CALPHAD Vol. 10, No. 2, pp. 175-185, 1986 Printed in the USA.

0364~5916/86 $3.00 + .OO (c) 1986 Pergamon Journals Ltd.

A THERMODYNAMIC EVALUATION OF THE CU-ZN SYSTEM

P.J. Spencer

Lehrstuhl fiir Metallurgle der Kernbrennstoffe und Theoretische Hiittenkunde der Rheinisch- Westfalisch Technischen Hochschule Aachen

D-510Q Aachen, W.-Germany

ABSTLPACT Experimental information on the thermodynamic properties and phase equilibria of Cu-Zn alloys has been used in carrying out a critical thermodynamic evaluation of the system. A set of coefficients is given which may be used to calculate ther- modynamic or phase boundary values in close agreement with experimental observations.

1. Introduction

Cu-Zn alloys are of major technological importance and have conse- quently been widely investigated. Apart from some uncertainty concerning the phase boundaries at lower temperatures, the Cu-Zn phase diagram is generally well-established [1,2]. A large number of thermodynamic investigations of solid and liquid alloys have also been made. These provide values which are in qualitative agreement, but there is significant scatter in some of the ex- perimental data, particularly for liquid alloys.

To provide a thermodynamically-consistent description of the Cu-Zn system which may also be used in calculations of phase equilibria in multi- component alloys containing Cu and Zn, a critical evaluation of the available experimental data has been made.

2. Experimental phase diagram information

The experimental phase diagram shown in Figure 1 and used as basis for the evaluation is essentially that proposed by Raynor in 1944 [3] from a review of the then available experimental results and reproduced with minor modifications by Hansen [I].

An eutectoid decomposition of 6' at 523 K, reported by Shinoda and Amano 141, has been refuted by the work of Massalskl and Kittl [5], Rao and Anantharaman [6] and Mack and Yang [7]. The a/a+B', a+6'/6' and B'/B'+y boundaries shown in Figure 1 are those reported by [6].

The range of the E phase has been modified slightly here from that suggested by Hansen to comply with the observation made by Massalski and King '81 697 K:

that the solubility of Zn in this phase extends to 88 at.% Zn at

Possible variations in the temperature and concentration ranges of the 6 phase are discussed later in conjunction with the results obtained from the evaluation. _______________^____--~-_--~-------------------- Received 2 October 1985, Revised 28 January 1986

175

176 P.J. SPENCER

F1G'JQ.E 1

Cu-Zn phase diagram used as basis for the evaluation.

3. Experimental thermodynamic information

3.1 Liquid alloys

The relative chemical potential o f Zn in liquid Cu-Zn alloys (Figure 2) has been determined by Schneider and S&mid [9] (923-1123 Kj, Everett et al. [IO] (1069-1303 K), Downie [II] (1200 K) and Azakami and Yazawa [IZ] (1073- 1473 K) using a variety of vapour pressure techniques, by Leitgebel [13] (1188-1773 K) and by Baker [14] (1373-1473 K) using the boiling oint method, and by Kleppa and Thalmayer [15] (900 K) and GerLing and Predel P 161 (1073- 1374 K) from emf investigations. There is general qualitative agreement between the different studies, but the values determined by Everett et al. are more negative than those reported by other investigators.

Calorimetrically measured enthalpies of mixing for the Liquid phase (Figure 3) have been obtained by von Samson-Himmelstjerna [17] (1273 K) and by Parameswaran and Healy [ 181 (1400 K). Bowever, apart from alloys at high copper concentrations, the results obtained from the two investigations differ greatly (s 5 kJ/g-atom at xZn = 0.5).

3.2 Solid alloys

Most of the experimental investigations of the relative chemical

The fee phase has also been investigated by K) using an atomic absorption method.

-5000

10000

-15000

E CI

9 - -20000 Ed

4’

-25000

-30000

-35000

THERMODYNAMIC EVALUATION OF THE CU-ZN SYSTEM

xZ" 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

1 1 1 I

FIGURE 3

Enthalpy of rnlxing of liquid Cu-Zn alloys.

177

FIGURE 2

Pelative chemical potential of Zn in liquid Cu-Zn alloys.

cu 0;’ 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 ( , , I I I I I

178 P.J. SPENCER

x2n cu 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 , , , , , ,

-60000

0

.

.

.

l

0.

