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Thermodynamic properties of solid nickel-platinum alloys
THERMODYNAMIC
PROPERTIES
OF SOLID
NICKEL-PLATINUM
ALLOYS*
and J. B. DARBY, JR.?
R. A. WALKER71
The activities and free energies of formation of solid Ni-Pt alloys at 1625°K were calculated from vapor-pressure measurements obtained with a torsion-effusion apparatus. Heats of formation at 298°K were determined by liquid-tin solution calorimetry. The entropies of formation at 1625°K were computed from the experimental free energy and heat-of-formation values, assuming Neumann-Kopp behavior between 298 and 1625°K. The heat-of-formation results appear to reflect the ordered structures that exist at low-temperatures in the Ni-Pt system. The entropies are believed to be largely configurational in origin. PROPRIETES
THERMODYNAMIQUES
DES
ALLIAGES
SOLIDES
NICKEL-PLATINE
Les activites et les 6nergies libres de formation des alliages solides Ni-Pt & 1625°K ont 6tB caloulees 21 partir des mesures de pression de vapeur obtenues avec un appareil de torsion-effusion. Lea chaleurs de formation & 298°K ont 6tB dbterminees par calorimetric B bain d’&ain. Les entropies de formation PL 1625°K ont 6tB calculbes 8. partir des valeurs exp&imentales de 1’Bnergie libre et de la chaleur de formation, en supposant un comportement de Neumann-Kopp entre 298 et 1625°K. Les rbsultats obtenus pour les chaleurs de formation semblent lies aux structures ordonnbes existant aux basses temperatures dans le systbme Ni-Pt. Les auteurs pensent que les entropies sont dans une large mesure liees aux configurations. THERMODYNAMISCHE
EIGENSCHAFTEN
FESTER
NICKEL-PLATIN-LEGIERUNGEN
Die Aktivitiiten und freien Energien der Bildung fester Ni-Pt-Legierungen bei 1625°K wurden aus den Ergebnissen van Dampfdruckmessungen in einer Torsione-Apparatur berechnet. BildungswBrmen bei Die Bildungsentropien bei 1625°K 298°K wurden in einem fliissig-Zinn-Lizjsungskalorimeter bestimmt. wurden aus experimentellen Werten der freien Energien und Bildungswarmen unter der Annahme eines Neumann-Kopp-Verhaltens zwischen 298 und 1625’K berechnet. DieErgebnisse fiir die Bildungswiirmen spiegeln die bei tiefen Temperaturen im System Ni-Pt existierenden geordneten Strukturen wider. Die Entropien riihren unserer Ansicht nach grijDtenteils von der Konfiguration her.
1. INTRODUCTION
An investigation of
the
Ni-Pd
system,
by
Bidwell
revealed an unusual behavior formation
same alloys from heat-of-solution
of the thermodynamic
properties
and
Speiser,(n
in the integral heats of
(AH) for this system.
The heat-of-forma-
tion values were small and varied from endothermic behavior
for nickel-rich
for palladium-rich
alloys to exothermic behavior The general similarity of
alloys.
elemental palladium and platinum, in terms of atomic size and electronic structure, led to the question Ni-Pt
of whether system
position
a
dependence.
properties
from
The
only
force
To compare
of Ni-Pd
and Ni-Pt
the integral thermodynamic system were required.
in the literature values
measurements
at
the thermodynamic alloys quantitatively,
properties
of the Ni-Pt
The present work was initiated to determine the free energies of formation for a series of solid Ni-Pt alloys by the torsion-effusion vapor-pressure nique, and to determine the heats of formation
techof the
* Received April 13, 1970. Work performed at the Argonne National Laboratory under the auspices of the U.S. Atomic Energy Commission. t -4110~ Properties Group, Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439. $Now at: Center for Metal and Mineral Technology, Nicol Hall, Queen’s University, Kingston, Ontario. ACTA
METALLURGICA,
VOL.
18, DECEMBER
was attractive
platinum
calorimeter.
measurements The
for this investigation,
form a complete
Ni-Pt
in a
system
since nickel and
series of solid solutions
at
two high temperatures. (2) At lower temperatures, superlattices are formed near the compositions Ni,Pt and NiPt at 853 and 918”K, respectively.
The vapor
pressures
sufficiently
of
different
nickel
and
platinum
in magnitude
torsion-effusion
com-
thermodynamic
and the excess free-energy
solution
to
permit
are the
use of
the
technique.
of the
asymmetrical
published
electromotive
1273 and 1473°K. properties
similar
of this system
were the activities derived
the heats of formation
had
liquid-tin
1970
2. EXPERIMENTAL
Torsion-effusion The
PROCEDURES
method
torsion-effusion
apparatus
has
been
scribed(3-s) in detail and only a brief description presented.
A cylindrical
of high-purity eccentrically
alumina,
de-
will be
effusion cell was constructed and two orifices
in the cell allowed
positioned
the nickel vapor to
escape. The cell was suspended at the end of a fme tungsten wire, and the total torque produced was proportional to the total vapor pressure within the cell (assumed equal to the equilibrium vapor pressure of nickel).
The slightly irregular shape of the orifices
did not allow an accurate calculation of the FreemanSearcy correction factor 6, for an orifice of non-zero length. The constants were determined”) by comparing the measured vapor pressure of copper in the alumina cell with values determined in an independent experiment.@) 1261
ACTA
1262
Several
calibration
experiments
METALLURGICA,
were
that
induced
by
a small,
fairly reproducible additional Without
unidirectional
the furnace.
The induced
and corrections
rotation corrections,
was
torque
was
were made for the wire.
the largest error in vapor pres-
sure was only about 5 per cent for the platinum-rich alloys (and hence for small deflections), and only approximately 2 per cent for the nickel-rich alloys. The vapor
pressure
of nickel in equilibrium
the pure solid nickel, and in equilibrium of
solid
Ni-Pt
temperatures. computed ployed
alloys
at
several
of nickel and platinum,
the vapor-pressure
subsequently
with
with a series
determined
The activities
from
formation
was
to calculate
data,
were em-
the free energies of
of the alloys.
of an alloy was calculated
of the heat-of-solution
of the pure
components and of the alloy in liquid tin. twin-well, liquid-metal solution calorimeter earlier.cg)
The samples,
tained at a temperature
The was
which were main-
Ti (approximately
298”K),
were dropped into a bath consisting of 4 moles (65 ml) of liquid
tin maintained
proximately
698°K).
