• PDF / 2,206,884 Bytes
• 22 Pages / 590.28 x 785 pts Page_size
• 40 Downloads / 237 Views

DOWNLOAD

REPORT

---

I.

INTRODUCTION

A. Previous Work in the Co-W-C System

SEVERAL investigations of the Co-W-C phase diagram have been presented over the years, starting with the work by Takeda[q in 1936. The information available in 1952 was reviewed by Rautala and Norton, t2] who performed a new study using X-ray diffraction, metallographic techniques, and DTA. They presented tentative phase diagrams of stable and metastable phase equilibria. In 1954 Gurlandt31 presented a vertical section of the Co-W-C phase diagram corresponding to wt pct Co: wt pct WC = 16 : 84, determined by X-ray and metallographic techniques, whereas GrtiterE4J-reported in 1959 isothermal sections for 1073, 1323, 1473, 1533, and 1673 K, and five vertical sections. In 1970 Pollock and Stadelmaier [5] performed a study of the M6C carbide in the Co-W-C system using X-ray diffraction and metallographic techniques. They presented two isothermal sections, for 1273 and 1673 K, respectively. Ettmayer and Suchentrunk [6] also investigated the stability of the MrC carbide by using X-ray and metallographic methods, and constructed isothermal sections for 1373 and 1573 K. In 1974 Hoffman and Mohs[7] reported another study based on using X-ray and metallographic methods, and constructed an isothermal section for 1523 K. Whereas these works were, in general, aimed at defining the equilibrium boundaries of the Co-W-C phase diagram by using classical metallographic techniques, Johansson and Uhrenius [8] reported in 1978 a study of the phase equilibria at 1423 K based on combining experimental and thermodynamic methods. They used a technique involving heat treatment of samples at controlled carbon activities, X-ray and microprobe analysis. The results, which involved the properties of fcc,/z-phase, M12C, M6C, and WC, were analyzed using Hillert and Staffansson, s model,[9] and a calculated isothermal section for 1423 K was presented. The phase equilibria at high temperatures and the liquidus surface were studied by Uhrenius, Carlsson, and Franzrn,[l~ using DTA, X-ray and microprobe measurements. They

pARMANDO FERNANDEZ GUILLERMET, formerly with the Department of Physical Metallurgy, The Royal Institute of Technology, S-10044 Stockholm, Sweden, is with CONICET, Centro Atrmico Bariloche, 8400 S.C. de Bariloche, Argentina. Manuscript submitted April 6, 1988. METALLURGICAL TRANSACTIONS A

also performed a thermodynamic analysis. Further solidification studies were made by Johansson, Elll who also presented new results on the phase relations at 1573 and 1713 K. Westin and Franzrn[lzl studied the solidification of Co-W-C alloys containing 10 to 17 at. pct W and 8 to 14 at. pct C. Two recent experimental studies of the Co-W-C system are due to Tuma and CiznerovaE131and/~kesson. B41 Tuma and Ciznerova E~31studied the Co-rich region of the Co-W-C system using a technique involving the carburization of samples in silica ampoules. Their results, which concern the fcc phase and the WC and M12C carbides, were analyzed using the model due to Hillert and Staffansson. [91 They reported is