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 V
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: Air Force Materials Laboratory
Date of Publication: 1964-10
Pages: 64
Contract: AF 33(657)-8025
DoD Project: 7350
DoD Task: 735001
Identifier: AD0608301

The work summarized in the report is the result of investigations of phase equilibria in the binary systems, tantalum-carbon, hafnium-carbon, and boron-carbon. A completed phase diagram which encompasses experimental results and considerations heretofore unreported in the literature, is presented for the hafnium-carbon system. A phase diagram for the tantalum-carbon system is included, which, in general, resembles the version released by Ellinger in 1943. Preliminary studies on the hafnium carbide-tantalum carbide quasi-binary were limited to temperatures of 3700°C at which no liquid formation was evident. Results for the boron-carbon system depicting maximum boron solubility in the graphite lattice at and near the eutectic temperature of approximately 2390°C, are also included.Determination of the hafnium-carbon phase diagram by metallographic and X-ray diffraction studies on appropriate alloys constitutes the bulk of effort for this period. The only intermediate phase observed in this binary system was HfC. This phase was found to be homogeneous between 34.0 at 48.0 atomic per cent at 2200°C and between 36.0 and 49.3 per cent carbon at 3150°C. The lattice parameter variation was also determined for HfC1-x compositions prepared at 2200°C and 3150°C. The most refractory composition with a melting point of 3830°C, was established at 47.5 atomic per cent carbon from melting point data. Solidus temperatures of 2240°C and 3150°C occur on the high hafnium and high carbon sides of the monocarbide, respectively. The invariant point between HfC and C is located at 66.0 atomic per cent carbon; wheras, the 2240°C solidus correspnds to the pertectic temperature at which β-Hf is formed from HfC and hafnium-rich liquid. β-Hf has a melting temperature of 2208°C and is capable of taking carbon into solution to the extent of 10.5 atomic per cent at this temperature.The tantalum-carbon system is characterized by two high temperature phases, TaC and Ta2C. A eutectic is observed at 2825°C and 12.5 atomic percent carbon between TaC and Ta2C. Ta2C has low carbon limits of 26.5 and 29.0 atomic per cent at 2850° and 3150°C, respectively. The Ta2C phase field boundary apparently does not deviate from the stoichiometric composition on the high carbon side. A peritectic reaction at 3240°C leads to formation of Ta2C 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. The phase field broadens at higher temperature as exemplified by limits of 39.5 and 49.8 atomic per cent carbon at 3240° and 3375°C, respectively. The eutectic formed between TaC and C corresponds to 3375°C and 61.2 atomic per cent carbon. TaC, containing 49.8 atomic per cent carbon, melts in the temperature region of 3600°-3700°C. The highest melting temperature for TaC is above 3800°C when the carbon content is approximately 48.5 per cent. Preliminary studies on the HfC-TaC solidus were limited to four specimens and temperatures below 3700°C. Experimental difficulties prevented melting point measurememnts.A solid solution of boron in graphite was found and the limits of solubility as a function of temperature were determined.  The solubility reached a maximum of 2.35 atomic per cent at 2350°C  The lattice constants varied with boron content as follows:ao = 2.46023 + 0.00310 KB,co = 6.71163 - 0.00594 KB, where KB is the atomic fraction of dissolved boron, co and ao being measured in angstroms. A comparison of measures and calculated densities of the solutions indicated that they were substitutional.

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