Solubility of antimony in cobalt, nickel and CoNi alloys

Journal of the Less-Common

SOLUBILITY ALLOYS

OF ANTIMONY

K. ISHIDA, M. HASEBE, Department

Metals, 114 (1985)

361

361 - 373

IN COBALT,

NICKEL AND Co-Ni

N. OHNISHI and T. NISHIZAWA

of Materials Science,

Faculty

of Engineering,

Tohoku

University, Sendai

fJapW (Received

February 27, 1985)

Summary

The solid solubility of antimony in cobalt, nickel and Co-Ni alloys has been examined by electron probe microanalysis. It has been ascertained that the solubility of antimony decreases abnormally below the Curie temperature. The compound phase y-(Co, Ni)Sb equilibrates with the f.c.c. Co-Ni phase at less than 20% Ni, while /I-(Ni, Co),Sb, appears in a region of higher nickel content. These experimental data have been analysed thermodynamically and the phase equilibria in the Co-Ni-Sb system below 50% Sb have been calculated.

1. Introduction

Cobalt and nickel are both ferromagnetic and are similar in many respects, forming a thermodynamically ideal solid solution. As shown in Fig. 1 [ 1 - 31, however, the character of solid solubilities of antimony in cobalt and in nickel seems to be somewhat different. In the first instance, the compound phases in equilibrium with nickel and cobalt phases are &NisSb, and y-CoSb respectively. In the second, the solubility of antimony in nickel is almost independent of temperature above 500 ‘C, while the solubility of antimony in cobalt is expected to decrease with temperature because of the effect of the magnetic transition [4 - 121. Because of this, the features of the phase equilibria in the Co-Ni-Sb system excite our interest with regard not only to the relative stability of /? and y phases, but also with regard to the abnormal change in the solubility of antimony near the Curie temperature. In addition, it has been recognized that antimony is one of the metalloid elements which have a harmful effect on the mechanical properties of steels as well as superalloys; sometimes these elements have been discussed in connection with grain boundary segregation [ 13 - 161. The purpose of the present study is to determine exactly the equilibrium composition 0022-5088/85/$3.30

0 Elsevier Sequoia/Printed

in The Netherlands

362

(a)

c0

20

40

Fig. 1. Phase diagrams [2] and Shunk [3]):

60

80

for (a) Co-Sb

I

and (b) Ni-Sb

systems

(from

Hansen

[l],

Elliott

of the matrix phase and the intermetallic compounds in Co-Sb, Ni-Sb and Co-Ni-Sb alloys, and to analyse thermodynamically the experimental data, taking into account the influence of the magnetic transition on the Gibbs energy of the matrix phase.

2. Experimental

procedure

2.1. Materials Co-Sb and Ni-Sb binary alloys and Co-Ni-Sb ternary alloys containing 10 - 50 wt.% Sb were made of electrolytic cobalt (purity, 99.98%), electrolytic nickel (purity, better than 99.95%) and pure antimony (purity, 99.99%) by induction melting under an argon atmosphere. They were cut into discs, approximately 10 mm in diameter and 3 mm thick, and then heated in flowing hydrogen at 800 “C for 16 h in order to remove carbon and nitrogen. These specimens were equilibrated at fixed temperatures between 700 and 1100 “C for 100 - 1400 h, and were subsequently quenched in iced brine. Diffusion couples consisting of antimony and cobalt, nickel or Co-. (25,50,75 wt.%)Ni alloys were also prepared to examine the equilibrium composition between 700 “C and 900 “C [17]. The couples were assembled as shown in Fig. 2, and were sealed in transparent quartz capsules under vacuum and then heated for 700 - 1400 h.

363

Ga,Ni or Co-Ni

Fig. 2. Scheme of the diffusion couple.

