Solid intermetallic phases structures

Another characteristic point is the special attention that in intermetallic science, as in several fields of chemistry, needs to be dedicated to the structural aspects and to the description of the phases. The structure of intermetallic alloys in their different states, liquid, amorphous (glassy), quasi-crystalline and fully, three-dimensionally (3D) periodic crystalline are closely related to the different properties shown by these substances. Two chapters are therefore dedicated to selected aspects of intermetallic structural chemistry. Particular attention is dedicated to the solid state, in which a very large variety of properties and structures can be found. Solid intermetallic phases, generally non-molecular by nature, are characterized by their 3D crystal (or quasicrystal) structure. A great many crystal structures (often complex or very complex) have been elucidated, and intermetallic crystallochemistry is a fundamental topic of reference. A great number of papers have been published containing results obtained by powder and single crystal X-ray diffractometry and by neutron and electron diffraction methods. A characteristic nomenclature and several symbols and representations have been developed for the description, classification and identification of these phases. 

The situation in the solid state is generally more complex. Several examples of binary systems were seen in which, in the solid state, a number of phases (intermediate and terminal) are formed. See for instance Figs 2.18-2.21. Both stoichiometric phases (compounds) and variable composition phases (solid solutions) may be considered and, as for their structures, both fully ordered or more or less completely disordered phases. This variety of types is characteristic for the solid alloys. After a few comments on liquid alloys, particular attention will therefore be dedicated in the following paragraphs to the description and classification of solid intermetallic phases. 

In the previous chapter we looked at some questions concerning solid intermetallic phases both terminal (that is solubility fields which include one of the components) and intermediate. Particularly we have seen, in several alloy systems, the formation in the solid state of intermetallic compounds or, more generally, intermetallic phases. A few general and introductory remarks about these phases have been presented by means of Figs. 2.2-2.4, in which structural schemes of ordered and disordered phases have been suggested. On the other hand we have seen that in binary (and multi-component) metal systems, several crystalline phases (terminal and intermediate, stable and also metastable) may occur. 

As a starting point in the description of the solid intermetallic phases it is useful to recall that their identification and classification requires information about their chemical composition and structure. To be consistent with other fields of descriptive chemistry, this information should be included in specific chemical and structural formulae built up according to well-defined rules. This task, however, in the specific domain of the intermetallic phases, or more generally in the area of solid-state chemistry, is much more complicated than for other chemical compounds. This complexity is related both to the chemical characteristics (formation of variable composition phases) and to the structural properties, since the intermetallic compounds are generally non-molecular in nature, while the conventional chemical symbolism has been mainly developed for the representation of molecular units. As a consequence there is no complete, or generally accepted, method of representing the formulae of intermetallic compounds. 

In the previous paragraphs a brief account has been given of the fundamental aspects of the crystallographic description of the structures and structure types of solid phases. A number of symbols and names have been defined and their application to intermetallic compounds exemplified. It must, however, be underlined that both for historical reasons and for the need to improve classification and interpretation of the structural characteristics of intermetallic phases, other symbols and nomenclature criteria have been invented. Some of them have a mathematical basis, others are more colloquial. A selection of these criteria will be given in the following. 

Some aspects of the mentioned relationships have been presented in previous chapters while discussing special characteristics of the alloying behaviour. The reader is especially directed to Chapter 2 for the role played by some factors in the definition of phase equilibria aspects, such as compound formation capability, solid solution formation and their relationships with the Mendeleev Number and Pettifor and Villars maps. Stability and enthalpy of formation of alloys and Miedema s model and parameters have also been briefly commented on. In Chapter 3, mainly dedicated to the structural characteristics of the intermetallic phases, a number of comments have been reported about the effects of different factors, such as geometrical factor, atomic dimension factor, etc. on these characteristics. 

Notes on the alloy crystal chemistry of the 6th group metals. A selection of the intermetallic phases, and of their structures, formed by Cr, Mo and W is shown in Table 5.35. Attention has been given in this list to the presence of several tetrahedrally close-packed alloys, often corresponding to ranges of solid solutions. 

The A2 metals and the elements of the earlier B subgroups (Bj metals) form the electron compounds already discussed. With the metals of the later B subgroups the A2 metals, like the Aj, tend to form intermetallic phases more akin to simple homopolar compounds, with structures quite different from those of the pure metals. The nickel arsenide structure has, like typical alloys, the property of taking up in solid solution a considerable excess of the transition metal. From Table 29.12... 

The structural chemistry of intermetallic phases containing the group 13 metals is extremely rich. The structure and bonding in such phases have been reviewed and discussed in R.B. King, Inorg. Chem. 1989. 28, 2796 J.K. Burdett, E. Canadell, J. Am. Chem. Soc. 1990, 112, 7217 C. Belin, M. Tillard-Charbonnel, Prog. Solid State Chem. 1993, 22, 59. 

The Cu-Au system is the classic textbook example for discussing ordering reactions in solid solutions and the effects of atomic order on properties (see, e.g., Schulze, 1967 Honeycombe, 1968). At higher temperatures above 410 °C the Cu-Au alloys form the disordered Al structure with complete mutual solid-solubility of Cu and Au, whereas at lower temperatures ordering reactions occur which produce various intermetallic phases, depending on temperature and composition, with broad... 

Figure 3a shows the microstructure of the initial parent metal the structure of d-phase inclusions (CuAh intermetallic compound-based solid solution) is shown in Fig. 3b. As a result of the welding heat effect, most of the inclusions of the intermetallic phase in the HAZ were subjected to partial local melting and were converted into clusters and interlayers of eutectics (Figs. 3c, 3d, 4a, and 4b), while others changed shape only slightly. This is probably explained by the fact that in the condition of nonequilibrium primary solidification, they had a composition considerably different from that prescribed by the equilibrium diagram [%...

Separate intermetallic inclusions were absent in the weld metal structure. In the fusion zone, the eutectic consisted of isolated clusters and interlayers along the grain boundaries (Figs. 4a and 4b) in the central areas of the weld, interlayers predominated (Figs. 4c and 4d). The structure of the eutectics formed in the HAZ, both within the fusion zone and in the weld, was practically the same. The areas of the basic solid solution appear to be homogeneous, while the intermetallic phase, as in the initial material, is distinguished by its own substructure (Figs. 3b, 4b, and 4d). The nature of the uniformly distributed spherical formations in the structure of the intermetallic inclusions is not known. 

The amount of Mn and additions of Fe and Si in solid solution for both alloys was almost the same. The inclusions of the intermetallic phase in the initial base metal and in the HAZ (arrows 3-5, 12, and 13) contained 38.0 to 43.3% Cu. The possibility of the phase existing with a wide range of Cu concentration under conditions of nonequilibrium formation of structure was confirmed. In this case, some inclusions with lower Cu content may have been partially preserved in the HAZ (arrows 5 and 6) without their noticeable conversion into eutectic clusters (arrow 7). The amount of Fe, Si, and Mn in some clusters of intermetallides considerably exceeded (by one order) their total content in the normal alloy (arrows 12 and 13). In the higher-purity alloy, the cluster of Fe and Si, along some intermetallic phase boundaries, was only 2 to 3 times higher than their average concentration in the alloy (arrows 5 and 6). 

Landolt-Bornstein, Numerical Data and Functional Relationships in Science and Technology, K.-H. Hellwege (Ed.), Group III. Crystal and Solid State Physics, Vol. 6, Structure Data of Elements and Intermetallic Phases, P. Eckerlin, H. Kandler, and A. Stegher (Eds), Springer, Berlin, 1971. 

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