A calorimetric study of the cobalt + vanadium system

J. Chem. i’%ermodynamics 1976,8,551-556

A calorimetric study vanadium system

of the cobalt

+

P. J. SPENCER” and F. H. PUTL.ANDb Division of Chemical Standards, MoVIesex TWll OL W, U.K.

National

Physical

Laboratory,

Teddington,

(Received 22 December 1975)

Enthalpiesof formationhavebeenmeasured for ninealloy compositions, comprisingthe f.c.c., CT,b.c.c., and “CO~V” phases,of the cobalt + vanadiumsystem.The resultsare discussed in termsof the generalcorrelationbetweenthe thermodynamicpropertiesand o-phasestability of o-formingsystems.Estimationof entropiesfor Co + V alloys has enableda set of thermodynamicvaluesto be obtainedwhich are consistentwith the publishedphaseboundariesfor the Co + V systemin the temperaturerange 1100to 1513K.

1. libodllction Alloys of cobalt and vanadium are of particular interest in view of the very stable o-phase which exists in this system over a wide range of compositions and temperatures. The Co+V phase diagram as given by Hansen(‘) is shown in figure 1, which also includes amended o and (V) phase boundaries determined by Zegler and Downey,(2) and a different V&o peritectoid temperature determined by these authors. Marcone and C011(~) have suggested that, at the mole fraction x = 0.5, and at a temperature of approximately 823 K, the o-phase exhibits an order-disorder transformation. The suggested transformation is not shown in the phase diagram. The o-phase occurs in a number of important steels and superalloy systems and frequently has serious embrittling effects on the alloys in which it forms. It is therefore of great importance to define the composition ranges in which o-phases will be found in alloys which have important technological applications. Such information can be obtained by thermodynamic calculation of the phase equilibria in the alloys of interest.(4*s) To carry out the calculations, information is needed on the thermodynamic properties of the constituent phases of the alloys and, in the case of the o-phase, such information is often not readily available. The present investigation represents part of a continuing programme to determine thermodynamic properties and phase equilibria in alloy systems of technological importance and, in particular, to obtain values for the thermodynamic properties of o-phase alloys in such systems. Previous studies have produced results for the o Presentaddress : Lehrstuhlftir Metahurgieder Kembremrstoffe undTheoretische Htjttenkunde, Rheinisch-Westfallische Technische Hochschule Aachen,51Aachen,Kopernikusstr~e16, Federal Repubhcof Germany. b To whomcorrespondence aboutthis papershouldbeaddressed.

P. J. SPENCER AND F. H. PUTLAND

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Y FIGURE 1. The phase diagram for Co,V,. -, Zegler and Dowmy.@)

----,

Hansen and Anderko;(l) -.-a-,

o-phase in the Fe + Cr,@) Fe + V,(‘) and Co + Cr,(‘) systems, permitting calculations to be made of phase boundaries in the Fe+ Cr+V,(4) Fe+ Co + Cr,(s) and Ni+Co+Cr,“) systems. The results of the present study will allow similar calculations to be made. No previous thermodynamic studies of the Co+V system have been reported in the literature. In the present work, enthalpies of formation at 1473 K have been measured for nine alloy compositions between xv = 0.10 and xv = 0.95 comprising cobalt-rich (f.c.c.), vanadium-rich (b.c.c.), and o-phase alloys. The enthalpy of formation of the “Co3V” phase has also been determined at a temperature of 973 K. 2. Experimental The present measurements were carried out using a high-temperature adiabatic calorimeter, for which details of the construction, method of operation, and specimenpreparation procedures have been described by Dench.tg) The materials used in the experiments were carbonyl cobalt powder and vanadium powder (stated to be of 99.5 mass per cent purity) supplied by Halewood Chemicals

