A revised thermodynamic description of the Co-W-C system

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Section I: Basic and Applied Research

A Revised Thermodynamic Description of the Co-W-C System Andreas Markstro¨m, Bo Sundman, and Karin Frisk

(Submitted January 26, 2004; in revised form October 26, 2004) A thermodynamic reassessment of the Co-W-C system is presented. New information on the liquid (L) + face-centered cubic (fcc) + graphite + WC and L + fcc + WC + M6C equilibria has recently been published. Because these equilibria are very important for extrapolation to higher-order systems, the ternary system has been revised. A revision of the Co-W system also has been performed.

1. Introduction Over the last century, there have been several investigations concerning the Co-W-C system. The investigations have shifted from using only classic metallography toward combining experimental information with theoretical descriptions to define equilibrium phase boundaries. Ferna`ndez Guillermet (Ref 1) has made the most thorough Calphad analysis of the Co-W-C system. The analysis was based on experimental information that was available at the time and has been used extensively to calculate phase equilibria in cemented carbides (Ref 2). For the four-phase equilibrium temperatures liquid (L) + graphite + facecentered cubic (fcc) + WC and L + fcc + WC + M6C, different experimental values were published, and Ferna`ndez Guillermet (Ref 1) selected the values (1548 and 1628 K, respectively) from Åkesson (Ref 3). Recent differential thermal analysis (DTA) of both temperatures by Kruse et al. (Ref 4) showed that the values 1571 and 1641 K, also found in older publications, were the most reliable. Furthermore, the values by Kruse et al. (Ref 4) are in better agreement with measurements in higher-order systems. To incorporate the new measurements into the thermodynamic description of Co-W-C, it was necessary to perform a complete new optimization, although most of the selection of experimental information could be adopted from the article by Ferna`ndez Guillermet (Ref 1). When performing an optimization on a ternary system, it is important that the binary systems are well described. The selected binary information is presented below. Ferna`ndez Guillermet (Ref 1, 5) performed thermodynamic assessments of the Co-W and Co-C systems, and Gustafson (Ref 6) performed assessments of the W-C system. The Co-W system is revised in the present work. Jonsson (Ref 7) has revised the W-C system. The main difference compared with the previous description by Gustafson (Ref 6) was a new description of the cubic WC phase. This

A. Markstro¨m and K. Frisk, Swedish Institute for Metals Research, Drottning Kristinas Va¨g 48, SE-114 28 Stockholm, Sweden; and B. Sundman, Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden. Contact e-mail: [email protected]


phase is only stable at very high temperatures (above 2800 K) in the binary W-C system, but the low-temperature properties are important when describing the solubility of W in cubic carbides (for example, TiC, NbC, and TaC). When describ