Thermodynamic properties of carbon in molybdenum and tungsten at solubility limi

Gupta, D. K.

Seigle, L.

Srivastava, S. C.

English

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* 
State University of New York at Stony Brook 
THERMODYNAMIC PROPERTIES OF INTERSTITIAL ELEMENTS 
I N  THE REFRACTORY METALS 
D. K. Gupta, S. C. Srivastava, and L. Seigle 
Seventh and Eighth Progress Reports for  the periods 
June 1, 1969 - August 31, 1969, and September 1, 1969 - 
November 30, 1969. 
NASA Research Grant NGR-33-015-035 
TABLE OF CONTENTS 
Abstract 
I. Introduction 
11. Determination of the Free Energies of Formation 
of the Lower Carbides 
111. Determination of the Solubility of Oxygen in 
Mo and W 
IY. Future Work 
References 
ABSTRACT 
Details of the experimental techniques for determining 
the act ivi ty  of C i n  Mo and W a t  the solubili ty l i m i t  by 
measuring the vapor pressure of Ca over a mixture of Me, 
Me2C, and CaC2, and by measuring the emf of a galvanic c e l l  
of the type Me + MezC I CaC12 + CaC2 I C are reviewed. 
It is concluded that the emf method would be simpler t o  apply, 
and work has been started t o  construct a cell.  Work has also 
begun t o  obtain missing information about the solubili ty of 
oxygen i n  Mo by equilibrating Mo rods i n  a mixture of 
Mo + Moo2. 
- 1 -  
* 
I. INTRODUCTION 
Calculation of the thermodynamic properties of C ,  0, and N dis-  
solved i n  solid Mo, W, Nb, and Ta from existing phase equilibrium 
data has proven t o  be a useful first step in  obtaining free energies, 
enthalpies, and entropies of solution of the in t e r s t i t i a l  elements i n  
the refractory metals. However, it has become clear i n  carrying out 
this work that  additional experimental data are required for  some of 
the systems i n  order t o  obtain a complete and reliable s e t  of thermo- 
dynamic parameters for  the terminal solid solutions. The gaps and 
uncertainties i n  the available data were discussed in  recent progress 
reports. (l) The most important are lack of sufficient information 
concerning the free  energies of formation of the lower carbides of 
the refractory metals, and insufficient lmowledge of the solid sol-  
ub i l i t i es  of oxygen i n  Mo and W. 
been directed t o  experimental methods for  obtaining these data. 
Attention therefore has recently 
11. DETERMINATION OF THE FREE ENERGIES OF FORMATION OF THE LOWER 
CARBIDES 
Methods for  determining the activity of C i n  a mixture of Me + 
Me$ were discussed i n  the l a s t  progress report. (2) Measurement of 
the act ivi ty  of C is equivalent t o  determination of the free energy 
of formation of MeZC of the specific composition in  equilibrium with 
the terminal solid solution,* since the solubi l i t ies  of C i n  the re- 
fractory metals are quite low. 
give directly the chemical potential of C i n  the terminal solid sol-  
A t  the same time, the measurements 
* The lower carbides are of variable composition 
- 2 -  
ution at the solubili ty l i m i t .  
In  the last progress report it w a s  concluded that an attractive 
method for  determining the free energy of formation of the lower car- 
bides, o r  the act ivi ty  of C i n  the Mo-C and W-C systems, was measure- 
ment of the Ca (or Ba) vapor pressure over a mixture of Me + Me$ + 
CaC2 (or BaC2). 
atm. between approximately 1000 and 1800°K. 
This vapor pressure w a s  estimated t o  be t o  1 
Use of such a method was 
f e l t  t o  have an advantage over determination of carbon act ivi t ies  by 
equilibration with CO-CO2 mixtures, inasmuch as it avoids possible 
complications due t o  the formation of oxycarbides. Another possible 
method with the same advantages-is measurement of the emf of a galvanic 
cell of the type 
Me + Me2c I CaC2 + Caclz c I 
During the current report period a study has been made of the 
detai ls  of apparatus and technique involved in  both of the above meth- 
ods. 
ized as follows: 
pressures, it was  judged that the dynamic methods (394) most commonly 
used t o  determine vapor pressures a t  elevated temperatures, such as 
the transpiration method, the method of Bodhansky and Schins, and even 
