Research Study To Determine The Phase Equilibrium Relations Of Selected Metal Carbides At High Temperatures

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Report Number: WADD TR 60-143 Part IV
Author(s): Sara, R. V., Lowell, C. E., Dolloff, R. T.
Corporate Author(s): Research Laboratory of National Carbon Company Division of Union Carbide Corporation
Laboratory: Directorate of Materials and Processes
Date of Publication: 1963-02
Pages: 41
Contract: AF 33(657)-8025
DoD Project: 7350
DoD Task: 73500
Identifier: AD0402797

The work summarized in this report is the result of investigations of phase equilibria in the binary systems, zirconium-carbon, tantalum-carbon, and boron-carbon. A completed phase diagram which encompasses experimental results and considerations heretofore unreported in the literature, is presented for the zirconium-carbon system. A tentative phase diagram for the tantalum-carbon system is included, which, in general, resembles the version released by Ellinger in 1943. Preliminary results for the boron-carbon system, depicting maximum solubility in the graphite lattice below the eutectic temperature of approximately 2390°C, are also included.The zirconium-carbon system is characterized by eutectic temperatures of 1850° and 2850°C on the zirconium-rich and carbon rich sides of ZrC, respectively. ZrC is the only carbide phase which has been observed in this system, but preliminary considerations of lattice parameter variation with temperature suggest a possible order-disorder transformation. The melting temperature of ZrC appears to maximize at 3420°C and at a composition containing 46 atomic per cent carbon and is invariant with temperature in the region of 1900° to 3300°C. The high carbon boundary between 2850° and 3300°C is located at 48.9 atomic per cent carbon.The tantalum-carbon system is characterized by two high temperature phases, TaC and Ta2C. A eutectic is observed at 2825° between Ta and Ta2C. Ta2C has approximate compositional limits of 29 and 33 atomic per cent carbon above 2850°C and is formed by a peritectic reaction at 3240°C from tantalum-rich liquid and TaC. TaC is face-centered cubic and has boundary limits of 41.7 and 49.5 atomic per cent carbon at 2250°C. Preliminary experiments suggest that the low carbon boundary contains les than 39.5 atomic per cent carbon at 3300° to 3400°C. The existence of a eutectic between TaC and C has been verified, but the temperature and composition remain to be characterized. Below the B4C-C eutectic temperature of 2390°C, maximum boron solubilities in graphite are found to be 1.50, 2.25, and 2.75 atomic per cent at temperatures of 2000°, 2200°, and 2350°C, respectively.

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