50 cal.
A sample
at a temperature
Tf (ap-
Each sample had a mass that
would yield a measured
3. EXPERIMENTAL
heat effect of approximately
was completely
of the surrounding
a larger
RESULTS
Torsiow-effusion The temperature of nickel
dependence
in equilibrium
of the vapor pressure
with the Ni-Pt
alloys
dissolved
block
was complete
was
determined from at least two independent
experiments
per alloy.
of the vapor
The temperature
pressure
of pure
dependence
solid nickel
separate experiments
was measured
in six
during the course of the present
investigation. The consistency of the data for pure nickel (Fig. 1) is indicative of the reliability of the apparatus
throughout
relationships
for
the investigation.
each
The linear
set of vapor-pressure
derived by the least-squares
method,
data,
are presented in of pure
within
good agreement
with the average value of 102.80 f
0.50 kcal/g-atom
reported
temperature
dependence
nickel obtained
by Hultgren
et aZ.(lo) The
of the vapor pressure of pure
in the present investigation,
shown in
Fig. 2, compares favorably with published data for the vapor pressure of solid(11-15) and liquid’16J7) nickel. The
activities
of nickel
of
culated
the vapor-pressure
from
sponding
1575,
in the Ni-Pt
temperatures
from 3 to 5 min and the exponential decay of the temperature of the calorimetric cell to the temperature imately
to provide
Table 1. The value for the heat of sublimation
The heat of formation
described
small pieces ratio.
nickel at 298°K is 101.85 f 0.20 kcal/g-atom, calculated by means of the third-law method, and is in
Calorimetry from measurements
1970
surface-to-volume
torque
of the cell and suspension
18,
0.004 in. foil into
performed
during the course of the present work, and the results indicated
VOL.
platinum
1625 and
activities
TEMPERATURE 14x)
I400
1380
1360
1675°K data.
alloys
at
were calThe
corre-
were determined
by
(“C)
1340
in approx-
150 min. II:
I
Alloy preparation Samples of the same alloy were employed the torsion-effusion
and the calorimetric
I
in both
experiments.
The alloys were prepared from pure nickel (99.999%) and pure platinum Mineral
(99.999%),
and Chemical
on a water-cooled
supplied by the United
Corporation,
copper
by arc melting
hearth in a helium-argon
atmosphere, followed by a homogenization anneal for 1 week at 1473°K. The alloys were rolled into sheets of approximately
0.1 in. in thickness,
a few hours at approximately
re-annealed
1473”K,
for
and given
a
final reduction to foil of about 0.004 in. in thickness. For the calorimetric measurements, samples with the appropriate mass were cut from the foil, rolled into a cylinder of about 1 in. in height and 0.125 in. in diameter, given a final heat treatment at 1473°K for 4 hr, and water quenched. effusion measurements
Samples for the torsion-
were obtained
by cutting the
FIQ.
1. The vapor pressure of nickel as a function of temperature.
WALKER
DARBY,
AND
JR.:
THERMODYNAMIC
PROPERTIES
TABLE 1. Vapor pressure of nickel in equilibrium Standard deviation Of log pNi (a x 104)
b
with Ni-Pt
Temperature
Ni-Pt
ALLOYS
range
Number of points
a
1.0 0.9
9.9180 9.8695
-21,255 -21,297
92 28
1572-1682 1574-1676
49 18
8:;
9.7148 9.8281
-21,375 -21,370
62 79
1600-1704 1574-1677
f ::
8:: o”.:
9.4524 9.3324 8.6622 9.3841
-21,221 -21,331 -21,823
78 92 101 97
1650-1769 1625-1703 1716-1836 1676-1795
;: 18
0:2 0.1
7.6326 6.8274
--21,034 19,708 -19,160
167 200
1755-1863 1809-1891
8: 36
= a + b/T.
integration of the Gibbs-Duhem equation with the aid of the u function,(ls) aNi = In YNi/Nrt2. The uncertainty in the nickel activities is estimated to be +4 per cent and for the platinum activities f5 per cent. The partial and integral free energies, heats of formation and entropies were calculated for 1625°K from the activities and the data are listed in Table 2. The integral free energies of formation are accurate to within f 115 Cal/g-atom. The integral heats of formation and the integral entropies have considerably larger errors than the free-energy values and are listed for completeness; estimated errors are f2.26
TEMPERATURE PC) 1500
(OK)
1300
I100
1000
TNESMEYANOV (1963)
\
Cal/g-atom-deg for the entropies and &3900 Cal/gatom for the heats of formation. Calorimetry
The heats of solution at 298’K, obtained in the present work, for pure nickel or platinum in liquid tin are presented in Table 3 together with pertinent data reported in the literature. (1s-23) In order to compare the various sets of data, the heat contents for the pure elements over the temperature interval 29%914’K were evaluated from equations given by Hultgren et al.(lO) and were combined with the present results at 298°K to obtain the calculated heats of solution at 914°K. The smaller values for platinum reported by Oriani and Murphy(ls) are perhaps the result of incomplete dissolution of the solute ; the data reported by Geikent20)are in better agreement with the present work. The present results at 298°K for the heat of solution of nickel in tin are in good agreement with the values at 273°K reported by Leach and Bever.f21) The nickel results at 910°K reported by Day and Hultgren,(22) and at 913°K by Oriani and Murphy(23) are much smaller than the present data extrapolated to 914°K. The integral heats of formation of the Ni-Pt alloys at 298°K were determined from the heats of solution at 298°K (AH,,,,) by means of the equation AH = NNi AHZI, + N,t AHPt S&l
5.0
6.0
7.0
6.0
lOOOO/ TeK) Fra. 2. Comparative
data for nickel.
1263
alloys
NNI
Log p (mm Hg)
1700
OF
the
vapor
pressure
of
-
AH$T.
