Carbides structure

Fig. 3.23. Scanning SIMS image of the carbide structure of a high-speed steel [3.52]. In the V distribution different phases (MC and M2C) are visible. The Al distribution shows the shell struc-...

The group at Aarhus have reported carbon-induced structures at Ni(lll) and Ni(110) surfaces resulting from the dissociation of ethylene at high temperatures.27 Between 400 and 500 K, the Ni(l 10) surface is seen to form two carbidic structures with (4 x 3) and (4 x 5) domains present arising from surface reconstruction with substantial transport of nickel taking place. At higher temperatures (560 K), the surface becomes dominated by the (4 x 5) structure, which is well ordered and can be observed clearly by LEED. Ion scattering studies provide additional information which enables models to be constructed for both the (4 x 3) and (4 x 5) phases. 

The production and isolation of the first carbide metallofullerene was reported on Sc2 Cg6 (=Sc2C2 Cg4) in 2001 (Wang et al., 2001). In fact, this study has revealed that the major part of the di-scandium metalloful-lerenes may have Sc2C2 C2 2 carbide structure rather than Sc2 C2 including Sc2 Cg4 (isomers III) (liduka et al., 2006 Nishibori et al., 2006a,b). 

Figures 7 — 9 Carbide structure. Note the influence of cooling rate on carbide coarseness, (compare figure 12).

Sintered silicon carbides are formed by all the traditional methods as well as standard plastic forming techniques such as injection molding. These compacts are sintered with small amounts of additives at very high temperatures in inert atmospheres to form essentially a single phase silicon carbide structure. 

The metal forms a stable carbide, V4C3, and is used to refine the grain and carbide structure of steel and to improve its hardness at elevated temperatures. Amongst its numerous applications are incorporation in high-speed tool steels and exhaust-valve springs. 

The jj-phases have the general formula Tl4T22C or Tl3T23C, where T1 and T2 are transition metals. There are carbides, nitrides, and oxides with the -carbide structure. Ward " has summarized the structure of these phases, which are characterized by TeC octahedra connected by common faces. 

Structures of transition-metal borocarbides have been reviewed, in particular those of yttrium, i.e. YB2C2, YBgC, YBC, and YB0.6C, in a general article on carbide structures. ... 

Nakagomi, S., Tobias, R, Baranzahi, A., Lundstrom, I., Martensson, P. and Lloyd Spetz, A. (1997), Influence of carbon monoxide, water and oxygen on high temperature catalytic metal-oxide silicon carbide structures. Sensors and Actuators 5,45,183-91. 

A covalent model has been provided to explain the limiting range of carbon composition and the possible structure of the R3SnC t compounds (McColm et al. 1971). The perfect perovskite carbide structure would have a carbon atom in the octahedral site at the center of the unit cell. However, a random distribution of lanthanide and tin atoms occurs over all the fee sites. In this situation, a probability... 

Tungsten carbide has a low resistance to oxidation. It is the result of low protective properties of tungsten oxides. Dissolution of oxygen in the carbide structure starts at low temperatures, and WO2 and WO3 form upon oxidation at 500°C and higher. The oxidation rate increases drastically at 800°C due to vaporization ofW03 [108]. 

The basic element of the silicon carbide structure is the tetrahedron [17] due to sp hybridization of the atomic orbitals. This tetrahedron consists of a silicon or a carbon atom at the spatial center, surrounded by four atoms of the other kind. The SiC- bond is 88% covalent. The tetrahedra are arranged in such a way that units of three silicon and three carbon atoms form angled hexagons which are arranged in parallel layers as shown in Fig. 4. 

Tungsten carbide (WC), the hard phase, together with cobalt (Co), the binder phase, forms the basic cemented carbide structure from which other types of cemented carbide have been developed. In addition to the straight tungsten carbide - cobalt compositions - cemented carbide may contain varying proportions of titanium carbide (TiC), tantalum carbide (TaC), and niobium carbide (NbC). These carbides are mutually soluble and can also dissolve a high proportion of tungsten carbide. Also, cemented carbides are produced which have the cobalt binder phase alloyed with, or completely replaced by, other metals such as iron (Fe), chromium (Cr), nickel (Ni), molybdenum (Mo), or alloys of these elements. 

Carbon steels are especially susceptible to hydrogen cracking when heat-treated to form martensite, but are less so if the structure is pearlitic. Carbon steel heat-treated to form a spheroidized carbide structure is less susceptible than pearlite, bainite, or martensite [56]. Austenitic steels— for example, 18-8 and 14% Mn steel (face-centered cubic), in which hydrogen is more soluble than in ferrite and the diffusion rate is lower—are immune under most conditions of exposure [57]. 

Copies of the report entitled "Characterization of Sintered Silicon Nitride and Silicon Carbide Structural Ceramics." prepared by Mr. M. E. Hatcher of KemaNord IndustrlkemI In Sweden and printed at ORNL, were distributed to all participants on December 8. 

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