Phorr

ICUI -

P.r ---

c _._

Argenl.Wokemon 1221110001~

Mossan.Sheu 123111063K1

Se1lh.XraurslZ0l11073K.1lZ3KI

Horgreovrs [I91 1100011

Olandet 12511673-67311

PcmslPr.Kopperporl11~11666-66511

IOOOK

7731

I

From Eralual~on

6731:

n

FIGURE 4

Relative chemical potential of Zn in solid Cu-Zn alloys.

The investigations of [I91 and [20] included measurements for the 8 -phase, while dlander r25] (685-899 K), for 8, y, 6 and s-phase'alloys.

using an emf method, obtained data

The results for the fee (Cu) phase are rather scattered, the scatter between the different investigations increasing with increasing Zn concen- tration.

For the 8 phase, the Au values of blander [25] lie between those of Seith and Krauss [20] and H&?greaves [19] which scatter by Q 10 kJ/g-atom

at ?Zn = 0.45 (ignoring the temperature dependence of the data).

The results obtained by blander represent the only available data for y, 6 and s-phase alloys.

Enthalpies of formation of fee alloys (Figure 5) have been determined from the calorimetric measurements made by von Samson-Himmelstjerna [17] (293 K), Korber and Oelsen [26] (298 K), Weibke [27] (363 K), Kleppa and King [28] (298 K), Orr and Argent [29] (573 K) and Blair and Downie [30] (573 K, 673 K). Apart from the values of von Samson-Himmelstjerna, which are Q 1 to 2 kcal/g-atom less exothermic, the results are in good agreement.

AH values for ordered bee (8') and for y-phase alloys have also been determined by several authors [17,26,27,30]. Since it is difficult to estab- lish equilibrium in the alloy specimens used for calorimetric measurements near room temperature, there must be some doubt as to whether the experimen- tal values for 8' and y apply to homogeneous single phase alloys - particu- larly in the investigations of [17,26,27].

THERMODYNAMIC EVALUATION OF THE CU-ZN SYSTEM 179

‘Zn

cu 0;’ 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 , ( , h

I I = BlaN DOWW 1101 15731: 671X1

0 Kleppo.K,nglZ9I1Zg9K1

. Orr. Argenl I291 f579XI

0 Korber Oelsen I 26 I I299 K I

n

-2ooo-

-4ooo-

5

$ -6OOO- =G

z

-aooo-

-1oooo-

-12000-

ICUI ,lrur.p’,p; p’. 1 , r+ , ‘I .E , F , r.iZnl

Phase ranges at 600K

FIGURE 5

Enthalpy of formation of solid Cu-Zn alloys.

4. Method of evaluation

The optimization program written by Lukas et al. [31] was used to obtain a set of GE coefficients for the liquid, (Cu), B,$',Y,G,E and (Zn) solution phases of the system. The input data for this program were perimental values reported in references 1 to 30. GE is represented kedlich-Kister polynomial equation of the form

the ex- by a

GE n

= xixj :=, (Xi_Xj) v L:;) (T) (1)

(i = Cu, j = Zn)

(T) are linear functions of temperature, i.e. correspoha to the temperature-independent values of the

e@halpy a?ia excess entropy of mixing.

For the pure components Cu and Zn, a conventional Gibbs Energy func- tion of the form

G = a + bT + cTlnT + dT2 + eT3 + fT -1

(2)

was used to represent the data of the stable and metastable phases. Data for the stablephases of Cu (fee, liquid) and of Zn (hcp, liquid) were taken from the Barin, Knacke, Kubaschewski Tables [32]. For the metastable phases of the pure elements (required to express the excess properties of mixing of the different solution phases in the system), Gibbs energies were calculated

180 P.J. SPENCER

by combining data for the stable phases of Cu and Zn with Kaufman's values for the transformation energies Cu(fcc * hcp) and Zn(hcp_t bee) [33], with Miodownik's value for Cu(fcc + bee) (4017 - 1.255 T Jmol &r34] and with Xurray's optimized value for Zn(fcc) (2281 - 1.6928 T Jmol ) 1351. The y phase, with the prototype D82 structure, has been treated as bee for the purpose of the evaluation.