(1)
The heats of solution and the heats of formation at 298’K are listed in Table 4. The uncertainty in the heats of formation was determined as the maximum deviation of the individual measurements from the average value for each composition, and did not exceed f3 per cent and usually was less than f 1 per cent. The integral thermodynamic quantities for the Ni-Pt system, determined by using data obtained from torsion-effusion and calorimetry measurements,
ACTA
1264 TABLE 2. Thermodynamic
METALLURGICA,
quantities for Ni-Pt
NNi
aNi
apt
AQN~ A&t (Cal/g-atom)
0.9 0.8 0.7
0.843 0.691 0.528
0.012 0.037 0.083
-552 -1193 -2063
8::
0.359 0.233
0.170 0.289
:.: 0:e 0.1
0.131 0.076 0.046 0.016
0.463 0.622 0.731 0.880
alloys at 1625°K
AHN~ AHpt (Cal/g-atom)
18,
1970
calculated
ASNI ASPt (Cal/g-atom-deg)
from vapor-pressure AG (callg-at::)
data AS (Cal/g-atom-deg)
- 14,282 - 10,646 -8037
-260 -446 -584
+2 $2841 +2525
+0.1s 1-0.46 f0.91
$8.79 $8.30 +6.50
-1925 - 3084 - 3852
-235 +215 +357
1.04 2.03 2.59
--4704 3308
-5722 -4008
+137 -447
+2346 +9os
+2.12 +2.62
+4.0s +3.91
-4274 -4356
+438 +95s
2.90 3.27
-6563 -8322 -9943 - 13,353
-2486 -1533 -1012 -413
+1363 -2403 $7883 +6569
+ +5.96 2.56 + 10.97 + 12.26
14.16 +2.54 +0.93 +0.33
-4117 -3570 -2799 -1707
+1603 +2231 +1978 +763
3.52 3.57 2.94 1.52
TABLE 3. Heats
Solute
of solution
Experimental heat of solution at 298°K
+4274 +;I;; + 123
of pure nickel and platinum expressed in Cal/g-atom
in liquid-tin.
The data are
Heat content of solute from 298 to 914°K
Calculated heat of solution at 914°K
Heat of solution at indicated temperature
Ni
-7989
*
10
4557
- 12,b46
-7470 (273’K) Leach and Beverce” -9920 (910°K) Day and Hultgren’az’ -9464 (913°K) Oriani and Murphy’23’
Pt
-24,734
*
3b
4051
-28,785
-26.951 (900°K) Geiken’a@’ - 24,800 (914°K) Oriani and Murphy”O’
TABLE 4. Heat of solution and heat of formation values of Ni-Pt alloys at 298°K
NNi
VOL.
(c*lgtom)
AHzws (Cal/g-atom)
0.9
-8988 -9023 -9021 -9014 - 9003 -9028
-676 -641 -643 -650 -661 -636
0.8
- 10,007 -9988
-1331 -1350
0.7
-11,127 -11,122 -11,174 -11,158
-1885 -1890 - 1838 - 1854
0.6
- 12,519 - 12,567 - 12,566 -12,522
-2168 -2130 -2121 -2165
0.5
-14,144 -14,154 -14,145 -14,148
-2218 -2208 -2217 -2214
0.4
-lb,872 - lb,877 - 15,883
-2164 -2159 -2153
0.2
-20,129 -20,138 -20,165 -20,211 -20,157
-1256 -1247 -1220 -1174 - 1228
0.1
-22,285 -22,313 -22,297
-775 -747 -763
are summarized in Table 5. The entropy values at 1625°K were calculated by means of the GibbsHelmholtz relationship from the tabulated free energies at 1625°K and from the heat of formation values at 298°K by assuming AC, = 0 between the two temperatures. The entropy data listed in Table 5 are preferred to those presented in Table 2, since large uncertainties are involved in deriving entropy values from the temperature coefficient of the free energies. 4. DISCUSSION
The activities at 1625°K for the Ni-Pt system (Fig. 3) are in good qualitative a&reement with the activities determined by Schwerdtfeger and Muan,(24) over the temperature range 1273-1473”K, and with the activities obtained by Alcock and Kubik’l’) from 1843-1905°K. The activities of nickel and platinum exhibit negative deviations from ideality and are consistent with the formation of superlattices in the Ni-Pt system at low temperatures. The integral free energies, heats of formation (calculated from calorimetric measurements), and entropies of formation, obtained in this investigation, are presented in Fig. 4. The integral heats of formation for the Ni-Pt system are exothermic over the entire range of composition, in contrast to the endothermic values obtained for the nickel-rich alloys in the Ni-Pd
WALKER
AND DARBY,
THERMODYNAMIC
JR.:
PROPERTIES
OF
Ni-Pt
1265
ALLOYS
TABLE 5. Summary of integral thermodynamic quantities for the Ni-Pt alloys
NNI
(oa$z’atom)
(oa$!tom)
ASS (Cal/g-atom-deg)
A@* (Cal/g-atom)
ASxd$ (Cal/g-atom-deg)
:.: 0:7
--3084 1925 -3852
-1340 -651 -1867
0.78 1.07 1.22
- -869 1463 -1877
0.6 0.5 :.:
-4274 -4356 -3570 -4117
-2146 -2214 -2159
1.31 1.32 1.20
-2102 -2121 -1936
+0.13 +O.OS +0.01 -0.03 -0.06 -0.14
0:2 0.1
-2799 -1707
- 1225 -762
0.97 0.58
-1186 -655
-0.02 -0.07
* From present work at 1625°K. t From present work at 298°K. $ From AB at 1625”K, and AH at 1625°K (assuming AC, = 0 between 298 and 1625°K).
system.(l) Similar differences between the Fe-Pt and Fe-Pd systems were attributed(24*25) to the greater
transformation
in the nickel-rich
tendency
rections
be small,
for clustering
the formation
in the palladium
of ordered
structures
alloys or to
in the platinum
alloys.
contribution
from
the integral
at 1625°K were
free energy
1625°K and the heat of formation
values
data at 298°K.
would
formation only
The integral entropies of formation calculated
the
that
arises
from
since
the
at the Curie temperature of pure nickel is The calculated en20 cal/g-atom.(26)
tropies of formation
(Fig. 4) are very close to the ideal
entropy
at 1625”K,
In
excess entropies of formation The
of mixing interpretation
of
and,
entropies
configurational
determined
suggests that the assumption may not be valid. However, the errors in the heats of formation calmeasurements
to the heats of formation
The magnetic
magnetic
so that
+ AS vib+ AS,,,f.
contribution
is and (2)
to the entropy
AS,,,
are so
great that the change in sign may not be real. corrections
contributions,
AS = AS,,,
the
of formation
the algebraic sign of the heats of formation
in this investigation
therefore,
are small.
generally made in terms of vibrational,
culated from the vapor-pressure
cor-
of trans-
140 f
Kopp behavior was assumed, although the disparity in employed
The
the heat
at
the absence of adequate specific heat data, Neumann-
by the two techniques
magnetic
alloys.
