TERNARY PHASE EQUILIBRIA IN TRANSITION METAL-BORON-CARBON-SILICON SYSTEMS. PART I. RELATED BINARY SYSTEMS. VOLUME XI. FINAL REPORT ON THE MO-C SYSTEM.
AEROJET-GENERAL CORP SACRAMENTO CA MATERIALS RESEARCH LAB
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The binary alloy system molybdenum-carbon was investigated by means of X-ray, metallographic, thermoanalytical, and melting point techniques on chemically analyzed specimens. The system is characterized by three congruently melting, intermediate phases, Mo2C, Eta-MoC1-x, and Alpha-MoC1-x, of which only Mo2C is stable at temperatures below 1650 C. Substoichiometric is greater than 32.5 At C dimolybdenum carbide undergoes a homogeneous sublattice order-disorder transformation at temperatures of approximately 1400 C, whereas hyperstoichiometric compositions undergo a discontinuous phase-change. The order-disorder transitions in the Me2C phases are discussed in terms of the structural changes involved in the transformation processes and the absence of long range sublattice coherency in stoichiometric or hyperstoichiometric compositions attributed to the impossibility of obtaining long range order in a linear chain of alternating carbon atoms and vacancies. From the experimental phase relationships and the known thermodynamic data for Mo2C, limits for the free energies of formation for the high temperature phases Eta-MoC1-x and Alpha-MoC1-x are derived and found to be in good agreement with data previously obtained from phase equilibria in ternary metal-carbon systems. Author
- Properties of Metals and Alloys