2.2. Electron probe microanalysis The determination of the concentration was carried out by a Shimizu ARL-EMX electron probe microanalyser using a LiF crystal for Co KCYand Ni Ka radiation, and an ADP crystal for Sb La radiation. The accelerating voltage for the electron beam and the sample current were kept at 20 KV and 10 nA respectively. The take-off angle of the spectrometer was 52.5”. The relative intensity of radiation Ki for component i was converted to the weight fraction Ci according to the following equation proposed by Ziebold and Ogilvie [ 181. 1----i

1 - Ci

-=%K, i where 6 is the conversion &=

olijcj + cj

%k

parameter

for the i-j-k

system and is expressed

as

ck

+ Ck

where (Yij and o!ik are the conversion parameters for component i in the binary i-j and i-k systems respectively. These values were determined by calibration experiments and are listed in Table 1. The equilibrium composition of individual phases in two- or threephase specimens was determined by point counting the X-ray intensities at a minimum of 15 positions. In the case of diffusion couples, the equilibrium TABLE 1 Conversion parameters ff~

i

CO

Ni

Sb

-

0.91 1.39

0.88 0.72 -

i CO Ni Sb

1.01 1.37

364

composition was estimated by extrapolating the concentration profile to the phase boundary between the o and compound phases. Typical examples of the concentration profile in diffusion couples for Co-Sb and Ni-Sb systems are shown in Fig. 3.

2

6,,-

E

* 2.0 -

#

I

=: 1.5. 1.0. 0.5 0-6o-U)-M

(4

0 Dirtancr

20

40

60 ,

&ml

OL--J---60 -40

(b)

-20 0 20 40 Distance (rm)

60

Fig. 3. Concentration profile in Co/Sb and Ni/Sb diffusion couples: (a) Co/Sb equilibrated at 900 “C for 740 h;(b) Ni/Sb equilibrated at 800 “C for 890 h.

2.3. Curie temperature measurement The Curie temperature was determined using an automatic balance in a field of 3 kOe at the heating rate of 10 ‘C min-’ .

3. Experimental

results

3.1. Solubility of antimony in cobalt and in nickel The experimental results on the solubility of antimony in nickel

magnetic

are summarized

in Tables

2 and 3 respectively.

in cobalt and The solubility of

TABLE 2 Equilibrium composition of a-(CoSb) and ‘y-(CoSb) in the Co-Sb system Temperature WI

01phase (at.% Sb)

y phase (at.% Sb)

1100 1075 1050 1000 950 900 800 700

2.2 2.0 1.6 1.1 0.8 0.6a 0.3a 0.2a

48.9 48.9

aDetermined by the diffusion couple method.

48.2 49.2 49.0 48.2a 48.0a 50.3a

365 TABLE 3 Equilibrium composition of cr-(NiSSbz) and fl-(Ni,Sb,) in the Ni-Sb system Temperature CC)

(ILphase (at.% Sb)

/3phase (at.% Sb)

1100 1000 950 900 800 700

9.4 9.6 9.0 8.8a 8.9= 8.ga

28.6 29.0 26.7 26.1a 25.5a 26.4a

aDetermined by the diffusion couple method.

antimony in nickel is inconsistent with the Hansen diagram, while that for cobalt is much smaller than the earlier data [ 191, as shown in Fig. 4. It should be mentioned that the composition of the y phase in the Co-Sb system is nearly 50:50, and that the composition of the fl phase in equilibrium with the cy phase in the Ni-Sb system shows an exceptional change, probably because of an ordering reaction in the fl phase.

cooNi

(b)

10

20 Antimony

30 Content

40

50 (at

60

‘h)

Fig. 4. Experimental results for (a) the Co-Sb and (b) the Ni-Sb binary systems.

3.2. Solubility of antimony in Co-Ni The experimental results on the Co-Ni-Sb ternary system are given in Table 4 and Fig. 5. It is clear in Fig. 5 that the solubility of antimony in the 01phase increases with increase in the nickel content. Furthermore, the /3-(Co, Ni),Sb, phase is stable over a wide range of composition, while the

366 TABLE

4

Equilibrium Temperature (“C)

composition

of (Y, /? and y phases in the Co-Ni-Sb

01phase

(at.% Sb)

flphase

(at.% Sb)

system

‘yphase (at.% Sb)