5. Results and Discussion

Tables 1 and 2 summarise the Gibbs energy values (phase stabilities) for Cu and Zn and the GE coefficients for the solution phases of the Cu-Zn system respectively. The complete, evaluated set of coefficients has been used to calculate the thermodynamic properties of the solid and liquid phases as well as the Cu-Zn phase diagram. As can be seen from Tables 1 and 2, the 8 and 6 bee phases have been fitted by the same set of coefficients, and 8' (ordered bee) has been treated as a separate bee phase with the same phase stabilities of Cu and Zn as for 8.

TABLE 1

Phase stabilities of Cu and Zn

G = a + bT + cTlnT + dT2 + eT3 + f/T (J moi -1 )

a b C d e f

Liquid Phase

Cu(liq) -48.7 173.911 -31.38 0 0 0 Zn(liq) -3629.1 161.624 -31.38 0 0 0

(Cu)Phase

Cu(fcc) -8044.1 135.257 -24.8530 -1.89325x10 -3 0 69454.5 Zn(fcc) -4178.2 98.8181 -20.7359 -6.2551 xIO-~ 0 -41631

B,B',y,G Phases

Cu(bcc) -4027.1 134.002 -24.8530 -1.89325x10 -3 0 -6.2551 xIO-~ 0

69454.5 Zn(bcc) -3572.2 98.0015 -20.7359 -41631

E, (Zn)Phases

cu(hcP) -7416.1 136.512 -24.8530 -1.89325x10 -3 0 69454.5 Zn(hcP) -6459.2 100.5115 -20.7359 -6.2551 x~O-~ 0 -41631

InFigure 2 a comparison is made between the different experimental results for the relative chemical potential of Zn in liquid Cu-Zn alloys and the corresponding values at 1173 K obtained from the evaluation.

Figure 3 compares experimental and evaluated enthalpies of mixing for liquid alloys. Only the results reported by [17] and [18] were obtained by calorimetry, but in view of the large discrepancies between these two sets of data AH values derived from the temperature coefficients of Gibbs energy measurements (a generally rather unreliable procedure) are also plotted in the Figure. It is evident from the plot that there is considerable uncertainty with regard to enthalpies of mixing in the liquid, but the AH curve obtained from the evaluation tends to support the calorimetric results obtained by Parameswaran and Healy [18].

In Figure 4 evaluated and experimental values of Au phases of the system are compared. The calculated results !%r'%e%f~~%t phases are presented for different temperatures to correspond more closely with the temperatures of the relevant experimental investigations.

THERMODYNAMIC EVALUATION OF THE CU-ZN SYSTEM 181

TABLE 2

Excess Gibbs energy, GE, coefficients for the liquid, (Cu), 8,8',y,G,~ and (Zn) phases of the Cu-Zn system

GE ; (x = XCuXZn v=. CumXZn Iv L&(T)

L&(T) = A;;;, + B&*T (v = 0,1,2)

Phase V

Liquid

(Cu) (fee)

8 (bee)

B'(ordered bee)

y (882, bee)

6 (bee)

E (hcp)

(Zn) (hcp)

0

1 2

0 I 2

0 I 2

0 1 2

0 I 2

0 1 2

0

Aw CuZn

-41104.6 13.31409 3325.1 -5.486 19355.1 -15.05038

-41661.1 12.83485 -6160.7 -2.9274 14034.4 -7.30663

-48994.7 11.0502 4294.6 -3.48868 19531.2 -20.44399

-54217.6 18.13889 4474.6 -3.48868 19531.2 -20.44399

-47463.5 54529.8 91010.5

See a (bee)

-20225.7 59259.8 64914.9

-18926.0

11.74479 -27.15161 -37.87594

12.9084

40.64098 25.56498

-3.4786

(v) BCuZn

Figure 5 presents a comparison of the exper.mental and evaluated data for the enthalples of formation of solid alloys, There is good agreement for the fcc(Cu) phase, but the evaluation produces values of AH for the 8' and y phases which are of the order of 800-IO00 J/g-atom less exothermic than the experimental data.

The transformation enthalpy, AH (8'+ 8, from the evaluation for the compositiokrx

= 1.25 kJ/g-atom, obtained = 3.47 may be compared with the

value of ~1.86 kJ/g-atom obtained from thg"Hultgren tabulations [2]. Attempts to make the evaluated AH values for 8' and y more negative result in un- acceptable shifts in phase boundary positions. The evaluated data for the E phase are in modest agreement (? ~1 kJ/g-atom) with the results obtained from experimental measurements in this composition range.

