Indian Journal of ChemistryVol. 25A. March 1986. pp. 263-265

Thermodynamic Properties ofAluminium-Chromium System at 1423K

N C OFORKA

Department of Applied Chemistry.University of Port Harcourt P.M.B. 5323.

Port Harcourt. NigeriaReceived 7 Nooember 1984; revised and accepted 17 July 1985

Acitivity measurements have been made with a Knudsen cell massspectrometer on aluminium-chromium alloys over the temperaturerange 1173-1483K. The activities of aluminium and chromium in allthe alloys show pronounced negative deviation from ideality at1423K. The properties !lGA" !lGc" !lG'!:" !lGt·" so and sc- havebeen derived from the experimental data.

While investigating the phase equilibria andthermodynamic properties of the ternary aluminium-chromium-nickel alloys. we felt the need of measuringaccurate thermodynamic properties of the aluminium-chromium binary alloys. Very few papers' -4 havebeen published in recent years on the binaryaluminium-chromium alloys and they do not giveconsistent set of thermodynamic data. The presentwork re-investigates the thermodynamic properties ofaluminium-chromium alloys by the Knudsen cell massspectrometric technique and the results are comparedwith selected literature data.

Experimental methods used were generally the sameas described in recent papers published by Moore etal. 5 and Argent et al. 6. 7 Some difficulty wasexperienced with alloys having high aluminium vapourpressures which gave rise to rapid deterioration of thethermocouples and of tantalum used in theconstruction of the Knudsen cell furnace. This wasavoided by using a narrow temperature range, 1173K-1483K. All the alloys were melted under anatmosphere of argon chill cast into ingotsapproximately 25 mm in diameter and 150 mm long.The compositions of alloys were determined bychemical analysis. Carbon and metallic trace elementswere not present at levels in excess of 0.01'1.'•.

Fine turnings or crushed samples of the more brittlealloys were contained in an alumina cell sealed with analumina orifice plate containing an orifice appro-ximately 0.35 mm in diameter. The orifices wereslightly diverging truncated cones, and Clausingfactors reported by Freeman and Edwards" were usedto convert ion intensities to those expected from anorifice of standard geometry. The temperaturestabilities of the specimens were ± 0.5 Kover 0.5 hr at1423K and the absolute accuracy of the temperaturewas ± 2 K as determined by melting point

determination of nickel and iron. An argon referencewas used to correct for day-to-day variations ininstrumental sensitivity.

The activity of a volatile component in.a liquid orsolid mixture can be readily measured by measuringthe partial pressure of the constituent in the vapourphase in equilibrium with the mixture. A measure ofthe partial pressure of the component i (PJ, over thesolution together with a knowledge of the partialpressure of the pure component i (Pi). yields the value(a; = P;/ Pi) of activity of the component i. In theKnudsen cell mass spectrometric technique, the partialpressure of the species is related to the intensity of thespecies observed, by the equation P,= K. It. T, whereK is a constant and contains all instrumental andgeometrical factors. It is the intensity of the species i(counts per second) and T is the absolute temperatureat which the measurement is made. The measurementswere repeated at various temperatures and the resultsof least-squares fits to the standardised intensities inthe form In(ff/Ks-t)=A+B/(T/K). where I is theintensity, T is the absolute temperature, A and Bareconstants, were used to calculate intensities at 1423K.The ratio of the intensity of a species in alloy to that inpure standard gives the activity of that species. Thestandards used were solid chromium and pure liquidaluminium. The activities determined were treated toget partial and excess partial molar quantities. Thepartial molar free energy change for the species i whendissolved in a solvent, is given by /lG; = RTlnaj. whereR is the gas constant. T is the absolute temperature andaj is the activity of the component i. The deviation of apartial quantity from its value in an ideal solution is theexcess partial quantity.

/lGfS = /l(/; - /lGjd = RTlna; - RTlnxj = RTlni'j.where /lGfs is the excess partial free energy of mixing./lGjd is the ideal partial free energy of mixing. Yj is theactivity coefficient of i (=aJxJ and x, is theconcentration of component i, in mole fraction.

The compositions of the alloys. Xj. are shown inTable 1. together with the activities of the species i. aj.activity coefficients. Y;. the partial molar free energy ofmixing. /lG;, the excess partial molar free energy ofmixing. /lGis• the integral free energy of mixing. /lG,and the excess integral free energy of mixing. AGxs.

The activities of the species were measured with aprecision of about 1O~<,.For high temperature activitydetermination an accuracy of ± 15% and a precision ofthe same magnitude are accepted. The possible errorsthat can occur in the activity determination of a

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INDIAN J. CHEM .• VOL. 25A. MARCH 1986

-----Table I - Mole Fraction. Activity. Activity Coefficient. Partial Free Energy of Mixing. Partial Excess Free Energy of Mixing.