Ni

Sb

Ni

Sb

Ni

Sb

1100

25.5 46.1 68.2

2.4 3.4 5.6

41.3 56.2 64.1

26.9 28.9 29.2

-

-

1050

24.6 45.9 68.5

2.2 3.0 5.1

46.4 56.4 65.9

30.0 29.0 27.8

-

-

1000

13.3 16.7 23.2 44.5 60.5 68.2 80.2

1.6 1.9 1.8 2.6 3.6 4.7 6.5

-

-

40.1 47.1 55.9 64.2 65.0 70.2

29.4 29.5 30.8 27.8 29.4 27.0

31.9 36.6 -

44.5 43.9 -

950

22.4 44.8 68.3

1.2 2.2 4.3

45.1 57.4 65.9

31.7 31.1 29.0

-

-

900

6.0* 12.1 18.4 18.8a 41.2a 59.7 80.3

0.6= 0.9 1.0 o.ga 1.68 2.6 6.0

-

-

52.7 46.8a 61.8a 65.2 70.6

28.4 28.2a 28.5a 28.3 27.3

18.1 33.7 42.6 -

46.1a 45.8 44.2 -

800

20.1 30.ga 58.8 80.3

0.5 0.4s 1.7 5.4

54.4 60.8a 66.1 69.9

30.7 29.1a 28.9 28.3

40.1 -

48.4 -

700

20.2 81.1

0.4 4.1

59.2 70.4

29.7 28.3

41.8 -

46.8 -

aDetermined

by the diffusion

couple

y-(Co, Ni)Sb phase equilibrates irrespective of temperature.

method.

only with the 01 phase at less than 20% Ni

3.3, Effect of antimony on Curie temperature of Co-Ni The effect of antimony on the Curie temperature of Co-25at.%Ni, Co-50at.%Ni, Co-75at.%Ni and nickel is illustrated in Fig. 6. These results are described by the following equation [ 91. Tea = “Tcff + AT,”

(1)

367 (b)

Sb

CO

20

(c)

CO

40 at

60

Ni CO

60 at

20

% Ni

Sb

9OO'C

20

60

WWC

(d)

60

AT,” = [ATI + Arz(1 -YNi)]XNi

40 at % Ni

BOOT

60

00

Ni

Sb

Ni at %

%Ni

Fig. 5. Experimental results on phase 1000 ‘C, (c) 900 “C and (d) 800 “C.

Sb

equilibria

+ ATsbx,

in Co-Ni-Sb

Ni

system:

(a) 1100 ‘C, (b)

(2)

where Xi is the atom fraction of component i, yNi = xNi/(xco + XNi) and ‘Tea (= 1394 K) and Tea are the Curie temperatures of pure cobalt and the Q phase in the Co-Ni-Sb system respectively. AT, (= -3000 K) is the rate of depression of the Curie temperature with the addition of antimony to the Co-Sb binary system and Ar, (= -763 K) and AT, (= 420 K) are the parameters in the Co-Ni binary system. According to the above expressions, the gradients of the Curie temperature can be written as a T,” =AT, axsb

- &YN~~

(3)

368

970 ‘

.u ?

630<,

3

.? 5

620. 4

"

360;

‘,

340. 0

0.2

0.4

0.6

0.8

( at '1. 1

Sb

Fig. 6. Effect of antimony on Curie temperature of Co-Ni alloys.

-

= AT~ + ATE - 2AT,y,i

(4)

axNi

4. Thermodynamic

analysis

4.1. Description of the Gibbs energy It has been established that the Gibbs energy of a ferromagnetic cyphase can be described by separating it into paramagnetic and ferromagnetic terms [4,5,7,9,111. G” = [G”lp + [AG”lf

(5)

where [G”lP is the Gibbs energy of the CYphase in the fully paramagnetic state and [ AG”lf is the extra term due to the magnetic transition. As reported in a previous paper [ll], the ferromagnetic term is approximated by the following equation. [AG”(T)lf

T*=T-

= (1 - xfiX,&) g

“T,”

[A”G,,“(T*)]f

(6)

(7) TC” where [ A”GCoQ] f is the ferromagnetic term in the Gibbs energy for pure cobalt, which is evaluated from the data on ferromagnetic specific heat by Normanton [ 201. fi is a factor representing the effect of component i on

369

the size of the ferromagnetic term, and it may be assumed here that fs,, = 1 and fNi = 0. The ferromagnetic terms for enthalpy and entropy are expressed as [AHa(T

= (1 -CfiXia)

[AS’“(Z’)]f =

(1- CfiXitij

$$

[A”Hc,“(T*)]f

(8)

[A”Sc,“(Z’*)]’

(9)

This formulation is the same in principle as that of the method presented by Inden [ 211, and both approximations yield almost the same results. The paramagnetic term in the Gibbs energy of the (11phase is approximated by the regular solution model as follows: [GalP = Z)[“GiUlP3ti + ~~[SZ~jO]“XiXj

