Metals body-centered cubic

Vanadium [7440-62-2] V, (at. no. 23, at. wt 50.942) is a member of Group 5 (VB) of the Periodic Table. It is a gray body-centered-cubic metal in the first transition series (electronic configuration When highly pure, it is very soft and dutile. Because of its high melting point, vanadium is referred to as a... 

Draw the simple (111), (100), and (110) surfaces of face-centered cubic metals, the (110) and (100) surfaces of body-centered cubic metals, and the (001) surface of an hexagonally-dosed packed surface. 

Figure 17 contains three chromium spectra of type III, the low amplitude spectra characteristic of metals, and one of type I. Chromium, a body-centered cubic metal, produces extended fine structure. 

The transition metal carbides do have a notable drawback relative to engineering applications low ductility at room temperature. Below 1070 K, these materials fail in a brittle manner, while above this temperature they become ductile and deform plastically on multiple slip systems much like fee (face-centered-cubic) metals. This transition from brittle to ductile behavior is analogous to that of bee (body-centered-cubic) metals such as iron, and arises from the combination of the bee metals strongly temperature-dependent yield stress (oy) and relatively temperature-insensitive fracture stress.1 Brittle fracture is promoted below the ductile-to-brittle transition temperature because the stress required to fracture is lower than that required to move dislocations, oy. The opposite is true, however, above the transition temperature. 

Metallic membranes for hydrogen separation can be of many types, such as pure metals Pd, V, Ta, Nb, and Ti binary alloys of Pd, with Cu, Ag, and Y Pd alloyed with Ni, Au, Ce, and Fe and complex alloys of Pd alloyed with more than one metal [3], Body-centered cubic metals, for example, Nb and V, have higher permeability than face-centered cubic metals, for instance, Pd and Ni [26-29], Even though Nb, V, and Ta possess a permeability greater than that of Pd, these metals develop oxide layers and are complicated to be used as hydrogen separation membranes [29], Especially, the Pd and Pd-based membranes have in recent times obtained renovated consideration on account of the prospects of a generalized use of hydrogen as a fuel in the future [26], We emphasize on these types of membranes in this chapter. 

Body-centered cubic metals contain no close-packed planes, but do contain four close-packed directions, the four [111] body diagonals of the cube. The most nearly close-packed planes are those of the 1 10 set. In BCC crystals, slip has been observed in the [1 I 1] directions on the [1 10], [1 12], and 12 3 planes, but that, attributed to the latter two planes, may be considered the resultant of slip on several different (1 1 0) type planes (Weertman and Weertman, 1992). The von Mises criterion is satisfied, but higher shearing stresses than those of CCP metals are normally requited to cause slip in BCC metals. As a result, most BCC metals are classified as semibrittle. 

Obtaining from Andersen and Jepsen is not quite so unambiguous. They list the energy difference between the center of the d band and the center of the s band, but is defined as the difference between the center of the d band and the bottom of the, v band. We must therefore add half the, s-band width, proportional to // /(m,ro). The i, values from Andersen and Jepsen (1977) have been used and the coefficient has been chosen to be 1.08, which is the average of the three values fitted to obtain the E, values for the three body-centered cubic metals listed in Table 20-3. This gives the values listed in Table 20-4. To obtain the values listed in the Solid State Table, E, has been written to equal h k]l2m, using the m, values... 

Kovenskii, Diffusion in Body-Centered Cubic Metals, (1965) A.S.M., Metals Park, Novelty, Ohio, p. 283. 

Polarization curves for iron, chromium, and alloys with 1, 6, 10, and 14 weight percent (wt%) chromium in iron are shown in Fig. 5.24 the environment is 1 N H2SO4 at 25 °C (Ref 21). Iron and chromium are body-centered-cubic metals, and the alloys are solid solutions having this structure. The passivation potential (Epp), the active peak current density (icrit), and the passive state current density (ip) are decreased... 

The experimentally determined surface self-diffusion constants for face-centered cubic and body-centered cubic metals are plotted in Figures 4.12 and 4.13 as a function of 7 ,/7 , where T, is the absolute melting temperature. For face-centered cubic metals this dependence is approximated by two functions ... 

Figure 4.13. Surface self-diffusion constants for body-centered cubic metals as a function of T ,/T, where T , is the melting temperature [96],...

Determine the packing efficiency by calculating the fraction of space occupied by atoms in a body-centered cubic metal. 

This equation is usually used for the determination of the surface diffusivity A from the measurements of t and X Y - Determination of the activation energy is made in the usual manner using Eq. 6.23. For many cubic metals (Gjostein 1967), is of the order of 40 kcal/mole, and the sum of (A d + AE/) can often be approximated at high temperatures by 0.54 A A, where A A is the heat of sublimation. The correlations for surface self difTusion coefficients are shown in Figures 6.3 and 6.4 for face-centered cubic metals and body-centered cubic metals, respectively. These correlations are for the diffusion of metal on the same metal surface. The neighboring site distance is of the order of a few angstrom, and Vo is of the order of 10 sec". ... 

Figure 6.4 Correlations between surface self-diffusion coefficient and reduced temperature for body-centered-cubic metals. (From Principles of Surface Chemistry, Somorjai 1972.)...

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