The phase diagram calculated with the evaluated set of coefficients in Tables 1 and 2 is illustrated in Figure 6. There is excellent agreement with the experimental liquldus curve of Figure 1 across the entire composi- tion range. The equilibrium phase boundaries between the liquid phase and

P.J. SPENCER

FIGURE 6

Cu-Zn phase diagram calculated with evaluated data

the (Cuf,B,y,~ and (znf phases , as well as theboundariesbetween (Cu) and (3, and B and yf are all reproduced to within about 1 at.%, while the (Cu)/B', B'/y and s/(Zn) equilibrium boundaries are reproduced to within about l-2 at.%. Less satisfactory agreement between experimental and evaluated boundaries is obtained for the equilibria Involving the 6 phase and for the y/e equilibrium (up to 4-5 at.3 discrepancy),

A summary of experimental and calculated three phase equilibria is given in Table 3.

Of

1)

21

The major difficulties encountered in achieving an optimum description the system were:

Equilibria involving the ordered L3' phase

Thermochemical data for the B' phase were fitted by the Redlich-Rister equation rather than by a more physically-realistic ordering model. This must be viewed as a compromise which nevertheless enables the $'+ B transformation temperatures as well as the (Cu)/B' and B'fy phase boundaries to be moderately well reproduced. The experimentally ob- served tendency of the 8' phase field to move to higher Zn concen- trations with decreasing temperature could not be reproduced by the calculations without observing a similar trend in the (Cu)/(Cu)+B'phase boundary. Since experimental observations in fact show that this boundary tends to higher Cu concentrations with decreasing temperature, the (Cu)/B' and S'/y equilibria illustrated in Figure 6 were finally accepted as a compromise,

The y/yfs phase boundary

It was found impossible to achieve a fit of the available experimental data which resulted in agreement with the reported y/y+~ phase boundary. By accepting a less satisfactory description of the s/liquid and sf(Zn) equilibria it is possible to move the y/y+s phase boundary to lower Zn concentrations, but not to values around x = 0.66 as shown in the phase diagram illustrated by Hansen [l]. The v/y@ boundary resulting from the evaluation lies at compositions up to -5 at.% richer in Zn than those shown in Figure 1. Whether this is simply due to a poor description of

THERMODYNAMIC EVALUATION OF THE CU-ZN SYSTEM 183

TABLE 3

Experimental and calculated 3-phase equilibria in the Cu-Zn system

Equilibrium Experimental Results [l] Calculated Results

Peritectic

(Cu) + liq c+ 8

Peritectic

8 + liq c-f Y

(Cc) T= 1175 KxZn = 0.319

x!n = 0.361

x;fq = 0.368

T = 1107 K x!, = 0.558

XY Z?

= 0.591

x4;q = 0.591

(W T = 1175 K xZn = 0.300

xzBn = 0.359

xi;9 = 0.373

T = 1110 K x;!' = 0.549

xY Zn

= 0.583

xiiq Zn

= 0.590

Peritectic

y + liq +-+ 6

Peritectic

6 + liq ++ E

Peritectic

E + liq - (Zn)

Y T= 973KxZn = 0.692 Y T = 963 K xZn = 0.680

x& = 0.7245 xin

= 0.726

xi$q = 0.8005 xi? = 0.802

6 T= 871KxZn = 0.76 6 T =857.5K xZn = 0.793

%n = 0.78

*z"n = 0.795

xi$q = 0.882 xz'f4 = 0.899

T= 697Kx;, = 0.88 T = 697 K x;, = 0.892

xi;"' = 0.9723 xi?' = 0.980

x;;q = 0.983 x;;q = 0.986

Eutectoid

6++y+E.

B+v - B'+y

T= 831Kxzn = 0.70 Y T = 825 K xZn = 0.701

'i-t = 0.735 X2n

= 0.761

xk = 0.78 x!L

= 0.779

T = 741 K x8 = 0.482 8 Zn

T =736.5K xZn = 0.493

XB’

x;I

= 0.482 xzB:,

f 0.493

= 0.565 XV Zn

= 0.565

T= 729KxZn . (cu) = 0 383 T = 735 K x;:"' = 0.390

(Cu)+B c-f (Cu)+B XB

xi;

= 0.448 XB = 0.463

= 0.448 xi; = 0.464

the thermodynamic properties of Y at lower temperatures, or in fact also represents a larger uncertainty in the limited experimental information for this boundary than indicated by Bansen, is uncertain. In any case, the fit of the data for the y phase cannot be changed without simultaneously affecting the present satisfactory representation of the B/y, B'/y and liquid/y equili- bria.