+ RTxXi

In Xi

(10)

where [“Gi”]” is the paramagnetic term in the molar Gibbs energy of component i in the (Y state and [aij”]’ is the paramagnetic term in the interaction energy between i and j atoms. The Gibbs energy of the B-(Co, Ni),Sb, phase is approximated as follows [ 221: G* = oG~o,~,eY~oe + mRT(Yc,*

+ “GNi,,,~,*YNiB

+ mficoNi*Yco’YNi*

In ycoe + YNi* ln yNi*)

where ycO e_- Xco*/(XNi* + xcoe) tions of Co,Sb, and Ni,Sb, “G%?I %I and ‘GNirns are the Ni,Sb, phases and ~coNi* is cobalt atoms in the 8 phase.

and yNi* = XNi*/(XNi* + 3tcoe) are the fracin the f3-(Co, Ni),Sb, phase respectively. molar Gibbs energy of 8-Co,Sb, and 8 the interaction energy between nickel and

4.2. Chemical potentials of components The chemical potentials of cobalt, nickel and antimony are calculated from the following equations. dGff

PC0(L=Ga-xNi-

axNi

in the (11phase

(12) (13)

PNi

kba = VCoQ +

aG” -x

(11)

aG”

04)

zg Sb

Inserting eqns. (10) and (6) into eqns. (12) - (14), the paramagnetic and ferromagnetic terms of the chemical potentials of components in the CYphase are obtained as follows:

370

bCoalP*

[*G~o~l~ -

+ [fzcoNicyIp(l

[aNiSbCY]PXNiXs + RTln

+ [aNiGJb’]‘(l EIISbalP =j *Gs~ -

-xNi)x&

+ [aN,“]‘xNi(l

Pk,OLlf =

$

(15) -

[%~s~~]~%oXSJ

XNi

(16)

+ E~~o~b~l~~~o(1

-xs\,)

-XC~&SI

+ RTh

[A”Gc,“(T*)lf

f “zYN;;-

[AOG,,“(T*)lf

+

-XS~)

XSb

(17)

‘TC [A”H,,“(T*)]f

(18)

c

[&Ni*If a $ =

+ RTln

[%!oNifffP%oXNf

+ [~~o~b~]~(1

xc0

E”GNilP ’ [~~~Ni~]‘~C~(l-~Ni)

lPNialP x

[4Glf

-XCo)xNi

ATI+ A:2yNi2

-AT,

IAoH

OTC

c

ATsb-AT,

(T*)]f COa:

cw

~~“H,,“P’*)lf

o_

The chemical potentials of 8-Co,Sb, obtained from eqn. (11) as follows:

(19)

and B-Ni,Sb,

in the 8 phase are

4.3. Equilibrium between (Yand 0 phases The ~qu~ib~urn between the rx and 8 phases is given by kk+,$+,* = mkoDL + w-hba

(23)

pNi,Sb,@ = mPNi@ + wSbQ

(24)

The solubility of antimony in the cy phase in equilibrium obtained from eqns. (15) - (23) as follows: a:

XSb

~

with the 8 phase is

(IQ]’ + [Qlf)

exp

(25)

RT +fsZ n

m+n +[acoNi”]’ 1 - 7XNi

XNi -

[a~o~b”l~(l

CoNi*(yNi* )2

-xNi)-

[‘nNisboL]pxNi

i (26) [Qlf = -

;

[A”Gc,“(T*)lf

$

-

AT,

e

“T,-’ [A”Hc,“(T*)]f

+ z Arz(YNi )2 (27)

371

where

eW”Gco,,,sJP = oG~o,~b,

m

[‘GcoQIP - n”GSbO

4.4. Calculation of the solubility The thermodynamic parameters in the above equations are evaluated from the experimental information and thermochemical data [ 231, as listed in Table 5. The solubility of antimony in Co-Ni alloys is calculated from eqns. (25) - (27) assuming that the /3 and y phases are stoichiometric compounds, i.e. m/n = 5/2 and l/l for the fl and y phases respectively. Figure 7 shows the calculated solubilities of antimony in Co-Ni alloys at constant TABLE 5 Thermodynamic

parameters for calculating the phase diagram in Co-Ni-Sb

Parameter

J mol-‘,

T (K)

0 40000 -20000 0 2290 -40160 133800

-1OOT -36.64T

system

1720 -222’ 4550 -5.90T

( ec1

Tmprraturc 1100 1000 lO.Oe 8.0

SW

MO

600

Ni

.._

-

$

700

10.0 8.0

P,s_72.,.Lli

2.0

t

'; 1.0

7

8

9 l/T

10 (~10‘~)

11

12

Fig. 7. Effect of magnetic transition on the solubility of antimony in Co- -Ni alloys.

