40

2.210/35 2.122/2

2.008/X

2.120/3 2.086/15 2.008/X

2.120/40 2.081/21 2.065/10 2.054/42 2.010/X 1.929/12 1.865/8

1.830/1 1.807/< 1

1.712/<1 1.632/2 1.616/1

1.710/1 1.632/3 1.615/2

1.418/12

1.285/1

1.285/2 1.278/1

1.06/< 1

1.047/2 1.003/18

2.005/X

1.202/4 1.19/23 1.158/34

1.91/20

1.866/7

1.729/2 1.707/2 1.630/3

1.771/<1 1.754/< 1 1.732/1 1.710/2 1.631/3

1.594/5 1.561/2 1.457/3 1.422/18 1.331/2 1.308/3 1.282/4

1.595/5 1.562/< 1 1.462/3 1.420/19 1.334/3 1.310/4 1.283/5

1.719/2 1.637/1

1.808/46

1.83/30 1.808/15

1.63/18 1.62/18

1.63/10

1.28/8

1.315/10 1.28/25

1.418/15

1.278/20 1.260/5 1.250/6

1.25/15

1.232/6 1.208/2

1.192/7 1.160/42 1.116/2 1.106/4

1.194/7 1.160/39 1.118/2 1.100/5

1.087/2

1.090/4

1.20/25 1.19/16

1.19/9

1.157/35 1.11/15

1.091/< 1 1.067/< 1 1.047/2 1.043/2 1.030/10 1.003/18

2.087/X

2.01/X

1.83/8 1.81/4

1.258/5 1.248/7 1.230/6

1.094/1

2.088/X

1.828/1 1.807/2

1.419/13

1.203/4 1.19/3 1.158/37

2.085/41 2.066/17 2.055/24 2.008/X 1.940/4 1.925/3 1.915/1

1.09t4

1.090/17 1.09/10

1.06/3 1.05/11 1.043/5 1.004/17 0.996/3

1.004/17 0.998/3

1.00/19 0.95/10

N. Mattern et al.

/

Materials Science and Engineering A 194 (1995) 77-85

XRD patterns clearly indicate for TA>--500°C the simultaneous formation of Fe~B and f.c.c. Cu from the remaining amorphous matrix phase (Fig. 5). The d spacings and intensities of the observed reflections of crystallized Fe76Si15.sB7CuL5 are given in Table 1. However, the most important point of the effect of Cu addition on the crystallization of FeSiB alloys is the increase in the (metastable) temperature region of coexistence of crystalline Fe3Si and the amorphous matrix phase. The dependences of the crystallization temperatures and the phases formed on the Nb addition are given in

57~

I"-<

5251

p

,5o0

C)

o

"

•

475

E •

~ : 429

37N

329

30O

®[[+ve3si ~°'"°~ i o .... 0

e®

®

•o " -- Q

o

o

1Sufface [e~/stalliza~onlO

- -..... o

0 0

0 0

0 0

0,5

1

1,5

5O0 3

-t

C u -content x in Fez7.5_x Si15.5B7Cu x

Fig. 3. Phases formed by annealing for 1 h of amorphous Fe77.5_xSils.sBvCut vs. temperature T A and crystallization temperature T~so, determined from DSC measurements (crosses connected by lines).

8|

Fig. 6. Txi where Fe3Si is formed is shifted from 4 2 5 ° C < T x 1 < 4 5 0 ° C (without Nb) to 5 2 5 ° C < Txl < 550 °C (with 3 at.% Nb), as also reported elsewhere [12]. The second stage of crystallization takes place at higher annealing temperatures TA= Tx2 with increasing Nb content up to 1.5 at.% Nb. A higher Nb content (over 1.5 at.%) reduces the crystallization temperature T~2. The phase composition after annealing above Tx2 is changed. For the alloy with 0.5 at.% Nb, only Fe2B is observed; for 1.5 at.% Nb, Fe2B is formed simultaneously with a new phase U1 at 650 °C. The alloy containing 3 at.% Nb exhibits the simultaneous formation of F%Si and U1 at TA=550 °C. The d values and intensities of annealed amorphous Fe74.5Sils.sBTNb 3 are given in Table 1. The additional reflections of U1 are not in agreement with any of the known phases such as Fe23B6 or Fe3 B (Table 1), as discussed in refs. [16] and [17]. Nevertheless, the composition of U1 should be about Fe60Nbl0B30 [18]. The temperature region of the coexistence of b.c.c. Fe3Si and the amorphous matrix phase depends on the Nb content (Fig. 6). Between 0 and 1.5 at.% Nb, the temperature range increases; it decreases in width; in amorphous alloys with 3 at.% Nb, there is no or only a very small temperature range of the coexistence of b.c.c. Fe3Si and the amorphous matrix phase. Combined additions of Cu and Nb at a ratio of 1:3 lead to increases in T,I and T,2. Fig. 7 shows the dependence of the phase formation on 7), of amorphous Fevy.5_4xSi13.sB9CuxNb3x (x = 0, 0.25, 0.5, 0.75,

