Investigation of (Cr,Co) 7 C 3 -fcc-graphite equilibrium in the temperature interval 1373 to 1473 K
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INTRODUCTION
THE ternary system C-Co-Cr has proved to be of industrial interest because of the monovariant eutectic reaction that forms an alloy containing a CoCr matrix reinforced with carbides, which gives mechanical advantages.[1] Alloys of C-Co-Cr have also a wide application area both in the hard material industry as well as in medical engineering. The C-Co-Cr system is of interest as a subsystem to the quaternary W-C-Co-Cr. Cr is one of the most frequently used WC grain growth inhibitors when sintering cemented carbide with submicron WC grains. In other words, the three-phase triangle Cr7C3 1 fcc 1 graphite is of great importance in the manufacturing of cemented carbides to avoid formation of brittle, unwanted carbides during the sintering binder solidification step. The first investigation on the ternary system C-Co-Cr involving the 7-carbide was made by Ko¨ster and Sperner[2] in 1955. This study was followed by experimental investigations using thermal analysis and microprobe studies,[3,4,5] measurements of the carbon activities,[6] and a combined differential thermal analysis and differential scanning calorimetric study.[7] These six investigations were later used for the thermodynamic evaluation of the C-Co-Cr system performed by Kusoffsky and Jansson.[8] In Figure 1, the isothermal section of the C-Co-Cr system at 1423 °K, calculated using the description from Kusoffsky and Jansson,[8] is presented. The earlier experimental studies[2–7] were most often made at temperatures higher than the temperature range of industrial interest. Zackrisson et al.[9] found that the primary precipitate during cooling after sintering in the THE´RE`SE STERNELAND, formerly with the Department of Material Science and Engineering, Royal Institute of Technology (KTH), Stockholm, Sweden, is now with the Outokumpu Stainless AB, Avesta, Sweden. ¨ M is Contact e-mail: [email protected] ANDREAS MARKSTRO with KIMAB, Stockholm, Sweden. SUSANNE NORGREN is with Sandvik Tooling R&D, Stockholm, Sweden. RAGNHILD E. AUNE, Researcher, and SESHADRI SEETHARAMAN, Professor, are with the Department of Material Science and Engineering, Royal Institute of Technology (KTH), Stockholm, Sweden. Manuscript submitted July 14, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
quaternary system W-C-Co-Cr at high carbon activities is the M7C3 phase. Their results also indicated that M7C3 had a high solubility of Co, and that the Cr solubility in the Co-rich fcc phase was higher than predicted by calculations. In view of this, it was strongly felt that an experimental confirmation of the solubility of Co in Cr7C3 was necessary, and the current work was initiated. To our knowledge, a systematic experimental investigation of the Co solubility in Cr7C3 has not been previously carried out. Having an accurate thermodynamic description of lower order systems is very important when designing new alloys. Accurate thermodynamic descriptions of binary and ternary systems are necessary when predicting phases and properties of new, multicomponent alloys or when des
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