Phase equilibria, carbide systems

Thermodynamic calculations by Dorner [78], Lukas [93] and Lim and Lukas [52], however, clearly demonstrated the existence of a binary phase equilibrium of boron carbide and a Si- and B-containing melt above 1560°C. The theoretical results were confirmed by hot pressing, liquid phase sintering and infiltration experiments by Lange and Holleck [75], Telle [83], Telle and Petzow [94], and Telle [54], which also yielded more details on the extension of the homogeneity field of boron carbide towards the Si-rich corner of the system B-C-Si. 

Suppose that a two-phase system consists of a fine dispersion of a carbide phase in a matrix. The carbide particles are in equilibrium with C dissolved interstitially in the matrix phase, with the equilibrium solubility given by... 

The crystallography of the f.c.c.— b.c.t. martensitic transformation in the Fe-Ni-C system (with 22 wt. %Ni and 0.8 wt. %C) has been described in Section 24.2. In this system, the high-temperature f.c.c. solid-solution parent phase transforms upon cooling to a b.c.t. martensite rather than a b.c.c. martensite as in the Fe-Ni system. Furthermore, this transformation is achieved only if the f.c.c. parent phase is rapidly quenched. The difference in behavior is due to the presence of the carbon in the Fe-Ni-C alloy. In the Fe-Ni alloy, the b.c.c. martensite that forms as the temperature is lowered is the equilibrium state of the system. However, in the Fe-Ni-C alloy, the equilibrium state of the system in the low-temperature range is a two-phase mixture of a b.c.c. Fe-Ni-C solid solution and a C-rich carbide phase.5 This difference in behavior is due to a much lower solubility of C in the low-temperature b.c.c. Fe-Ni-C phase than in the high-temperature f.c.c. Fe-Ni-C phase. If the high-temperature... 

In most systems the martensitic reaction is geometrically reversible. On heating, the martensite will start to form the higher temperature phase at the As temperature and the reaction will be complete at an Af temperature, as illustrated in Figure 11.19. Martensite in the iron-carbon system is an exception. On heating, the iron-carbon martensite decomposes into iron carbide and ferrite before the As temperature is reached. Martensite can be induced to form at temperatures somewhat above the Ms by deformation. The highest temperature at which this can occur is called the Md temperature. Likewise, the reverse transformation can be induced by deformation at the Ad temperature somewhat below the As. The temperature at which the two phases are thermodynamically in equilibrium must lie between the Ad and Md temperatures. 

Figure 4.14 A very simplified melting diagram of the Fe-C system where a stationary metastable phase can form during the catalytic graphitization of amorphous carbon. FesC (cementite) and Fe2C are stoichiometric iron carbides. A is the equilibrium eutectic point (T = 1420 K, x = 0.173), and B is the stationary oversaturated state (T = 920 K).

Bond energies of RuC(g) and RhC(g) suggest that TcC(g) is a thermodynamically stable molecule present in significant amounts in the equilibrium vapor of the technetium-carbon system at temperatures above 2000 K. A graphite effusion cell was used to vaporize the technetium carbide into a quadrupolc mass spectrometer. The TcC(g) molecule was observed abt)vc a liquid technetium carbide phase. A bond energy of 148 2 kcal - mole at 2450 K was derived for TcC(g) [13],... 

As is known in the case of binary systems, when their composition is close to equiatomic, d-ina-via metals form, as a rule, stable phases with Bl-type structures. But, as was observed by Vereshchagin and Kabalkina (1979) using a high-pressure treatment, it is possible to get B1 B2 (NaCl-CsCl)-type structural transitions with some oxides. The question of whether this could happen with d-met carbides was discussed by Ivanovsky et al (1988). These authors carried out LMTO band structure calculations for hypothetical TiC, VC and CrC compounds with a B2 structure. The lattice parameters were determined from the condition that the unit cell volumes of the CsCl- and NaCl-type phases were equal. In order to consider the influence of uniform isotropic compression the B2 VC calculations were carried out for crystal lattice volumes of 5 and 10% less than the equilibrium one. 

Figure 6 Phase fields for solid species deposited at equilibrium in the Ti-C-H-Cl system, showing the iso-concentration curves in the titanium carbide single-phase domain. (From Ref. 125.)...

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