2.O_8/

I [cps]

i .04

o.oo, . _I ~ ~ -l ~ 30.0

50.0

I

__.L.___ '_ -

I 70.0

I

.,.

I 90.0

"1

i ll0.0

,,

,---.--,,/~400

I

t''~'350 130.0

28[ ° ] Fig. 4. XRD patterns of amorphous Fe76Sijs.5B7Cul. 5 vs. annealing temperature TA.

TA[ C.

82

N. Mattern et al. xlO 6 3.00

/

Materials Science and Engineering A194 (1995) 77-85

1

>t 2.701

43 2.40 I

0 Fe3S i

H 2.10

I~ Fe2B

O Cu

I . 80 I I

i.50 1

r i.20

0.90 J

c)

0.60 i 0.30 1

4~.o

~o'.o

~o

6o~o

55~o

Bragg-angle 2(9

Fig. 5. XRD patterns of annealed amorphous Fe76Si15.sB7Cul.5: (a) 425 °C; (b) 525 °C; (c) pattern of (a) intensities multiplied by factor of 20.

•

•

E 6541

6751

•

•

,_< ~

ot~

+~-~. Io

Icrystamnel

~

~

•

•

,® ~

E

•

•

on:~i

6SO ~

®

•

•

OY

800

~. s ~

O_- - - -

®

®

o,, - m

•

57~

3

= 47S

,~

0 O

45o ~ " ' - O 42,51 ) O

4oo )-----O

o

o,~

;

O

~,~

I. . . . . . . . . .

2

I

~;

0 O

5oo

_~

~

475~

~

~

4,~o~

42~

4oc~

O--

~

~,~

Nb --contentx in FeT/.5-x Si15.5B7Nbx

® ---

. ....•

-

--0

OO

0

0 ~ 0

0

0 I- - " ~'~10

0

0

0

0,25

0,5

0,75

01:3 OCl 0E1-1

x in Fe77.5_4xSi135BgCuxNb3x

Fig. 6. Phases formed by annealing for 1 h of amorphous Fe77.5_xSi~5.sBTNbx vs. temperature TA and crystallization temperature T~sc, determined from DSC measurements (crosses connected by lines).

Fig. 7. Phases formed after annealing for 1 h of amorphous Fe77.5_xSis3.sB9CuxNb3x (x=0, 0.25, 0.5, 0.75, 1.0) and Fe73.sNiss.sg7CulNb 3 (squares) vs. TA.

1.0) with Si and B contents similar to those of the alloys discussed before. T h e values for the fe73.5Sils.sB7 C u l N b 3 alloy are also indicated, by the squares at x = 1.0. T h e X R D patterns of Fe73.sSils.sB7CulNb3 as a function of annealing treatment are shown in Fig. l(b). T h e a m o r p h o u s alloy with combined Cu and Nb additions crystallizes primarily with the formation of Fe3Si at 475 °C < Txl < 500 °C and, after annealing at about 600 °C < Txz < 650 °C, the phase U1 is observed. T h e

phase composition is similar to that of the annealed Fe74.sSils.sB7Nb3 alloy (Table 1 ). T h e formation of f.c.c. Cu, such as in amorphous Fe76Siz5.sBTCul. 5, could not be detected, but cannot be excluded, because of the coincidence of the reflections of the U1 phase. T h e Si content of the Fe3Si phase is about 21 at.% (Fig. 2). C o m p a r e d with the crystallization of Fe77.sSils.sB7, the range of the coexistence of Fe3Si and the amorphous matrix phase is extended by the combined Cu and Nb additions, and is shifted to higher annealing tempera-

N. Mattern et al.

/

tures of TA= 500-600 °C. The line broadening in the diffraction patterns indicates the small grain size of the Fe3 Si phase formed in this alloy. 3.2. Microstructure o f the partly crystallized state

