Thermodynamic investigations of Cr 3 C 2 and reassessment of the Cr-C system

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Cr, CrF2 |CaF2 | CrF2, Cr3C2, C (950 to 1150 K) Extreme precautions were taken during the experimental measurements to avoid errors. The reliability and reproducibility of the values were confirmed by careful repetition of several of the experiments. The following equation has been obtained by a linear analysis of the EMF results as a function of temperature: f G°Cr3C2(1600)  58,857 22.344T (J # mol1)

(950 to 1150 K)

The generated values are compared with those reported in the literature. The values of the enthalpy of formation were evaluated by using a third-law analysis, and an average value of  fH°298  71.7 kJ>mol was obtained. The ground-state energy of the hypothetic end-member compound CrC3 in the bcc structure at 0 K was calculated by using the ab initio method. The experimental results obtained, as well as the results from the ab initio calculations, were employed in a reassessment of the Cr-C system using the CALPHAD approach. A new set of parameters for the bcc phase was evaluated using first-principles calculations.

I. INTRODUCTION

CHROMIUM carbides are of considerable scientific and technical importance because of their extensive applications in the cutting tool and in the heat-resistant steel industries. A precise knowledge of their thermodynamic properties is essential for the understanding, as well as the prediction of, their chemical behavior in various environments. The chromiumcarbon system contains three stable intermediate phases, i.e., Cr23C6, Cr7C3, and Cr3C2.[1,2] The carbides are all nonstoichiometric compounds with very narrow homogeneity ranges below 1200 K.[3] Numerous experimental investigations have over the years been published on the high-temperature thermodynamic properties of the chromium carbides.[4–15] The available experimental data for the Gibbs energy of formation of the chromium carbides, however, are quite scattered.[15] Among the many previous measurements, there have been six determinations of the Gibbs energy of formation of Cr3C2 by the use of the galvanic cell technique (four of them using CaF2 as the electrolyte). It is especially surprising to find that the disagreement among the data generated by the galvanic cell technique is still quite large. The possible reasons for the large scattering of the electromotive force (EMF) results have already been discussed in the respective investigations.[11–15] As a preliminary study on the thermodynamic properties of the Fe-Cr-C transition metal carbides, the Gibbs energies L.D. TENG, Doctor, X.G. LU, Postdoctoral Student, R.E. AUNE, Doctor, and S. SEETHARAMAN, Professor, are with the Department of Materials Science and Engineering, Royal Institute of Technology, SE-10044 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted February 23, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A

of formation of Cr3C2 were determined in the present study by use of the EMF technique. A CaF2 single crystal was used as an electrolyte, and extreme precautions were taken to avoid experimental errors. Based on th

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