No
were made for
0.6
0
0.2
0.4
0.6
0.8
I.0
NNI
FIQ. 3. The activities of nickel and platinum at 1625’K. Open circles are experimental nickel activities, closed circles are the calculated platinum activities, closed triangles are experimental nickel activities reported by Schwerdtfeger and Muante*) (1273-1473”K), crosses are experimental nickel activities of Alcock and Kubik”‘) (1843-1905°K).
-6m1 0.0
’
’
a2
’
1
0.4
’
1
0.6
’
’
0.8
’
1
1.0
NNi
FIQ. 4. Integral free energies, heats of formation and entropies of formation of Ni-Pt alloys at 1625°K. Broken curve represents ideal entropy of formation at 1625OK.
is
ACTA
1266
METALLURGICA,
equal to $ ACgmas dT/T. In the absence of specificheat data for the Ni-Pt system, the magnetic contribution must be estimated and is assumed to be small for the Ni-Pt ment.
Weiss
and
magnetic entropy the form AS,,,
alloys,
from the following
Tauer(s’)
have
of formation
shown
argu-
that
the
of a binary alloy is of
N, ln (Pi + 1) -
NB ln (rug +
111 (3)
where p, and ,uB are the moments of the pure elements, ,L@‘Y and &uor are the moments of the elements in the alloy, and N, and NB are the mole fractions of the component elements. The rate of change of magnetic platinum dilution atom, solute
with
moment
nickel,@)
of a constant
per atom, upon alloying
is consistent
magnetic
with
moment
simple
sumption,
the contribution of
equation
the
of AS,,
Ni-Pt
to the entropy of
alloys,
estimated
(3), is small and has a maximum
approximately
0.05 Cal/g-atom-deg
40 at.% Ni.
The
changes
entropy
upon alloying
are determined
from Neumann-Kopp behavior. specific-heat data, the following
from
value of
in an alloy
taining
in
con-
vibrational
by deviations
In the absence of assumptions can be
made : (a) assume no deviations from Neumann-Kopp behavior,
thus
formation
become
AS,,
= 0;
temperature,
as suggested
obtained
the two
by
investigation, Ni-Pt
system
if the heats
of
with an increase
in
by comparing
the values
employed
in this
would be positive.
to form at low
(b)
techniques
then AS,,
The tendency
or
less negative
ordered
structures
temperatures
exists
in the to some
extent at higher temperatures, as indicated by the negative deviations in the activities from Raoult’s Law. AS~~~~ will be less than ideal by an amount depending upon chemical ordering amall quantity equal
to zero,
the extent of in the system.
high-temperature Since AS,, is a
and if AS,,, is assumed approximately the entropy
of formation
is almost
exclusively
determined by ASooni [equation (2)]. Then ASconi would be very close to ideal and, hence, the tendency to order chemically is small. Conversely, if ASvib is positive, ASzzefBB would be negative to
yield the integral entropy close to ASideal. It is not possible to distinguish between the two alternatives at this time in view of the inaccuracy associated with the high-temperature heats of formation obtained from the vapor pressure measurements. In conclusion, the heats of formation
are
for the Ni-Pt
1970
exothermic
composition,
over
the
entire
range
of
which is in contrast to the results for the
Ni-Pd system. The contrast in behavior also is observed between the Fe-Pt and Fe-Pa systems and probably
reflects the ordering tendency
systems containing
platinum.
tion of the Ni-Pt
system,
which
are close to ideal,
mainly
from contlgura-
The source of the difference
the thermodynamic cannot
that exists in
The entropies of forma-
to be derived
tional contributions.
quantities
be discerned
until
for Ni-Pt
appropriate
in
and Ni-Pd specific-heat
data are available. ACKNOWLEDGMENTS
The authors wish to thank Mr. R. E. von Massow for helpful discussions and Mr. A. P. Paulikas and Mr. S. D. Smith for their assistance with the measurements.
per nickel
i.e. essentially no magnetic moment on the platinum atoms. With this simplified as-
formation
system
18,
are considered
= R[ln (,uyl”r + 1) + In (,u$‘~Y + 1) -
VOL.
REFERENCES (1965). 1. L. R. BIDWELL and R. SPEISER, Acta Met. l&61 2. M. HANSEN and K. ANDBRKO, Constitution of Binary Alloys, 2nd edition. McGraw-Hill (1958). Report ANL-6657, Argonne 3. K. M. MYLES, USAEC National
Laboratory
(1963).
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Press (1963).
12. H. A. JONES, I. LAN~MVIR and G. M. J. MACKAY, Phya. Rev. 30,201 (1927). 13. G. P. KOVTVN, A. A. KRU~LICH and V. S. PAVLOV, Ukr. fiz. Zh. 7(4), 436 (1962) 14. H. L. JOHNSTON and A. L. MARSHALL, J. Am. them. Sot. 02, 1382 (1940). G. BRYCE, J. them. Sot. 2, 1517 (1936). ::: J. P. MORRIS, G. R. ZELLARS, S. L. PAYNE and R. L. KIPP, U.S. Bureau of Mines, Report of Investigation No. 5364 (1957). 17. C. B. ALCOCE and A. KUBIK, Trans. Inst. Min. Metallurgy 77[C](745), C220-C224 (1968). 18. R. W. CURRY and L. S. DARKEN, Physical Chemistry of Metals. McGraw-Hill (1953). 19. R. A. ORIANI and W. K. MURPHY, Acta Met. 10, 879
(1962). University 20. G. W. GEIKEN, USAEC Report UCRL-17615, of California (1967). 21. J. S. LL. LEACH and M. B. BEVER, Trans. Am. Inst. Min. Engrs 215, 728 (1959). 22. G. F. DAY and R. HULTOREN, J. phys. Chem. 66, 1532 (1962). R. A. ORIANI and W. K. MURPHY, Acta Met. 8,23 (1960). ii: K. SCHWERDTFE~ER and A. MUAN, Acta Met. 13, 509 (1965).
