Carbon – Tantalum – Tungsten

Abstract

Refractory carbides of transition metals are used as a basis for tool materials and as special small additions to greatly improve the high-temperature strength of tungsten alloys. The refractory interstitial phases, in particularly carbides, are characterized by high melting temperature, thermodynamic stability in equilibrium with the tungsten solid solution under alloy forming conditions and service temperatures, and low coagulation rate for long time high temperature exposure. In order to select the composition of materials and evaluate their stability under service conditions, it is essential to have data on the phase equilibria in the respective systems. One such system is the C-Ta-W system. The melting temperatures of C-Ta-W alloys along the W2C-TaC section were determined by [1930Agt] using the Pirani-Alterthum method. The solidus temperatures were measured by [1969Rud] along the W2C-Ta2C, WC0.7-TaC1.57 and WC0.64-TaC0.89 sections using the Pirani technique and DTA. The phase equilibria in the solid state along the TaC-WC section were studied by [1943Uma, 1946Kov, 1947Bro, 1947Now, 1966Fun, 1966Tot, 1967Den, 1968Hac, 1968Pan], using X-ray analysis of alloys after sintering at different temperatures, and also by [1968Hac] after furnace cooling or quenching alloys of compositions located along the section. The measurement of the variation in lattice parameters of the (Ta,W)2C and (Ta,W)C phases as a function of W content was conducted by [1962Rud, 1977Hol], and isothermal sections at 1500, 1700 and 2200°C were constructed by [1962Rud, 1966Rud]. Simultaneously, [1962Rud, 1963Rud, 1965Rud, 1969Rud] evaluated thermodynamically the isothermal sections at temperatures from 1500 to 3500°C. [1969Rud] also presented a number of vertical sections and the liquidus surface, which were determined experimentally. [1973Sav, 1982Sav] investigated the W corner of the C-Ta-W system. They described the phase equilibria of as-cast alloys and constructed isothermal sections at 1500 and 2000°C. [1999Fri, 2001Fri] presented a new thermodynamic analysis of the solid and liquid phases for C-Ta-W system based on the experimental data given by [1962Rud, 1966Rud]. Quite good agreement was found between the calculations and the available experimental information. Based on a thermodynamic analysis of the carbide phases in the binary systems, [1973Ere, 1988Vel] presented a hypothetical projection of the solidus surface and phase equilibria below the minimum solidus temperature in the C-Ta-W system. The presented solidus surface projection and the phase relations agreed with the experimental data of [1969Rud]. A summary of the experimental studies of the phase equilibria is given in Table 1. There are reviews on the C-Ta-W system [1981Hol, 1984Hol] in which the works of [1962Rud, 1965Rud, 1969Rud, 1968Hac, 1973Ere] are presented but without critical analysis. [1994Mch] described the C-Ta-W system as presented by [1965Rud]