Alloy systems interstitial structures

More commonly, the two metals A and B are only partially soluble in the solid state. The first additions of B to A go into solid solution in the A lattice, which may expand or contract as a result, depending on the relative sizes of the A and B atoms and the type of solid solution formed (substitutional or interstitial). Ultimately the solubility limit of B in A is reached, and further additions of B cause the precipitation of a second phase. This second phase may be a B-rich solid solution with the same structure as B, as in the alloy system illustrated by Fig. 12-2(a). Here the solid solutions a and P are called primary solid solutions or terminal solid solutions. Or the second phase which appears may have no connection with the B-rich solid solution, as in the system shown in Fig. 12-2(b). Here the effect of supersaturating a with metal B is to precipitate the phase designated y. This phase is called an intermediate solid solution or intermediate phase. It usually has a crystal structure entirely different from that of either a or P, and it is separated from each of these terminal solid solutions, on the phase diagram, by at least one two-phase region. 

To these may be added a fourth class (4) the interstitial structures, which resemble alloy systems in many of their properties, although they are structures in which one of the components is a non-metallic element. 

This completes our discussion of the three principal classes into which we have divided the alloy systems. As a summary, a condensed survey of the chief structural characteristics of these classes is given in table 13.10. The remaining class of alloys to be considered is that embracing the interstitial structures. These systems, however, differ materially from the alloy systems so far discussed, and a description of them is accordingly deferred until later in the chapter ( 13 37). 

For a range of simple substitutional solid solutions to form, certain requirements must be met. First, the ions that replace each other must be isovalent. If this were not the case, other structural changes (e.g., vacancies or interstitials) would be required to maintain electroneutrality. Second, the ions that replace each other must be fairly similar in size. From a review of the experimental results on metal alloy formation, it has been suggested that 15% size difference can be tolerated for the formation of a substantial range of substitutional solid solutions. For solid solutions in nomnetal-lic systems, the limiting difference in size appears to be somewhat larger than 15%, although it is very difficult to quantify this. To a certain extent, this is because it is difficult to quantify the sizes of the ions themselves, but also because solid solution formation is very temperature dependent. 

Virtually all of the reported structural data on titanium alloy hydrides and deuterides indicate that the solute atoms occupy tetrahedral interstitial sites in the metal lattice. Neutron diffraction data obtained for deuterium in Ti/34 atom % Zr and in Ti/34 atom % Nb (17) indicate tetrahedral site occupancy in the bcc /3-phase. Similarly, data reported for deuterium in Ti/19 atom % V and in Ti/67 atom % Nb (18) indicate tetrahedral site occupancy in the fee 7-phase. Crystallographic examination of the 7-phase Ti-Nb-H system (19) reveals that increasing niobium content linearly increases the lattice parameter of the fee 7-phase for Nb contents ranging from 0 to 70.2 atom %. Vanadium, on the other hand, exerts the opposite effect (6) at H/M = 1.85, the 7-phase lattice parameter decreases with increasing vanadium contents. 

Although the distinction is not always clear, ternary nitrides (and nitrides in general) often are classified into two groups (1) intermetallic-type and (2) ionic/covalent-type. Intermetallic nitrides are those in which metal-metal (M-M) interactions are dominant and where the nitrogen atoms are interstitial within the metal array.3 Because these phases are stabilized by M-M interactions, the structure and physical properties are similar to those of many other metallic systems, such as alloys, metals, and... 

The process by which these interstitial alloys form is similar in aU systems. A reactive gas, typically hydrogen (H2) for hydrides, methane (CH4) for carbides, or ammonia (NH3) for nitrides, decomposes on the metal surface. The atoms formed can then enter the structure, to occupy sites at random. The phases formed are often called a phases. Continued reaction leads to the formation of new structures, either by the ordering of the impurity atoms, as described for substitutional alloys (Section 6.1.3.1), or by more extensive structural rearrangements, as in cementite (Fe3C). 

One material that has wide application in the systems of DOE facilities is stainless steel. There are nearly 40 standard types of stainless steel and many other specialized types under various trade names. Through the modification of the kinds and quantities of alloying elements, the steel can be adapted to specific applications. Stainless steels are classified as austenitic or ferritic based on their lattice structure. Austenitic stainless steels, including 304 and 316, have a face-centered cubic structure of iron atoms with the carbon in interstitial solid solution. Ferritic stainless steels, including type 405, have a body-centered cubic iron lattice and contain no nickel. Ferritic steels are easier to weld and fabricate and are less susceptible to stress corrosion cracking than austenitic stainless steels. They have only moderate resistance to other types of chemical attack. 

A very detailed work was published recently on CuTi and CuTij body-centered tetragonal structure alloys (Shoemaker et al., 1991), unfortunately systems with no experimental information on point-defect properties. In both compounds, the removal of a Cu or Ti atom results in a vacant Cu site, with an adjacent Ti -u antisite defect in the latter case. Interstitials have complicated structures of the crowdion type, on a Cu (111) row, which involves seven Cu for six sites (CuTi case) or five Cu for four sites (CuTi2 case) and the creation of two or three antisite defects in the respective cases of Cu or Ti displacements. 

---