25. C. B. ALCOCK end A. KUBIK, Acta Met. 1’7, 437 (1969). 26. 0. KUBASCHEWSKI and E. LL. EVANS, Metallurgical Thermochemistry. Pergamon Press (1958). 27. R. J. WEISS and K. J. TAUER, Phys. Rev. 102,149O (1956). 28. J. CRANQLE and D. PARSONS, Proc. R. S’oc. A255, 609 (1960).
PROPERTIES
OF SOLID
NICKEL-PLATINUM
ALLOYS*
and J. B. DARBY, JR.?
R. A. WALKER71
The activities and free energies of formation of solid Ni-Pt alloys at 1625°K were calculated from vapor-pressure measurements obtained with a torsion-effusion apparatus. Heats of formation at 298°K were determined by liquid-tin solution calorimetry. The entropies of formation at 1625°K were computed from the experimental free energy and heat-of-formation values, assuming Neumann-Kopp behavior between 298 and 1625°K. The heat-of-formation results appear to reflect the ordered structures that exist at low-temperatures in the Ni-Pt system. The entropies are believed to be largely configurational in origin. PROPRIETES
THERMODYNAMIQUES
DES
ALLIAGES
SOLIDES
NICKEL-PLATINE
Les activites et les 6nergies libres de formation des alliages solides Ni-Pt & 1625°K ont 6tB caloulees 21 partir des mesures de pression de vapeur obtenues avec un appareil de torsion-effusion. Lea chaleurs de formation & 298°K ont 6tB dbterminees par calorimetric B bain d’&ain. Les entropies de formation PL 1625°K ont 6tB calculbes 8. partir des valeurs exp&imentales de 1’Bnergie libre et de la chaleur de formation, en supposant un comportement de Neumann-Kopp entre 298 et 1625°K. Les rbsultats obtenus pour les chaleurs de formation semblent lies aux structures ordonnbes existant aux basses temperatures dans le systbme Ni-Pt. Les auteurs pensent que les entropies sont dans une large mesure liees aux configurations. THERMODYNAMISCHE
EIGENSCHAFTEN
FESTER
NICKEL-PLATIN-LEGIERUNGEN
Die Aktivitiiten und freien Energien der Bildung fester Ni-Pt-Legierungen bei 1625°K wurden aus den Ergebnissen van Dampfdruckmessungen in einer Torsione-Apparatur berechnet. BildungswBrmen bei Die Bildungsentropien bei 1625°K 298°K wurden in einem fliissig-Zinn-Lizjsungskalorimeter bestimmt. wurden aus experimentellen Werten der freien Energien und Bildungswarmen unter der Annahme eines Neumann-Kopp-Verhaltens zwischen 298 und 1625’K berechnet. DieErgebnisse fiir die Bildungswiirmen spiegeln die bei tiefen Temperaturen im System Ni-Pt existierenden geordneten Strukturen wider. Die Entropien riihren unserer Ansicht nach grijDtenteils von der Konfiguration her.
1. INTRODUCTION
An investigation of
the
Ni-Pd
system,
by
Bidwell
revealed an unusual behavior formation
same alloys from heat-of-solution
of the thermodynamic
properties
and
Speiser,(n
in the integral heats of
(AH) for this system.
The heat-of-forma-
tion values were small and varied from endothermic behavior
for nickel-rich
for palladium-rich
alloys to exothermic behavior The general similarity of
alloys.
elemental palladium and platinum, in terms of atomic size and electronic structure, led to the question Ni-Pt
of whether system
position
a
dependence.
properties
from
The
only
force
To compare
of Ni-Pd
and Ni-Pt
the integral thermodynamic system were required.
in the literature values
measurements
at
the thermodynamic alloys quantitatively,
properties
of the Ni-Pt
The present work was initiated to determine the free energies of formation for a series of solid Ni-Pt alloys by the torsion-effusion vapor-pressure nique, and to determine the heats of formation
techof the
* Received April 13, 1970. Work performed at the Argonne National Laboratory under the auspices of the U.S. Atomic Energy Commission. t -4110~ Properties Group, Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439. $Now at: Center for Metal and Mineral Technology, Nicol Hall, Queen’s University, Kingston, Ontario. ACTA
METALLURGICA,
VOL.
18, DECEMBER
was attractive
platinum
calorimeter.
measurements The
for this investigation,
form a complete
Ni-Pt
in a
system
since nickel and
series of solid solutions
at
two high temperatures. (2) At lower temperatures, superlattices are formed near the compositions Ni,Pt and NiPt at 853 and 918”K, respectively.
The vapor
pressures
sufficiently
of
different
nickel
and
platinum
in magnitude
torsion-effusion
com-
thermodynamic
and the excess free-energy
solution
to
permit
are the
use of
the
technique.
of the
asymmetrical
published
electromotive
1273 and 1473°K. properties
similar
of this system
were the activities derived
the heats of formation
had
liquid-tin
1970
2. EXPERIMENTAL
Torsion-effusion The
PROCEDURES
method
torsion-effusion
apparatus
has
been
scribed(3-s) in detail and only a brief description presented.
A cylindrical
of high-purity eccentrically
alumina,
de-
will be
effusion cell was constructed and two orifices
in the cell allowed
positioned
the nickel vapor to
escape. The cell was suspended at the end of a fme tungsten wire, and the total torque produced was proportional to the total vapor pressure within the cell (assumed equal to the equilibrium vapor pressure of nickel).
