Thorium-carbon System

P. Chiotti

Jan 1950 · U.S. Atomic Energy Commission, Technical Information Division

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A study of the thorium-carbon system has been made and a tentative phase diagram constructed from metallographic X-ray, and melting point data. The existence of two compounds, ThC (4.92% carbon) and ThC2 (9.38% carbon), has been verified. No conclusive evidence was found for any other compound. The monocarbide has a sodium chloride type structure, and the lattice constant for this compound is shown to be 5.34 A. This is considerably larger than the lattice constant previously reported for the monocarbide. It is shown that the lattice constant previously measured was that of a solid solution of thorium in thorium monocarbide. The lattice constant at room temperature for the monocarbide phase increases from 5.29 to 5.34 A as the carbon content is increased from about 3.8 to 4.92%. The monocarbide melts at 2625 +/- 25 C and the dicarbide at 2655 +/- 25 C. All compositions between thorium and the monocarbide show complete miscibility in the liquid state and in the solid state at elevated temperatures. However, an immiscibility gap in the solid solution area appears to extend from room temperature to just under the solidus line. The peak of this gap is at about 2% carbon and 1975 C. The compositions of the thorium rich and carbon rich solid solutions for this gap at room temperature are about 0.25% carbon and 3.8% carbon, respectively. The monocarbide and dicarbide also form a complete series of solid solutions at elevated temperatures but show little or no mutual solid solubility at room temperature. The dicarbide forms a eutectic with graphite containing approximately 12.6% carbon which melts at 2500 +/- 35 C.