A CALORIMETRIC

STUDY OF THE COBALT

+ VANADIUM

SYSTEM

553

Ltd. Gas analysis showed the powders to have the following mole fractions of 0 and of N: Co: x0 = 0.0044, xN = 0.0005; V: x0 = 0.0280, xN = 0.0012. By carrying out measurements on each of the pure components, it was shown that, in the experimental temperature range, no gas + metal reactions or other impurity reactions were contributing additional thermal effects to the measured enthalpies. Before measurements were made, metallographic and X-ray examinations of compacted specimens were carried out to determine a temperature at which no observable alloying of the component metal powders takes place within a period of approximately 4 h, and also a temperature at which alloying of the cobalt and vanadium powders is complete within about 1 h. Suitable temperatures were found to be 973 K and 1473 K respectively. Full details of the experimental procedures used in carrying out measurements have been reported previously.(‘) 3. Results Enthalpies of formation of the alloys studied were obtained enthalpy changes for the following three processes:

from the measured

xCo(s, 973 K)+ yV(s, 973 K) = CoJ$(s, 1473 K), (1) CoJ$(s, 973 K) = Co,V,(s, 1473 K), (2) xCo(s, 973 K)+yV(s, 973 K) = xCo(s, 1473 K) +yV(s, 1473 K), (3) where x and y are mole fractions (x+y = 1). It can be seen from the above three equations that AH,(Co,V,, 1473 K) = AH, -AH3, AH,(Co,V,, 973 K) = AH1 -AHz, (Measurements for process 2 were only made in the case of the alloy “Co,V”.) Measured values of AH, and AH,, and the derived mean enthalpies of formation are given in table 1. The estimated accuracy of the AH, values (based on the present TABLE

38

1. Enthalpies of formation of alloys Co,V, (f.c.c. Co, b.c.c. V standard states)

Y

AH1/J mol-1

AH,/J mol-1

0.10 0.20 0.25

21880; 22610 17510 16440; 16710

0.30

15230

0.55 0.60 0.65

26570 ; 29070 11710; 11790 13860; 14460

0.90

18540; 18480

0.95

17520; 19800

25990;25680 24810; 25010 26150;24810 24280;25680 24230; 23500 23190 38370; 39790 22410;23070 25230;23650 23290 20220; 20930 21590 20140; 20420

AH:(1473 K)/J mol - 1 -3590 -7400 -8655 -8410 -11260 -10990 -9895 -2405 -1620

554

P. J. SPENCER AND F. H. PUTLAND

results and many previous experimental measurements made using the same apparatus) is +,850 J mol-‘. For the alloy composition y = 0.25, measured AH, values were 31040 and 30250 J mol-’ and hence AHF(973 K) = - 14070 J mol-‘. 4. DiscussioIl The present results, at 1473 K, show that there is a maximum negative enthalpy of formation in the Co+V system of approximately - 11300 J mol- ’ which occurs at a composition of about y = 0.55, i.e. within the o-phase (figure 2). Previous

0

0.2

0.4

0.6

0.8

Y FIGURE

2. Enthalpy of formation of Co,V, at 1473 K.

calorimetric measurements have been made for the o-phase in the Co + Cr system,(*) and since Cr and V are neighbours in the Periodic Table, it is of interest to compare the above negative AHf value for o-phase Co +V alloys, with the positive value (T = 1473 IS) of +3950 J mol-’ at the mean composition (xcr = 0.575) of the o-phase in the Co + Cr system. Values of AH, for o-phase alloys of iron with V and with Cr also show this difference of sign. (‘* @ Thus AH,(o-Fe, 5V,,5, 1273 K) = - 8380 J mol-’ and 1090 K) = +4800 J molSi. AW~-Feo.&ro.47, If the equation AG, = AH,- TAS, is used in conjunction with assessed thermodynamic data for Fe + Cr alloys in order to study the relative stabilities of the different phases of this system, it is found that there is instability of the o-phase at temperatures