Integral Free Energy and Integral Excess Free Energy of MixingXc, ac, aA, Yc, YA, llGcr llGA, llG~~ llG~, llG llG"

(kI/mol) (kl/mol) (kJ/mol) (kl/rnol) (kl/mol) (k.I/mol)

0.10 0.005 0.860 0.0500 0.9556 -62.6835 -1.7844 -35.4420 -0.5373 -7.8743 -4.02780.19 0.041 0.628 0.2158 0.7753 -37.7941 -5.5045 -18.1441 -3.0112 -11.6395 -5.88650.225 0.048 0.562 0.2133 0.7252 - 35.2117 -6.8183 -17.5622 -3.8024 -13.2068 -6.89300.36 0.048 0.562 0.1333 0.8781 -35.2117 - 6.8183 -23.1233 -1.5378 -17.0399 -9.30860.38 0.104 0.350 0.2737 0.5645 -25.4884 -12.4216 -14.0398 -6.7655 -17.3870 -9.52970.43 0.255 0.208 0.5930 0.3649 -16.1685 -18.5790 -6.1829 -11.9280 -17.5425 -9.45750.57 0.255 0.208 0.4474 0.4837 -16.1685 -18.5790 -9.5175 -8.5931 -17.2051 -9.12000.70 0.420 0.080 0.6000 0.2667 -10.2632 -29.8814 -6.0435 -15.6360 -16.1487 -8.92130.777 0.560 0.042 0.7207 0.1883 -6.8597 -37.5080 - 3.8750 -19.7564 -13.6945 -7.41660.80 0.650 0.030 0.8125 0.1500 -5.0965 -41.4855 -2.4565 -22.4445 -12.3743 -6.45410.90 0.875 0.005 0.9722 0.0500 -1.5793 -62.6835 -0.3336 -35.4420 -7.6902 - 3.8444

1.0~-------- --.

0.9

O.B

.& Present work(l423 K)

• Hultgren et 01{1273 K)

0.7

0.6

0.5

OAIO

.4

0.3

0.2

0.1

o 0.5 0.6 0.7 O.B 0.9 1.0

XCr

Fig. 1- Activity data of Cr and AI in Cr-AI binary alloys.

component by measuring the intensity in an alloy andstandard metal are discussed by Moore et al".

The activities of both aluminium and chromiumshow a negative deviation from ideality throughout thecomposition range studied. These values are in goodagreement with Hultgren's values", falling slightlycloser to ideality as would be expected on the basis thatthe present results are for 1423 K as compared to 1273K for the earlier assessment".

The values of Johnson et al. at 1273 K show apositive deviation from ideality at high aluminiumconcentration (Fig. O. The integral free energies ofmixing are slightly less negative than those obtained byKaufman and Nesor" and slightly more negative thanthe values obtained by Samokval and Vecher ' at 980 K{Fig. 2). The values obtained by Johnson et at? are ingood agreement with the values of Kaufman andNesor".

264

Cr-AI SystemX.L

ro ~ ~ ~ 00 ro ~

UCr

• Present workU423K)o Johnson et 0' U273K)

• Kaufman ond No"" U273K)

• Somokva' ond Ilechef(98QK)

·4

·8

Fig. 2 - Integral free energy of mixing-composition plot of AI-Cralloys.

At higher temperatures, the integral free energychange, I1G, is expected to be more negative. assumingenthalpy change (11/1) and entropy change (11S) to beconstant. But I1S is usually not constant for an orderedphase; as the temperature increases. the phase becomesmore disordered and higher I1S values result. whichimplies that I1G will still be more negative.

The present data are more consistent with the phasediagram data of Shunk9 and Hansen and Anderko '"than the earlier literature data.

The author wishes to express his thanks to Prof. 8.8.Argent of the University of Sheffield. England. forlaboratory facilities and advice. and to the Universityof Port Harcourt for financial grant.

ReferencesI Hultgren R. Desai P D. Hawkin D T. Gleiser M & Kely K K.

Selected ralues of the thermodynamic properties of metalbinarvallovs, Vol. 2 (ASM. metal Park. Ohio). 1973.33.

2 Johnson W. Komarek K & Miller E. Trans Met Soc AIME. 8(1968) 242.

3 Samokhval V V & Vecher A A. Russ Metal. 6 (1971) 118-120.4 Kaufman L & Nesor H. Bureau of mines report of iniestigation

(Department of Interior). (\ 975) 293-318.5 Moore R H. Robinson D & Argent B B. Proceedings of the

international conference on metallurg chem (jointly organized

by Brunnel University and the National PhysicalLaboratories) 14-16 July 1971.

6 Argent B B. Jones K & Kirkbride B J. The industrial use ofthermochemical data (the Chemical Society. London). 1980.379-390.

7 Argent B B. Ellis M & Effenberg G. High temperatures-highpressures. 14 (1982) 409-416.

NOTES

8 Freeman R D & Edwards T G. Characterization of hightemperature vapours, edited by J L Margrave (John Wiley.New York). 1967. 508-509.

9 Shunk F A. Constitution of binary alloys and supplement(McGraw-Hill. New York). 1969. 21.

10 Hansen M & Anderko K. Constitution ofbinaryalloys(McGraw-Hill. New York) 1958. 81.

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