Carbon interstitial element

The octahedral voids in the MnsSi3 type structure can be filled not only by a late transition metal. As an example Guloy and Corbett (1994) have tested this possibility for LasPb3. The voids could be filled with P, S, Cl, As, Se, Sb, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ru, and Ag atoms and also partially with boron or carbon. Filling of the voids leads to a small increase of the lattice parameters. On the other hand, lead can also act as the interstitial element on the octahedral site. The structural relationships of various M5X4 structures have recently been discussed by Guloy and Corbett (2005). 

For an interstitial alloy to form, the solute atoms must have a much smaller bonding atomic radius than the solvent atoms. Typically, the interstitial element is a nonmetal that makes covalent bonds to the neighboring metal atoms. The presence of the extra bonds provided by the interstitial component causes the metal lattice to become harder, stronger, and less ductile. For example, steel, which is much harder and stronger than pure iron, is an alloy of iron that contains up to 3% carbon. Other elements may be added to form alloy steels. Vanadium and chromium may be added to impart strength, for instance, and to increase resistance to fatigue and corrosion. 

The solid solubility of a number of the non-rare earth metals (including europium and ytterbium in this group) are more extensive in the trivalent rare earths than vice versa, see fig. 14. This group includes the small size divalent metals (M = Mg, Zn, Cd and Hg), the interstitial elements (M = H, C and O), the two divalent barides (M = Eu and Yb), and the trivalent group III B metals (In and Tl, and probably Ga). Except for carbon, M is more soluble in the bcc phase than in the close-packed lower temperature polymorph. Indeed it is possible to retain the bcc phase at room temperature by modest quenching techniques in the R-Mg (Miller and Daane 1964, Herchenroeder et al. 1985) and R-Cd (Herchenroeder et al. 1985) alloys. 

The 2500°C heat-treatment causes the reordering of the structure. The basal planes coalesce and become more parallel and closer together. The various crystallite imperfections such as vacancies, stacking faults, dislocations, and rotational disorders, tend to heal and disappear the crystallite size (Lg) increases the 002 line narrows considerably and becomes close to the position of the ideal graphite line as the interlayer spacing (d) decreases to approach that of the ideal graphite crystal (0.3354 nm). This observed reduction of the interlayer spacing is attributed in part to the removal of interstitial elements, mostly carbon.P l... 

This group of stainless steels (Table 1-3) has a chromium content of 11 to 30% and, in general, a low carbon content (<0.06%). Interstitial elements such as carbon and nitrogen are very poorly soluble in ferrite and so to avoid embrittlement, even at room temperature, the carbon and nitrogen content must be kept very low, especially at higher chromium levels. 

Examination of the compositions of stainless types 409 and 439 introduce an additional approach to improving corrosion resistance. It also underscores the importance of carbon in stainless alloys. The role of carbon as an alloy addition to steel is primarily that of increasing strength. Increasing carbon content, because it is an interstitial element, pins the movement of atoms within the matrix, resulting in higher stresses required to cause deformation. This is also a factor in stainless steels, but increasing carbon content can have a deleterious effect on corrosion resistance. 

The first significant alloy developed commercially to meet these requirements contained 26% chromium and 1% molybdenum. To obtain the desired corrosion resistance and acceptable fabrication characteristics, the material had to have very low interstitial element contents. To achieve these levels, the material was electron-beam rerefined imder a vacuum. It was known as E-Brite alloy. Carbon plus nitrogen contents were maintained at levels below 0.02%. 

Corrosion reactions can occur by a simple dissolution mechanism, whereby the containment material dissolves in the melt without any impurity effects. Material dissolved in a hot zone may be redeposited in a colder area, possibly compounding the corrosion problem by additional plugging and blockages where deposition has taken place. Dissolution damage may be of a localized nature, for example, by selective dealloying. The second corrosion mechanism is one of reactions involving interstitial (or impurity) elements such as carbon or oxygen in the melt or containment material. Two further subforms are corrosion product formation and elemental transfer. In the former the liquid metal is directly involved in corrosion product formation. In the latter the liquid metal does not react directly with the containment alloy rather, interstitial elements are transferred to, from, or across the liquid. 

Recall that in contrast to substitutional alloys, where one metal atom substitutes for another in the lattice, interstitial alloys contain atoms of one kind that fit into the holes, or interstitial sites, of the crystal structure of the other. In metals with the interstitial elements hydrogen, boron, nitfogen, or carbon, the aUoy that results retains its metallic properties. 

Thermochemical diffusion heat treatments that involve the introduction of interstitial elements, such as carbon, nitrogen, or boron, into a ferrous alloy surface at elevated temperatures... 

Failure of the nozzle weld was the result of intergranular corrosion caused by the pick-up of interstitial elements and snbseqnent precipitation of chromium carbides and nitrides. Carbon pick-up was believed to have been cansed by inadequate joint cleaning prior to welding. The increase in the weld nitrogen level was a direct resnlt of inadequate argon gas shielding of the molten weld puddle. Two areas of inadequate shielding were identified ... 

In order to preserve the structural integrity and corrosion performance of the new generation of ferritic stainless steels, it is important to avoid the pickup of the interstitial elements carbon, nitrogen, oxygen, and hydrogen. In this particular case, the vendor used a flow rate intended for a smaller welding torch nozzle. 

The same situation is met in R-M-N ternary nitrides in which the nature of the M element determines the dominating type of bond involved in the material. This is illustrated by the fact that with lithium (or barium) as a cationic element, the R-M-N corresponding nitride is essentially ionic in character, whereas with silicon, more covalent nitrido-silicates are formed. In addition, metallic nitrided alloys exist, with nitrogen located as an interstitial element in octahedral voids of the metal atom lattice. The presence of insertion nitrogen (as well as carbon) in such compounds is sometimes necessary for their existence, and can strongly modify the physical properties. 

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