The slightly irregular shape of the orifices
did not allow an accurate calculation of the FreemanSearcy correction factor 6, for an orifice of non-zero length. The constants were determined”) by comparing the measured vapor pressure of copper in the alumina cell with values determined in an independent experiment.@) 1261
ACTA
1262
Several
calibration
experiments
METALLURGICA,
were
that
induced
by
a small,
fairly reproducible additional Without
unidirectional
the furnace.
The induced
and corrections
rotation corrections,
was
torque
was
were made for the wire.
the largest error in vapor pres-
sure was only about 5 per cent for the platinum-rich alloys (and hence for small deflections), and only approximately 2 per cent for the nickel-rich alloys. The vapor
pressure
of nickel in equilibrium
the pure solid nickel, and in equilibrium of
solid
Ni-Pt
temperatures. computed ployed
alloys
at
several
of nickel and platinum,
the vapor-pressure
subsequently
with
with a series
determined
The activities
from
formation
was
to calculate
data,
were em-
the free energies of
of the alloys.
of an alloy was calculated
of the heat-of-solution
of the pure
components and of the alloy in liquid tin. twin-well, liquid-metal solution calorimeter earlier.cg)
The samples,
tained at a temperature
The was
which were main-
Ti (approximately
298”K),
were dropped into a bath consisting of 4 moles (65 ml) of liquid
tin maintained
proximately
698°K).
50 cal.
A sample
at a temperature
Tf (ap-
Each sample had a mass that
would yield a measured
3. EXPERIMENTAL
heat effect of approximately
was completely
of the surrounding
a larger
RESULTS
Torsiow-effusion The temperature of nickel
dependence
in equilibrium
of the vapor pressure
with the Ni-Pt
alloys
dissolved
block
was complete
was
determined from at least two independent
experiments
per alloy.
of the vapor
The temperature
pressure
of pure
dependence
solid nickel
separate experiments
was measured
in six
during the course of the present
investigation. The consistency of the data for pure nickel (Fig. 1) is indicative of the reliability of the apparatus
throughout
relationships
for
the investigation.
each
The linear
set of vapor-pressure
derived by the least-squares
method,
data,
are presented in of pure
within
good agreement
with the average value of 102.80 f
0.50 kcal/g-atom
reported
temperature
dependence
nickel obtained
by Hultgren
et aZ.(lo) The
of the vapor pressure of pure
in the present investigation,
shown in
Fig. 2, compares favorably with published data for the vapor pressure of solid(11-15) and liquid’16J7) nickel. The
activities
of nickel
of
culated
the vapor-pressure
from
sponding
1575,
in the Ni-Pt
temperatures
from 3 to 5 min and the exponential decay of the temperature of the calorimetric cell to the temperature imately
to provide
Table 1. The value for the heat of sublimation
The heat of formation
described
small pieces ratio.
nickel at 298°K is 101.85 f 0.20 kcal/g-atom, calculated by means of the third-law method, and is in
Calorimetry from measurements
1970
surface-to-volume
torque
of the cell and suspension
18,
0.004 in. foil into
performed
during the course of the present work, and the results indicated
VOL.
platinum
1625 and
activities
TEMPERATURE 14x)
I400
1380
1360
1675°K data.
alloys
at
were calThe
corre-
were determined
by
(“C)
1340
in approx-
150 min. II:
I
Alloy preparation Samples of the same alloy were employed the torsion-effusion
and the calorimetric
I
in both
experiments.
The alloys were prepared from pure nickel (99.999%) and pure platinum Mineral
(99.999%),
and Chemical
on a water-cooled
supplied by the United
Corporation,
copper
by arc melting
hearth in a helium-argon
atmosphere, followed by a homogenization anneal for 1 week at 1473°K. The alloys were rolled into sheets of approximately
0.1 in. in thickness,
a few hours at approximately
re-annealed
1473”K,
for
and given
a
final reduction to foil of about 0.004 in. in thickness. For the calorimetric measurements, samples with the appropriate mass were cut from the foil, rolled into a cylinder of about 1 in. in height and 0.125 in. in diameter, given a final heat treatment at 1473°K for 4 hr, and water quenched. effusion measurements
Samples for the torsion-
were obtained
by cutting the
FIQ.
1. The vapor pressure of nickel as a function of temperature.
WALKER
DARBY,
AND
JR.:
THERMODYNAMIC
PROPERTIES
TABLE 1. Vapor pressure of nickel in equilibrium Standard deviation Of log pNi (a x 104)
b
with Ni-Pt
Temperature
Ni-Pt
ALLOYS
range
Number of points
a
1.0 0.9
9.9180 9.8695
-21,255 -21,297
92 28
1572-1682 1574-1676
49 18
8:;
9.7148 9.8281
-21,375 -21,370
62 79
1600-1704 1574-1677
f ::
8:: o”.:
9.4524 9.3324 8.6622 9.3841
-21,221 -21,331 -21,823
78 92 101 97
1650-1769 1625-1703 1716-1836 1676-1795
;: 18
0:2 0.1
7.6326 6.8274
--21,034 19,708 -19,160
167 200
1755-1863 1809-1891
8: 36
= a + b/T.
integration of the Gibbs-Duhem equation with the aid of the u function,(ls) aNi = In YNi/Nrt2. The uncertainty in the nickel activities is estimated to be +4 per cent and for the platinum activities f5 per cent. The partial and integral free energies, heats of formation and entropies were calculated for 1625°K from the activities and the data are listed in Table 2. The integral free energies of formation are accurate to within f 115 Cal/g-atom. The integral heats of formation and the integral entropies have considerably larger errors than the free-energy values and are listed for completeness; estimated errors are f2.26
TEMPERATURE PC) 1500
(OK)
1300
I100
1000
TNESMEYANOV (1963)
\
Cal/g-atom-deg for the entropies and &3900 Cal/gatom for the heats of formation. Calorimetry
The heats of solution at 298’K, obtained in the present work, for pure nickel or platinum in liquid tin are presented in Table 3 together with pertinent data reported in the literature. (1s-23) In order to compare the various sets of data, the heat contents for the pure elements over the temperature interval 29%914’K were evaluated from equations given by Hultgren et al.(lO) and were combined with the present results at 298°K to obtain the calculated heats of solution at 914°K. The smaller values for platinum reported by Oriani and Murphy(ls) are perhaps the result of incomplete dissolution of the solute ; the data reported by Geikent20)are in better agreement with the present work. The present results at 298°K for the heat of solution of nickel in tin are in good agreement with the values at 273°K reported by Leach and Bever.f21) The nickel results at 910°K reported by Day and Hultgren,(22) and at 913°K by Oriani and Murphy(23) are much smaller than the present data extrapolated to 914°K. The integral heats of formation of the Ni-Pt alloys at 298°K were determined from the heats of solution at 298°K (AH,,,,) by means of the equation AH = NNi AHZI, + N,t AHPt S&l
5.0
6.0
7.0
6.0
lOOOO/ TeK) Fra. 2. Comparative
data for nickel.