the effusion method* would be doubtful f o r  the systems of interest ,  
because they a l l  require the transport of an appreciable quantity of 
Ca from the powder mixture t o  the vapor phase. 
The conclusions.derived from th is  study can be briefly summar- 
F i r s t ,  with regard t o  measurement of the Ca vapor 
I t  is not certain that 
diffusion and reaction rates i n  this mixture of refractory materials, 
* The vapor pressures are also too high f o r  the effusion method over 
most of the temperature range. 
- 3 -  
at temperatures far below their melting points, would be rapid enough 
t o  maintain equilibrium in the presence of a substantial removal of 
Ca. Attention was therefore directed toward the static methods of 
vapor pressure measurement. 
(I 
There are a number of static methods described in the literature 
which at first glance seem attractive. (3,  4, 
however, it is seen that none of these would be easy to apply to 
measurement of Ca vapor pressures at high temperatures. 
the reactivity of Ca vapor rules out the use of silica glass windows 
in the dew point method. 
of light absorption techniques. 
the difficulty of finding a fluid useful in contact with Ca vapor 
at very high temperatures. 
electrical conductivity measurements at high temperatures also are 
burdened with experimental difficulties. 
the use of radioactive Ca, in which the vapor pressure is determined 
by counting through the walls of the reaction vessel (3) seemed 
appealing, but since this called for the use of radiotracer techniques 
and apparatus it w a s  decided not to act on it until the emf method, 
which, as previously mentioned, is an alternative to the vapor pressure 
methods, was more closely examined. 
on closer inspection, 
For example, 
A similar problem arises in the application 
Direct manometric methods suffer from 
Methods involving weight measurements, or 
Finally, a method involving 
The emf method involves the use of a galvanic cell such as 
Me(s) + Me2C(s) 1 CaC2(1) + C ~ C ~ ~ O )  } 
Electrolytic conduction in the fused salt electrolyte takes place via the 
acetylide ion, Cz , and the chemical potential of C in the Me + MeZC - 
- 4 -  
mixture is proportional t o  the cell potential. 
recently used successfully by Hawkes and Morris 
A cell of t h i s  type w a s  
t o  determine the ac- 
t i v i t y  of C i n  austenite up t o  1000° C. Furthermore, Salzano, e t  a l .  , ( 6 )  
of the Brookhaven National Laboratories report good results i n  their  
use of similar cells. 
A limitation t o  the use of these cel ls  is the occurrence of the side 
reaction 
Me(s) + CaC,(l) = Me2C(s) + Ca(g) 
which determines the Ca vapor pressure above the ce l l .  
must be low i n  order t o  minimize the extent of the side reaction. 
idea of the order of magnitude of th i s  pressure can be obtained from the 
Ca pressures calculated to  be i n  equilibrium with a mixture of solid 
This pressure 
An 
Me + MezC + CaCz, given i n  Table 5 of the last progress report. ( 6 )  
A t  1000°K these are of the order of 
and 1011 - lo1' for  the Nb and Ta systems. In the cel ls  the CaCz w i l l  
be present i n  di lute  liquid solution and therefore the pressures in  
Table 5 represent an upper limit t o  the expected Ca vapor pressures. 
From these values it may be concluded that such ce l l s  are feasible for 
the Mo-C and W-C systems, but not for  the Nb-C and Ta-C systems. 
- lo-'  for the Mo and W systems, 
Another limitation is imposed by the vola t i l i ty  of the CaCl, 
i n  the electrolyte. 
and Morris t o  a temperature of 1000°C i n  their  experiments. 
the boiling point of CaClz is about 20OO0C, and it would seem that 
temperatures could be extended above 1000°C by the use of closed cel ls  
or perhaps admixture of CaCl, with a less vo la t i l e ' s a l t ,  such as 
Evaporation of this  constituent limited Hawkes 
However, 
CaF,. 
- 5 -  
Details of the experimental apparatus and technique have been 
discussed w i t h  Salzano. ( 6 )  Calcium vapor attacks ceramics such as 
A1  0 , but th i s  diff icul ty  is  easily avoided by ut i l iz ing metal, 
e.g., N i  or  Inconel crucibles and furnace tubes. 
2 3  
The calcium 