372 (1) mo’c

Sb

(b) 600-C

Sb

A 60

7

CO

20

40 at

60

60

Ni CO

20

Fig. 8. Calculated phase diagrams for Co-Ni-Sb

40

at

% Ni

60

60

Ni

7. N i

system: (a) 1000 “C and (b) 800 “C.

ratios of nickel and cobalt, where the broken lines are hypothetical solubilities of antimony provided that the cr phase was paramagnetic even below the Curie temperature. It is shown that the solubility of antimony in the ferromagnetic region is much smaller than that expected from the paramagnetic region. In other words, the effect of the magnetic transition on the solubility of antimony is quite remarkable in the Co-Ni-Sb system. The calculated phase diagrams for the Co-Ni-Sb system below 50 at.% Sb are illustrated in Fig. 8, which show good agreement with the observed ones and the calculated tie lines reflect well the experimental data.

5. Conclusions 1. The phase diagram of Co-Ni-Sb system below 50 at.% Sb was determined by electron probe microanalysis. 2. The solubility of antimony in Co-Ni alloy decreased abnormally below the Curie temperature. This fact was well explained by thermodynamic analysis taking the effect of the magnetic transition into account. 3. The compound phase p-(Co, Ni)$bz appeared over a wide range of compositions, while the y-(Co, Ni)Sb phase only equilibrated with the o phase at less than 20 at.% Ni.

Acknowledgment The authors wish to express their thanks help in carrying out the experiments.

to Mr. S. Yokoyama

for his

373

References 1 M. Hansen and K. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York, 1958, pp. 500,1036. 2 R. P. Elliott, Constitution of Binary AZZoys, McGraw-Hill, New York, 1965, pp. 322, 671. 3 F. A. Shunk, Constitution ofBinary AZZoys, McGraw-Hill, New York, 1969, p. 553. 4 C. Zener, Trans. AZME, 203 (1955) 619. 5 M. Hillert, T. Wada and H. Wada, J. Iron Steel Inst., 205 (1967) 539. 6 H. Harvig, G. Kirchner and M. Hillert, Metall. Trans., 3 (1972) 329. 7 C. Wagner, Acta Metall., 20 (1972) 803. 8 M. Ko and T. Nishizawa, Trans. Jpn. Inst. Met., 16 (1975) 369. 9 T. Nishizawa, M. Hasebe and M. Ko, Acta Metall., 27 (1979) 817. 10 T. Takayama (formerly M. Ko), M. Y. Wey and T. Nishizawa, Trans. Jpn. Inst. Met., 22 (1981) 315. 11 T. Nishizawa, S. M. Hao, M. Hasebe and K. Ishida, Acta Metall., 31 (1983) 1403. 12 M. Hasebe, H. Ohtani and T. Nishizawa, Metall. Trans., A, 16 (1985) 913. 13 M. P. Seah and E. D. Hondros, Proc. R. Sot. London, Ser. A, 335 (1973) 191. 14 R. A. Mulford, Treatise Mater. Sci. Technol., 25 (1983) 1. 15 K. Ishida, S. Yokoyama and T. Nishizawa, Acta Met&, 33 (1985) 255. 16 C. L. White, J. H. Schneibel and R. A. Padgett, Metall. Trans. A, 14 (1983) 595. 17 M. Hasebe and T. Nishizawa, in G. C. Carter (ed.), Applications of Phase Diagrams in Metallurgy and Ceramics, National Bureau of Standards, Washington, DC, 1978, p. 911. 18 T. 0. Ziebold and R. E. Ogilvie, Anal. Chem., 35 (1963) 177. 19 U. Hashimoto, J. Jpn. Inst. Met., 1 (1937) 177. 20 A. S. Normanton, Met. Sci., 9 (1975) 455. 21 G. Inden, Physica B, 103 (1981) 82. 22 M. Hillert, Phase Transformations, American Society for Metals, Metals Park, OH, 1968, p. 181. 23 R. Hultgren, P. D. Desai, D. T. Hawkins, M. Gleiser and K. K. Kelley, Selected Values of the Thermodynamic Properties of Binary Alloys, American Society for Metals, Metals Park, OH, 1973, pp. 683, 1234.