Melting transition metal carbides

The transition metal carbides are some of the highest melting known materials. 5-TaCo.89 melts congruently at 3985 °C and 5-HfCo.94 congruently at 3950 °C. The melting temperatures can be read from the phase diagrams presented in Section 5.2. 

Cemented carbides form one of the most important groups of hard materials [41,42], The carbides WC, TiC, and TaC are the technically most important ones. They are produced in amounts of several thousand tons per year. VC, NbC, ZrC, HfC, M02C, and the chromium carbides are somewhat less important. The extreme hardness and high melting points of many transition metal carbides were already recognized in the 1890s by Moissan [43]. 

Table 1. Bulk properties at room temperature of some important transition metal carbides (disordered state). For f.c.c. carbides (Pearson symbol cF8) the composition is near 50at-% C, except for VCo.ijs- The melting points do not apply to this composition. 

The mechanical properties and the failure mechanisms of transition-metal carbides are reviewed in detail by Toth.t l Generally, large spreads in the reported values found in the literature are common. This is particularly true in older reports which were mostly performed on sintered materials. More recently, testing has been switched to single crystals or polycrystalline materials obtained from the melt or by thin-film deposition. These are believed to yield more accurate and consistent information. Yet, any test must be carefully characterized in order to be meaningful. The following factors influence mechanical testing.l H ... 

This book presents a systematic description of the electronic and physicochemical properties of transition-metal carbides and nitrides. These materials possess remarkable physical and chemical properties, including extremely high hardness and strength, and high melting points, metallic conductivity and superconductivity. As a result, they have been extensively studied by scientists, and their properties widely exploited by engineers. 

Table 3.1 Crystal structure (space group), melting point crystal density g and color of some transition-metal carbides and nitrides, rs = rocksalt.

Small atoms such as C and N have radii less than 2/3 of some of the transitional metals and can form interstitial compounds. The C and N atoms form covalent bonds with transition metals, such as Ti, V, Zr, Nd, Flf, and Ta, to form carbides and nitrides that are extremely hard and have very high melting points. The best-known example of a transitional metal-carbide is Fe3C, also known as cementite, which is the primary hardening component in steel. It has a complex DOn structure similar to Al3Ni. There is another form of Fe3C known as bainite (Pearson symbol hP8—apparently no Strukturbericht symbol) which forms in steels at a lower temperature than cementite. 

Figure 7 The entropy-related quantity , the cohesive energy Es, and the melting temperature for transition metal carbides (filled symbols) and nitrides (unfilled symbols) in the NaCl-type crystal structure. Data as in Figs. 8-10.

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