The microstructure of the samples annealed at Txl < TA< Txz consists of crystalline Fe3Si and an amorphous matrix phase. From the intensities of the diffuse maxima of the amorphous phase and the crystalline reflections, the volume fractions V can be determined [19]. Fig. 8 shows the volume fractions of the Fe3Si phase obtained as a function of TA. The onset of crystallization is affected by the different additions but, after annealing at TA= 500 °C, about 75 vol.% of the crystalline Fe3Si phase and 25 vol.% of the amorphous matrix phase are reached in all alloys. Fig. 9 shows the dependence of the mean crystallite size D of the Fe3Si phase on the Cu content, as estimated from the line width [19]. Annealing at irA= 425 °C leads to the formation of Fe3Si with a grain size of about 150 nm. The mode of nucleation during crystallization of metallic glasses depends strongly on the annealing temperature [20]. Annealing at higher temperatures, such as Ta = 500 °C, gives smaller values of D, because of enhanced nucleation--about 40 nm is observed. With an increase in TA, D is found to increase also. The dependence of D on TA is in agreement with observations and calculations in ref. [20]. Fig. 9 shows clearly that Cu addition, as such, does not reduce the crystallite size of the Fe3 Si phase. In Fig. 10, the effect of the Nb content on the crystallite size is shown. The alloy with 0.5 at.% Nb exhibits a similar behavior to that without Nb. With higher Nb contents, the crystallization is shifted to higher temperatures. Samples annealed at TA=550°C show a decrease in D with the Nb content, from 40 nm (0.5

at.% Nb) to 20 nm (3 at.% Nb). The grain size of the alloy with 3 at.% Nb is similar to that in Fe73.sSils.5ByCUlNb3, which is also given in Fig. 10. The number N of crystallites per volume unit can be calculated assuming a uniform ensemble of spherical crystals of diameter D: N = 6V ~D 3

200 I'~ Fe-n.sShs.sB7 [ "~"Fe77.08il5.5B-/Ct.lo.5

I~

~ .8 .=

is0

't\

:

I*~~.

:

'~~c°'

100 5O

0 40o

i

r

r

450

500

550

600

Annealing temperature TA [°C]

Fig. 9. Mean grain size D of the F%Si phase of Fe77.5_xSi15.5BTCu x vs. annealing temperature TA.

140

"e"Fe77.sSi15.sB7 "~' Fe77.0Si15.sB7Nbo.5 l " Fe76.0Sil5.5B7Nbl.5 * Fe74.SS!15.5BTNb3

120

0,8

¢•

~> 15


Using the values determined for the volume fraction V and crystallite size D, the crystallite number density can be estimated (Fig. 11 ). Without any addition of Cu or Nb, the density reaches a maximum at TA= 500 °C. Above 500 °C, the density decreases according to the increasing particle size. The addition of Cu only does not change this behavior significantly. However, the number of crystal-

1

.o_ =

83

Materials Science and Engineering A 194 (1995) 77-85

0,6 0,4

g

100

/,~ ,,/ I / [ / I

115

..... _...

80

~ !,

.~_

0,2 20

II

•deFe73.5S15.sB7Cu1Nb3 0

0 4OO

450

500

550

600

Annealing temperature TA [°C]

Fig. 8. Volume fraction V of the Fe3Si phase vs. annealing temperature TA.

450

,

r

470

490

510

530

Annealing temperature

5,50 TA

570

590

[°C]

Fig. 10. Mean grain size D of the Fe3Si phase of Fe77.5_~Si15.5BTNbxand Fev3.sSi155B7CulNb3vs. annealingtemperature TA.

84

N. Mattern et al.

/

Materials Science and Engineering A 194 (1995) 77-85

100000 5,.e..Fe77,5Si 5 Bl ~ "V"Fe76"sSils'sBTCu1 10000

I I'*F~oSe~.sB, cu,.~ I

'°"011-+,,-,,,.o_+!+.+,,-,,,,o,.+ I

l " Fe76.oSi15.5B7Nb1.5 Fe74.5Sil 5.5B7Nb'3

~

l

f'",~.

-

yo -

~% -,-

8

1o0

:

/

I+

:

100

8 1

.#

i

10 0,1

,

as--qu.300 325 350 375 400 425 450 475 500 525 550 575 600

---g400

425

450

475

500

525

550

575

625

Annealing temperature TA [*C]

650

Annealing temperature TA [°C]

Fig. 12. Coercivity H c vs. annealing temperature TA.