1263
alloys
NNI
Log p (mm Hg)
1700
OF
the
vapor
pressure
of
-
AH$T.
(1)
The heats of solution and the heats of formation at 298’K are listed in Table 4. The uncertainty in the heats of formation was determined as the maximum deviation of the individual measurements from the average value for each composition, and did not exceed f3 per cent and usually was less than f 1 per cent. The integral thermodynamic quantities for the Ni-Pt system, determined by using data obtained from torsion-effusion and calorimetry measurements,
ACTA
1264 TABLE 2. Thermodynamic
METALLURGICA,
quantities for Ni-Pt
NNi
aNi
apt
AQN~ A&t (Cal/g-atom)
0.9 0.8 0.7
0.843 0.691 0.528
0.012 0.037 0.083
-552 -1193 -2063
8::
0.359 0.233
0.170 0.289
:.: 0:e 0.1
0.131 0.076 0.046 0.016
0.463 0.622 0.731 0.880
alloys at 1625°K
AHN~ AHpt (Cal/g-atom)
18,
1970
calculated
ASNI ASPt (Cal/g-atom-deg)
from vapor-pressure AG (callg-at::)
data AS (Cal/g-atom-deg)
- 14,282 - 10,646 -8037
-260 -446 -584
+2 $2841 +2525
+0.1s 1-0.46 f0.91
$8.79 $8.30 +6.50
-1925 - 3084 - 3852
-235 +215 +357
1.04 2.03 2.59
--4704 3308
-5722 -4008
+137 -447
+2346 +9os
+2.12 +2.62
+4.0s +3.91
-4274 -4356
+438 +95s
2.90 3.27
-6563 -8322 -9943 - 13,353
-2486 -1533 -1012 -413
+1363 -2403 $7883 +6569
+ +5.96 2.56 + 10.97 + 12.26
14.16 +2.54 +0.93 +0.33
-4117 -3570 -2799 -1707
+1603 +2231 +1978 +763
3.52 3.57 2.94 1.52
TABLE 3. Heats
Solute
of solution
Experimental heat of solution at 298°K
+4274 +;I;; + 123
of pure nickel and platinum expressed in Cal/g-atom
in liquid-tin.
The data are
Heat content of solute from 298 to 914°K
Calculated heat of solution at 914°K
Heat of solution at indicated temperature
Ni
-7989
*
10
4557
- 12,b46
-7470 (273’K) Leach and Beverce” -9920 (910°K) Day and Hultgren’az’ -9464 (913°K) Oriani and Murphy’23’
Pt
-24,734
*
3b
4051
-28,785
-26.951 (900°K) Geiken’a@’ - 24,800 (914°K) Oriani and Murphy”O’
TABLE 4. Heat of solution and heat of formation values of Ni-Pt alloys at 298°K
NNi
VOL.
(c*lgtom)
AHzws (Cal/g-atom)
0.9
-8988 -9023 -9021 -9014 - 9003 -9028
-676 -641 -643 -650 -661 -636
0.8
- 10,007 -9988
-1331 -1350
0.7
-11,127 -11,122 -11,174 -11,158
-1885 -1890 - 1838 - 1854
0.6
- 12,519 - 12,567 - 12,566 -12,522
-2168 -2130 -2121 -2165
0.5
-14,144 -14,154 -14,145 -14,148
-2218 -2208 -2217 -2214
0.4
-lb,872 - lb,877 - 15,883
-2164 -2159 -2153
0.2
-20,129 -20,138 -20,165 -20,211 -20,157
-1256 -1247 -1220 -1174 - 1228
0.1
-22,285 -22,313 -22,297
-775 -747 -763
are summarized in Table 5. The entropy values at 1625°K were calculated by means of the GibbsHelmholtz relationship from the tabulated free energies at 1625°K and from the heat of formation values at 298°K by assuming AC, = 0 between the two temperatures. The entropy data listed in Table 5 are preferred to those presented in Table 2, since large uncertainties are involved in deriving entropy values from the temperature coefficient of the free energies. 4. DISCUSSION
The activities at 1625°K for the Ni-Pt system (Fig. 3) are in good qualitative a&reement with the activities determined by Schwerdtfeger and Muan,(24) over the temperature range 1273-1473”K, and with the activities obtained by Alcock and Kubik’l’) from 1843-1905°K. The activities of nickel and platinum exhibit negative deviations from ideality and are consistent with the formation of superlattices in the Ni-Pt system at low temperatures. The integral free energies, heats of formation (calculated from calorimetric measurements), and entropies of formation, obtained in this investigation, are presented in Fig. 4. The integral heats of formation for the Ni-Pt system are exothermic over the entire range of composition, in contrast to the endothermic values obtained for the nickel-rich alloys in the Ni-Pd
WALKER
AND DARBY,
THERMODYNAMIC
JR.:
PROPERTIES
OF
Ni-Pt
1265
ALLOYS
TABLE 5. Summary of integral thermodynamic quantities for the Ni-Pt alloys
NNI
(oa$z’atom)
(oa$!tom)
ASS (Cal/g-atom-deg)
A@* (Cal/g-atom)
ASxd$ (Cal/g-atom-deg)
:.: 0:7
--3084 1925 -3852
-1340 -651 -1867
0.78 1.07 1.22
- -869 1463 -1877
0.6 0.5 :.:
-4274 -4356 -3570 -4117
-2146 -2214 -2159
1.31 1.32 1.20
-2102 -2121 -1936
+0.13 +O.OS +0.01 -0.03 -0.06 -0.14
0:2 0.1
-2799 -1707
- 1225 -762
0.97 0.58
-1186 -655
-0.02 -0.07
* From present work at 1625°K. t From present work at 298°K. $ From AB at 1625”K, and AH at 1625°K (assuming AC, = 0 between 298 and 1625°K).
