Binary alloys structural disorder

Phase separation in binary alloys, polymer blends, and fluid mixtures has been studied intensively for many years [148]. It takes place when a two-component mixture is quenched from a disordered state into a two-phase coexistence region. After such a quench, composition fluctuations are created that grow and form domain structures. In the late stages, the domain structure coarsens in time, a process that is driven by the interfacial tension coexisting phases. The growth of the characteristic length scale R t) follows a power-law behavior,... 

For definiteness we will examine the expansion in relation to two physical systems (1) the CuZn (jS-brass) binary alloy and (2) a ferro-magnet such as iron. It is well known that CuZn has a second-order, order-disorder transition at Tc = 742 K. The crystal structure at 0 K can be described by two interpenetrating simple cubic lattices each with No sites. Let us suppose that lattice 1 is occupied by Cu atoms and lattice 2 by Zn atoms. As the temperature is raised above... 

Fig. 17.12) the global shape and extent of the function envelope are related to the size and shape of the objects (the coherence length is not, however, a true size measurement since twinning or disorder also terminate the RDF) first peak(s) location is a direct measurement of bond-length in the material finally, the complete set of distances is a signature of the order inside the objeets and ean immediately be compared with reference structures. Metallic nanoparticles (single metal or binary alloys) and nanowires have been successfully studied by this method which also applies to well-crystallized oxides. 

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. 

From either hydrogenated binary Mg3lr alloy or by sintering powder mixtures of the elements at up to 773 K under a H2 pressure of up to 150 bar. Red colored hydride partially disordered structure (Figure 45) from npd on deuteride, P2 /c, Z = 8 contains four symmetry independent Ir hydride complexes a spy [IrHs]" " and three sad [lrFl4] complexes of which two are disordered, and five hydride anions are exclusively bonded to Mg all complexes are surrounded by Mg in distorted cubic... 

These results indicate that the FCD-EMTO-CPA method is a reliable method for electronic structure calculations, even in the case of systems with both configurational and magnetic disorder. I will now present a slightly larger case study of the binary MoRu alloy system, which I have been working on for some time. 

NiAl can be alloyed with further elements in order to form ternary phases with a B2 structure which is then known as an L2o structure, too, or to obtain additional phases in equilibrium with NiAl. Fe, as well as Co, can substitute for Ni in NiAl completely without affecting the B2 structure, as is expected in view of the binary B2 phases FeAl and CoAl. Correspondingly, the ternary Ni-Fe-Al phase diagram, which is of importance with respect to conventional high temperature alloys, shows the extended B2 phase field and the respective equilibria with NijAl and Al-rich phases on the one hand and with disordered b.c.c. Fe and f.c.c. Fe on the other (Bradley and Taylor, 1938 Dannohl, 1942 Bradley, 1951 Hao et al., 1984). The Ni-Al-Co system behaves in an analogous way (Hao et al., 1984 Ishida et al., 1991 a, 1993). Other ternary systems have been re-... 

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