184 P.J. SPENCER

3) The 6 phase

The range of existence of the very narrow 6 phase proved to be particularly difficult to describe in the present evaluation. The phase has the bee structure and has been fitted with the same set of coefficients as used for the 8(bcc) phase. The necessity of simultaneously describing the equi- libria of 6 with the liquid, y and E phases places severe constraints on the fit of the relevant experimental data, and an attempt to treat d as a stoichiometric compound did not result in an improvement in the calculated equilibria.

The position of the 6 phase as given by the evaluated set of coefficients is at compositions some 2 to 3 at.% richer in Zn than suggested in Hansen's compilation, while the peritectlc formation of 6 from liquid and y is cal- culated to occur 70 K Lower than observed experimentally. This less satis- factory description for 6 has been accepted here as a compromise, because amendments to data for other phases result in less satisfactory descriptions of phase equilibria in other parts of the system.

1.

2.

3.

4.

5.

6.

7.

8.

9.

.lO.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

References

M. Hansen, K. Anderko; 'Constitution of Binary Alloys', McGraw-Hill Book Co. Inc., New York (1958) R.P. Elliott, 1st Supplement (1965) F.A. Shunk; 2nd Supplement (1969)

R. Hultgren, P.D. Desai, D-T. Hawkins, M. Gleiser, K.K. Kelley; 'Selected Values of Thermodynamic Properties of Binary Alloys', American Society of Metals, Metals Park, Ohio (1973)

G.V. Raynor; 'Annotated Equilibriun Diagram Series', No. 3, The Institute of Metals, London, 1944

G. Shinoda, Y. Amano; Trans. Jap. Inst. Metals I_, 54-57 (1960)

T.B. Massalski, J.E. Kittl; J. Aust. Inst. Metals 1\, 91-97 (1963)

S.S. Rao, T.R. Anantharaman; Z. Metallkunde 9, 312-315 (1969)

D.J. Mack, W.J. Yang; Met. Trans A.$ 7&, 7798-1800 (1976)

T.B. Massalski, H.W. King; Acta Met. lo, 1171-1181 (1962)

A. Schneider, H. S∣ Z. Elektrochem. 2, 627-646 (1942)

L.H. Everett, P.W.M. Jacobs, J.A. Kitchener; Acta Met. 2, 281-284 (1957)

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E.H. Baker; Trans. Inst. Mining Met. z, 1 (1970)

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U. Gerling, B. Predel; Z. Metallkunde 2, 158-164 (1980)

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R. Hargreaves; J. Inst. Metals 04, 115-134 (1939)

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A.W. Herbenar, C.A. Siebert, 0-S. Duffendack; Trans. AIME 188, 323-326 (1950)

B.B. Argent, D.W. Wakeman; Trans. Faraday Sot. 2, 799-813 (1958)

D.B. Masson, J. Sheu; Met. Trans. 1, 3005-3009 (1970)

24. J.P. Pemsler, E.J. Rapperport; Trans. AIME 245, 1395-1400 (1969)

THERMODYNAMIC EVALUATION OF THE CU-ZN SYSTEM 185

25. A. blander; Z. physik. Chem. 164, 428-438 (1933)

26. F. Kijrber, W. Oelsen; Mitt. K.W.I. fiir Eisenforschung 19, 209-211 (1937)

27. F. Weibke; Z. anorg. Chem. 323, 289-296 (1937)

28. O.J. Kleppa, R.C. King; Acta Met. lo, 1183-1186 (1962)

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32. I. Barin, 0. Knacke; 'Thermochemical Properties of Inorganic Substances', Springer-Verlag, Berlin, Heidelberg, New York; Verl. Stahleisen, Dusseldorf 1973. Supplement with 0. Kubaschewski, 1977

33. L. Kaufman, H. Bernstein; 'Computer Calculation of Phase Diagrams', Academic Press, New York and London, 1970

34. A.P. Miodownik, private communication, Sept. I984

35. J.L. Murray, Bulletin of Alloy Phase Diagrams 4, 55-73 (1983)