system.(l) Similar differences between the Fe-Pt and Fe-Pd systems were attributed(24*25) to the greater
transformation
in the nickel-rich
tendency
rections
be small,
for clustering
the formation
in the palladium
of ordered
structures
alloys or to
in the platinum
alloys.
contribution
from
the integral
at 1625°K were
free energy
1625°K and the heat of formation
values
data at 298°K.
would
formation only
The integral entropies of formation calculated
the
that
arises
from
since
the
at the Curie temperature of pure nickel is The calculated en20 cal/g-atom.(26)
tropies of formation
(Fig. 4) are very close to the ideal
entropy
at 1625”K,
In
excess entropies of formation The
of mixing interpretation
of
and,
entropies
configurational
determined
suggests that the assumption may not be valid. However, the errors in the heats of formation calmeasurements
to the heats of formation
The magnetic
magnetic
so that
+ AS vib+ AS,,,f.
contribution
is and (2)
to the entropy
AS,,,
are so
great that the change in sign may not be real. corrections
contributions,
AS = AS,,,
the
of formation
the algebraic sign of the heats of formation
in this investigation
therefore,
are small.
generally made in terms of vibrational,
culated from the vapor-pressure
cor-
of trans-
140 f
Kopp behavior was assumed, although the disparity in employed
The
the heat
at
the absence of adequate specific heat data, Neumann-
by the two techniques
magnetic
alloys.
No
were made for
0.6
0
0.2
0.4
0.6
0.8
I.0
NNI
FIQ. 3. The activities of nickel and platinum at 1625’K. Open circles are experimental nickel activities, closed circles are the calculated platinum activities, closed triangles are experimental nickel activities reported by Schwerdtfeger and Muante*) (1273-1473”K), crosses are experimental nickel activities of Alcock and Kubik”‘) (1843-1905°K).
-6m1 0.0
’
’
a2
’
1
0.4
’
1
0.6
’
’
0.8
’
1
1.0
NNi
FIQ. 4. Integral free energies, heats of formation and entropies of formation of Ni-Pt alloys at 1625°K. Broken curve represents ideal entropy of formation at 1625OK.
is
ACTA
1266
METALLURGICA,
equal to $ ACgmas dT/T. In the absence of specificheat data for the Ni-Pt system, the magnetic contribution must be estimated and is assumed to be small for the Ni-Pt ment.
Weiss
and
magnetic entropy the form AS,,,
alloys,
from the following
Tauer(s’)
have
of formation
shown
argu-
that
the
of a binary alloy is of
N, ln (Pi + 1) -
NB ln (rug +
111 (3)
where p, and ,uB are the moments of the pure elements, ,L@‘Y and &uor are the moments of the elements in the alloy, and N, and NB are the mole fractions of the component elements. The rate of change of magnetic platinum dilution atom, solute
with
moment
nickel,@)
of a constant
per atom, upon alloying
is consistent
magnetic
with
moment
simple
sumption,
the contribution of
equation
the
of AS,,
Ni-Pt
to the entropy of
alloys,
estimated
(3), is small and has a maximum
approximately
0.05 Cal/g-atom-deg
40 at.% Ni.
The
changes
entropy
upon alloying
are determined
from Neumann-Kopp behavior. specific-heat data, the following
from
value of
in an alloy
taining
in
con-
vibrational
by deviations
In the absence of assumptions can be
made : (a) assume no deviations from Neumann-Kopp behavior,
thus
formation
become
AS,,
= 0;
temperature,
as suggested
obtained
the two
by
investigation, Ni-Pt
system
if the heats
of
with an increase
in
by comparing
the values
employed
in this
would be positive.
to form at low
(b)
techniques
then AS,,
The tendency
or
less negative
ordered
structures
temperatures
exists
in the to some
extent at higher temperatures, as indicated by the negative deviations in the activities from Raoult’s Law. AS~~~~ will be less than ideal by an amount depending upon chemical ordering amall quantity equal
to zero,
the extent of in the system.
high-temperature Since AS,, is a
and if AS,,, is assumed approximately the entropy
of formation
is almost
exclusively
determined by ASooni [equation (2)]. Then ASconi would be very close to ideal and, hence, the tendency to order chemically is small. Conversely, if ASvib is positive, ASzzefBB would be negative to
yield the integral entropy close to ASideal. It is not possible to distinguish between the two alternatives at this time in view of the inaccuracy associated with the high-temperature heats of formation obtained from the vapor pressure measurements. In conclusion, the heats of formation
are
for the Ni-Pt
1970
exothermic
composition,
over
the
entire
range
of
which is in contrast to the results for the
Ni-Pd system. The contrast in behavior also is observed between the Fe-Pt and Fe-Pa systems and probably
reflects the ordering tendency
systems containing
platinum.
tion of the Ni-Pt
system,
which
are close to ideal,
mainly
from contlgura-
The source of the difference
the thermodynamic cannot
that exists in
The entropies of forma-
to be derived
tional contributions.
quantities
be discerned
until
for Ni-Pt
appropriate
in
and Ni-Pd specific-heat
data are available. ACKNOWLEDGMENTS
The authors wish to thank Mr. R. E. von Massow for helpful discussions and Mr. A. P. Paulikas and Mr. S. D. Smith for their assistance with the measurements.
per nickel
i.e. essentially no magnetic moment on the platinum atoms. With this simplified as-
formation
system
18,
are considered
= R[ln (,uyl”r + 1) + In (,u$‘~Y + 1) -
VOL.
REFERENCES (1965). 1. L. R. BIDWELL and R. SPEISER, Acta Met. l&61 2. M. HANSEN and K. ANDBRKO, Constitution of Binary Alloys, 2nd edition. McGraw-Hill (1958). Report ANL-6657, Argonne 3. K. M. MYLES, USAEC National
Laboratory
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