halides used i n  the electrolyte must be free of water and oxygen. 
Highly purified salts are, however, commercially available. 
Considering a l l  of the available information, a f te r  careful 
study it has been concluded that  although a s t a t i c  vapor pressure 
method could probably be developed fo r  the Mo-C and W-C systems, it 
would be simpler t o  u t i l i ze  the emf technique described above, and 
at  the present time supplies are being obtained t o  proceed with this  
approach. 
Since the carbide c e l l  method is not suitable for the Nb-C and 
Ta-C systems, another technique must be found. 
CO pressures over a mixture of Me + MezC + MeO, by the effusion 
Measurement of the 
method is a possibil i ty,  as mentioned in  the last progress report. (2) 
However, th i s  possibil i ty requires further study. 
111. DETERMINATION OF THE SOLUBILITY OF OXYGEN IN Mo AND W. 
Knowledge of the solid solubi l i t ies  of the in t e r s t i t i a l  elements 
i n  the terminal solutions is essential t o  calculation of the thermo- 
dynamic properties from the phase diagrams. 
oxygen i n  Mo w a s  determined by Few and Manning c7) using a technique 
i n  which the sample w a s  equilibrated a t  high temperatures, quenched, 
The solubility of 
and examined metallographically for  residual oxide. 
solubi l i t ies  ranging from 0.004 w t .  % a t  1000°C t o  0.006 w t .  % a t  
Oxygen 
- 6 -  
16OO0C were determined in this way. 
for several reasons. 
carbon systems indicate that it is difficult to retain the inter- 
These results are questionable 
First, experiments with the refractory metal- 
stitial element in solution upon quenching from high temperatures. 
Second, the purity of the Mo used by Few and Manning was not specified, 
Third, the oxygen solubilities determined vary linearly, rather than 
exponentially with temperature, and extrapolate to a non- zero 
solubility at absolute zero. For these reasons it is felt that Few 
and Manning's data arexot reliable, and a new determination of the 
solubility of oxygen in Mo should be undertaken. 
the solubility of oxygen in W has been reported, (*I i.e., 0.005 
w t .  % at 17OO0C, and the need for additional data here is obvious. 
Only one value for 
The solubility of oxygen in Mo and W cannot easily be obtained 
by direct equilibration of the metal with gaseous oxygen at a known 
pressure, due to complications arising from the high vapor pressures 
of Mo and W oxides. Therefore, the method of equilibrating with 
solid Moo2 or W02 has been selected as that most likely to succeed. 
In this method, a small specimen of solid Mo, for example, is embedded 
i n  a mixture of powdered Mo + Moo2 contained in a sealed capsule. 
The whole is held at temperature for a long enough time to saturate 
the solid Mo with oxygen. 
the solid Mo is removed from the powder, cleaned and analyzed for 
oxygen. 
After cooling rapidly to room temperature, 
The analogous method of equilibrating thin Mo rods in a 
mixture of Mo + Mo2C powders was recently used by Rudman (9) to 
determine the solid solubility of C in Mo. 
Certain conditions must be fulfilled in order to assure the 
- 7 -  
2 
woo31 
2.9 x lo-' 
1 .4  
5.7 I O - ~  
1.6 x 
1.3 x 
3.7 x 
----- 
- - - - I  
success of th i s  method. F i r s t  the oxide pressure above the Me + Me0 
2 
mixture must not be too high. Blackburn ('O) and Burns, e t  al. ,  (11) 
have studied the Mo-0 system and report that  the evaporation of 
solid Mooz is governed by two reactions 
1) MoOz(s) = MoOz(g) 
and 2) 3/2 MoOz(s) = Mo03(g) + 1/2  Mo(s) 
Polymers of the trioxide, such as (MOO ) 
i n  the gas above solid Mooz. 
(g) and (Moo3) (g) also occur 
3 2  3 
The pressures of oxide species above MOO are given in  Table 1. 
From these values, it is evident tha t  there should be no problem in  
containing a mixture of Mo(s) + MoOz(s). 
2 
cMOO3) 
- 
----- 
6.9 
- - - - -  
7.1 x 
1 .4  
3.1 
----- 
- - - - -  
Table 1. PARTIAL PRESSURES OF SPECIES IN EQUILIBRIUM WITH 
SOLID Mooz 
TOK 
1566 
1632 
1675 
1716 
1732 
1777. 
*1905 
"202' 
MOO 
4.9 
1.6 
2.7 
3.7 
2.8 
1.5 
4 . 1  x lo-' 
1 .0  x 
Pres 
Moo3 
1.4 
6.5 
2.0 x 
4.1 x 
5.4 x 
1 .4  
6.4 
3.5 