Fig. 1 1. Crystallite number density in dependence on TA. Fe3Si of grain size 40 nm. However, in Fe74.5Sits.sBTNb3, the enhancement of H c at TA= 550 °C is

lites per unit volume is strongly correlated with the amount of Nb. The crystallite density after annealing at TA= 500 °C is enhanced by one or two orders of magnitude in the alloys with 1.5 or 3.0 at.% Nb. The Nb additions obviously affect the growth of the Fe3Si phase. The primary crystallization is diffusion controlled [21]. The diffusion coefficient in the amorphous alloy depends on its atomic structure and on the moving species. Amorphous F e - S i - B - C u - N b alloys are densely packed structures with a nearest neighbor distance of 0.255 nm [22], which corresponds to the atomic diameter of Fe. The movement of Nb requires collective rearrangements of atoms, as a result of the greater atomic diameter of Nb (0.286 nm), and the diffusion and the growth of b.c.c. Fe is impeded. Confirming this assumption, it was shown in ref. [23] that the grain size of the Fe3Si phase in amorphous Fe73.sSits.sB7CulR3 is controlled by the atomic diameter of the added refractory element R.

3. 3. Magnetic properties

Fig. 12 shows the behavior of the coercivity H c of different amorphous alloys at the annealing temperature TA. The crystallization of the amorphous alloys leads to drastic changes in H~, with the exception of Fe73.sSils.sBTCulNb 3. The increase in Hc by about one order of magnitude in Fe76Sils.sBTCuL5 at TA= 425 °C and in Fe77.5Sils.sB 7 at TA= 450 °C corresponds to the precipitation of Fe3Si with grain size 40 nm. The further increase in both alloys at TA= 500 °C is caused by the formation of Fez B. Surface crystallization is probably responsible for the increase in H~ for TA between 375 and 400 °C. In the Nb-containing alloys, the behavior of H e with annealing indicates crystallization. The increase in H~ in Fe76Sils.sBTNbl. 5 at TA=500°C corresponds to

caused by the boride phase which is simultaneously formed with Fe3Si of grain size 20 nm. There is no change in the low H c values resulting from crystallization in amorphous Fe73.sSi15.sB7 CulNb3 up to TA= 550 °C. At T= 500 °C, nanocrystalline Fe3Si with grain size 15 nm is formed. As a result of the small grain size, the crystal anisotropy averages out, resulting in low Hc values [2]. Low values of Hc are obtained also in Fe74.sSiIs.sB7Nb 3 in the amorphous state after annealing up to TA= 525 °C. Fig. 13 shows the variation of the saturation magnetostriction Zs with annealing temperature. The amorphous state has a saturation magnetization of about 2s= 20-30 x 10 -6. The decrease in ,~ at higher annealing temperatures corresponds to the values of formation of the Fe3Si phase given in Figs. 3, 5 and 6. The Fe3Si phase has a negative saturation magnetostriction, compensating for the positive 2 S value of the amorphous phase [2]. The advantage of the alloy Fe73.sSils.sB7Cu1Nb 3 is that a low coercivity (He=0.5 A m 1) and zero magnetostriction (2+=0) can be obtained in the partly crystallized state after annealing at TA= 550 °C. This combination of magnetic properties cannot be achieved by the alloys with separate additions of Cu or Nb.

4. Conclusions

The additions of Cu and Nb essentially affect the crystallization temperature, the phases formed and the microstructure of annealed amorphous Fe77.sSi~s.sB7 in quite different ways. Cu decreases the temperature Txt of the primary formation of Fe3Si, so enhances the range of the coexistence of Fe3Si and the amorphous matrix phase. Cu does not affect the nuclei density or the microstructure of the Fe3Si crystallites. However, at higher annealing

N. Mattern et aL

/

Materials Science and Engineering A194 (1995) 77-85

T h e authors thank C. Stiller for DSC measurements. Experimental assistance from U. Kiihn, Ch. Moschner and K. Stange is much appreciated.