A CALORIMETRIC

STUDY OF THE COBALT

+ VANADIUM

SYSTEM

555

below about 713 K.@) In the Co+Cr system, instability of o-phase alloys at lower temperatures is prevented only by a significant decrease in the magnitude of the AH, values for the o-phase with decrease in temperature.(loJ1l) On the other hand, the o-phases which occur in the Fe+V and Co + V systems, and for which the measured enthalpies of formation are strongly negative, are stable at all temperatures. Since no references to previous thermodynamic investigations of the Co +V system could be found in the literature, it is not possible to provide a complete set of reliable thermodynamic values for Co +V alloys. Nevertheless, an attempt has been made to estimate entropies and hence Gibbs energies for the system such that in the temperature range 1100 to 1513 K the present measured and estimated values give consistency (within say y = kO.02) with the phase boundaries reported by Hansen.“) Estimation of entropies was made in the following way. It has been shown, for example by Kubaschewski et al.(“) and by Chart,(r3) that a smooth functional relation exists between the enthalpy of formation and excess entropy of formation of alloy phases. Chart (13) has pointed out that the relation appears to be independent of the bonding mechanism involved, and indeed when experimental AH, and SF values for the o-phases of the Fe + Cr, Co + Cr, and Fe + V systems are inserted on the plot given by Chart, the resulting points lie very close to the curve of AH, against SF selected by that author. The curve has therefore been used in conjunction with the present experimental values of the enthalpies of formation of o-, f.c.c.-, and b.c.c.-phase Co+V alloys to obtain values of the excess entropy of formation at different alloy compositions. Finally, Gibbs energies of formation have been calculated by combination of the experimental and estimated data (assuming AC, = 0). Phase boundaries calculated in this way are consistent with the reported phase boundaries”’ to within y = kO.02 between 1343 and 1513 K, while a value of AS, = -2.72 J K-’ mol-‘, when used in conjunction with the experimental AH,(973 K) value for “Co3V” of - 14070 J mol-‘, gives the required transformation temperature of 1343 K for this phase as well as satisfactory agreement between

TABLE

Y

phase

0.1 0.2 0.3 0.35 0.45 0.5 0.6 0.65 0.9 0.95

f.c.c. f.c.c. f.c.c. f.c.c. 0 CT Q b.T.c. b.c.c.

2. Thermodynamic AHi(expt.> J mo1-1

values for alloys Co,V, at 1473 K A&@.) J K-’ moisl

-4100 -7030 -9040 -9750 -10880 -11255 (&850) -11005 -10000 -2845 -1445

1.07 1.52 1.81 1.91 1.96 1.91 (f2.0) 1.79 1.87 1.57 1.07

s&St.) J K-l mol-1 -1.63 -2.64 -3.26 -3.47 -3.77 -3.85 (zk2.0) -3.81 -3.51 -1.13 -0.59

AG&st.) ___ J mol-l -5676 -9269 -11706 -12563 -13767 -14068 (f3800) -13642 -12755 -5158 -3021

P. J. SPENCER AND F. H. PUTLAND 556 calculated and observed cr-to-(cr+Co,V) and o-to-@+ Co,V) boundaries at temperatures between 1100and 1343 K. A complete set of experimental and estimated thermodynamic values for Co+V alloys is given in table 2 (f.c.c. Co and b.c.c. V standard states).

The authors wish to thank Dr J. F. Counsel1 for his interest and support in this work, and Dr A. J. Head for helpful commentson the manuscript, REFERENCES

1. Hansen,M. ; Anderko, K. Constitution of Binary Alloys, McGraw-Hill: New York, 1958. 2. Zegler,S. T.; Downey,J. W. l’kans AZME 1963,227, 1407. 3. Marcone,N. J.; Coll, J. A. Acta Met 1961, 12, 742 4. Spencer, P. J.; Counsell, J. F. 2. Metallk. 1973, 64, 662. 5. Chart, T. G.; CounselI, J. F.; Jones, G. P.; Slough,W.; Spencer, P. J. Znl.Met. Rev. 1975,20,57. 6. Mtiller, F.; Kubaschewski, 0. High Temp. H&h Press. 1969, 1, 543. 7. Spencer,P. J.; Putland,F. H. J. Iron SteelInst. 1973, 211,293. 8. Bell, H. B.; Hajra, J. P.; Putland,F. H.; Spencer,P. J. Met. Science J. 1973, 7, 185. 9. Dench,W. A. Trans. Fan&y Sot. 1963, 59, 1279.

10. Spencer,P. J.; Putland,F. H. unpublished work. 11. Kaufman,L.; Nesor,H. Met. Zbs. 1975, in press. 12. Kubaschewski, 0.; Evans,E. LI.; Alcock, C. B. Metallurgical Thermochemistry, Pergamon: London.1%7, p. 65. 13. Chart, T. G. High Temp. High Press. 1973, 5, 241. 14. Hultgren, R.; Desai,P. D.; Hawkins,D. T.; Gleiser,M.; Kelley, K. K.; Wagman,D. D. Selected Values of the lkwnodynamic Properties of the Elements, ASM: MetalsPark, Ohio, 1973.