* From Blackburn, e t  a l .  ( lo) The rest of the data is from 
Burns, e t  a l .  (11) 
- 8 -  
Blackburn, e t  a l . ,  also studied the evaporation of WO I t  w a s  2 '  
concluded that  th i s  oxide evaporates almost completely t o  (W03) , 
according t o  the reaction 
3 
3/2 W02(s) = 1/3 (wo3)3cg) + 1/2  W(S> 
Table 2 gives the pressures of (W03) above a mixture of W(s) and 
3 
WO2 (SI 
Table 2. PRESSURE OF (w03) ABOVE SOLID W + W02 
3 
TOK I Pressure - abn 
1324 
1375 
150 7 
1603 
1658 
7.4 
1.0 
3.4 
1.3 
4.0 x 
A second conuLtion wh-2 must be fu l f i l l ed  order f o r  t ie 
given method to  work, is  that the diffusion of oxygen into the solid 
specimens must occur rapidly enough t o  achieve saturation in  a 
reasonable time. The diffusion coefficients of oxygen in  Mo and W 
are not known, but i f  we assume that they are of the same order of 
magnitude as the diffusion coefficients of C i n  these metals, then we 
can estimate the times for saturation. 
coefficient of C i n  Mo, 
Rudman gives for  the diffusion 
2 D(C i n  Mo) = 0.034 exp ( - 41,000) RT cm /sec 
while Pivani and Sandor (12) give fo r  c i n  w 
D(C in  = 0.31 exp ( - 59 RT ,Ooo) cm2/sec 
- 9 -  
W Mo 
Using these D values the estimated times for  saturation of a 1/8" 
diameter rod are given i n  the following table: 
1673 
1800 
Table 3. ESTIMATED TIMES FOR 90% SATURATION OF A l/S1' 
DIAMETER ROD 
1.5 
2.1 x 
1873 
2000 
Time 
5.6 I 
f 1.1 10-7 
! 
Mo 
3.5 ,days 
16 hrs. 
4.2 hrs. 
1.5 hrs. 
W 
4.0 days 
18 hrs.  
4.8 hrs.  
It appears that  the method could be feasible for  Mo above 
1500OK and W above 1800'K. 
The success of the proposed method also depends upon use of a 
highly pure s tar t ing material, and ab i l i t y  t o  determine oxygen content 
with sufficient precision. 
commercially available, with an oxygen content of < 4 ppm. According 
Floating zone purified Mo and W rods are 
t o  the supplier these may be even further purified i f  necessary, 
and no problem is foreseen i n  obtaining sufficiently pure s tar t ing 
material. 
vacuum %ion analysis t o  a sensi t ivi ty  of + - 2 micrograms. 
Oxygen contents i n  the ppm range may be determined by 
With a 
sample s ize  of 2 grams it should be possible t o  obtain oxygen 
contents with accuracy down t o  a t  l eas t  10 ppm. 
- 10 - 
Since the described method was judged t o  be feasible, work has 
been s tar ted t o  procure materials and s e t  up equipment for determina- 
t ion of the solubili ty of oxygen i n  Mo. A molybdenum wound muffle 
furnace which can reach a temperature of 18OO0C i n  iner t  atmosphere 
is available. 
being made ready. 
Crucibles, thermocouples, and other accessories are 
FUTURE WORK 
Work i n  the immediate future w i l l  be concentrated on completing 
the data for  the binary systems, i n  order t o  obtain a complete and 
reliable set of thermodynamic parameters for  carbon, oxygen, and 
nitrogen i n  a l l  four refractory metals. 
- 11 - 
REFERENCES 
1. 
2. 
3. 
4. 
5. 
6. 
7. 
8 .  
9. 
L. Seigle, e t  a l . ,  5th and 6th Semiannual Progress Reports 
under NASA Grant NGR-33-015-035, covering the periods 
6/1/68 - 11/30/68, and 12/1/68 - 5/31/69. 
6th Semiannual Progress Report, 12/1/68 - 5/31/69 under this 
grant. 
See "Metallurgical Thermochemistry" by 0. Kubaschewski , e t  al .  , 
Pergamon Press, 4th ed. (1967) and Vapor Pressure of the 
Chemical Elements" by Nesmeyanov, Elsevier Pub. Co. (1963) 
Bohdansky and Schins, J. Physik. Chemie. - 71 (1967) 215. 
G. L. Hawkes and D. R. Morris, Tr. ADE - 242 (1968) 1083. 
F. J. Salzano, Brookhaven National Lab., Private c o m i c a t i o n .  
W. E. Few and G. K. Manning, J. Metals - 4 (1952) 271. 
B. C. Allen and W. M. Albright, DMIC Memo No. 50, 1960, p. 10. 
P. S. Rudman, T r .  AIME - 239 (1967) 1949. 
10. P. E. Blackburn, M. Hoch, H. L. Johnston, J. Physik. Chemie. I 62 
(1958) 769. 
R. P. Burns, e t  a l . ,  J. Chem. Phys. - 32 (1960) 1363. 
M. Pirani, J. Sandor, J. Inst .  Metals - 73 (1947) 385. 
11. 
1 2 .  


Thermodynamic properties of interstitial elements in the refractory metals Progress report, 1 Jun. - 30 Nov. 1969

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