,50

•e- Fe77 5Sil s.sB7 <9

-1F-Fe76.0Sil 5.5 B7 Cu 1.5

°l 0

40

~:

'

;

ii. Fe76 0Sil 5 5B7Nbl. 5 •~ F074.5Sil 5.5B7Nb3

-

•\

---~-~._

I + Fe73.5Sil 5.5B7Cu1Nb~

References

20 10

0

as-qu

85

~'

400

475

500

Annealing temperature

425

450

T A [°C]

525

550

Fig. 13. Saturation magnetostriction 2~ vs. annealing temperature TA. temperatures, the mean grain size becomes smaller, as a result of enhanced nucleation rates. T h e r e is no indication of f.c.c. Cu before or during the first stage of crystallization. Crystalline f.c.c. Cu is formed in the second stage of crystallization, simultaneously with Fe2B. T h e addition of Nb increases Txl but, at higher Nb contents, the range of the coexistence of Fe3Si and the amorphous matrix phase is reduced and disappears. With the addition of 3 at.% Nb, the crystallization is characterized by the simultaneous formation of nanocrystalline Fe3Si and further unknown phases. Nb was found to be responsible for the increase in the crystallite density and the formation of nanocrystalline Fe3Si, resulting from the diffusion barrier of Nb atoms. T h e combined addition of Cu and Nb enhances the stability of amorphous Fe77.5Si15.sB7 against crystallization, shifts Txl to higher values, and stabilizes the t e m p e r a t u r e - t i m e region of the coexistence of nanocrystalline Fe3Si and the amorphous matrix phase. T h e magnetic properties are strongly correlated with the microstructure. Excellent soft magnetic behavior is only obtained with the combined addition of Cu and Nb, and after annealing for 1 h at 550 °C. Acknowledgments

This work was supported by the Federal Ministry of Research and Technology under Contract 03M 50105.

[1] Y. Yoshizawa, S. Oguma and K. Yamauchi, J. Appl. Phys., 64 (1988) 6044. [2] G. Herzer, 1EEE Trans. Magn., 25(1989) 3327. [3] M. Miiller, N. Mattern and L. Illgen, Z. Metallkd., 12 ( 1991 ) 895. [4] K. Hono, K. Hiraga, Q. Wang, A. Inoue and T. Sakurai, Acta Metall. Mater., 40(9)(1992) 2137. ]5] J.D. Ayers, V.G. Harris, J.A. Sprague and W.T. Elam, Appl. Phys. Lett., 64 (8)(1994)974. [6] S.H. Kim, M. Matsuura, M. Sakurai and K. Suzuki, Jpn. J. Appl. Phys., 32, Suppl. 32-2(1993)676.

[7] K. Suzuki, A. Makino, A. Inoue and T. Masumoto, J. Appl. Phys., 70(10)(1991)6232. [8] A. Makino, Y. Yamamoto, S. Hirotsu, A. Inoue and T. Masumoto, Mater. Sci. Eng., A, 179/180(1994) 495. [9] K. Narita, IEEE Trans. Magn., 16 (1980) 435. [10] W.B. Pearson, A Handbook of Lattice Spacings and Structures of Metals and Alloys, Pergamon, London, 1958, p. 656. [11] G. Herzer and H. Warlimont, Nanostruct. Mater., 1 (1992) 263. ] 12] T. Kulik, Mater. Sci. Eng., A 159 (1992) 95. [13] H.G. Wagner, M. Ackermann, R. Goa and U. Gonser, in S. Steeb and H. Warlimont (eds.), Rapidly Quenched Metals, North-Holland, Amsterdam, 1985, p. 247. [ 14] A. Gupta and S. Habibi, Mater. Sci. Eng., A 133 ( 1991 ) 375. [15] U. K6ster, Mater. Sci. Eng., 97(1988) 233. [16] G. Hampel, A. Pundt and J. Hesse, J. Phys. Condens. Matter, 4(1992) 3195. [17] J. Jiang, F. Aubertin, U. Gonser and H.R. Hilzinger, Z. Metallkd., 82 ( 1991 ) 698. [18] G. Herzer, J. Magn. Magn. Mater., 112 (1992) 258. [19] H.R Klug and L.E. Alexander, X-ray Procedures for Polycrystalline and Amorphous Materials, J. Wiley, New York, 1974. [20] U. K6ster, Z. Metallkd., 75 (1984) 691. [21] U. K6ster, U. Sch6nemann, M. Blank-Bewersdorff, S. Brauer, M. Sutton and G.B. Stephenson, Mater. Sci. Eng., A133(1991)611.

[22] N. Mattern, M. Miiller, C. Stiller and A. Danzig, Mater. Sci. Eng., A, 179/180 (1994) 473. [23] M. Miiller and N. Mattern, J. Magn. Magn. Mater., A, 136 (1994)79. [24] Powder Diffraction Standard Data Base, International Centre of Diffraction Data, Swarthmore, 1994.