Intermetallic systems

Since all known QC systems, with e/a of about 1.75-2.20 [25], lie close to the approximate border between the Hume-Rothery and polar intermetallic phase regions, a reasonable starting place for development of new QC/AC systems is to study selected polar intermetallic systems with nearby e/a values. Synthetic explorations of such polar intermetallics have been significant only in the past few decades [42,45], Knowledge and insights developed about the diverse interplays between composition-structure-electronic structure-physical properties for these phases were expected to be a considerable aid to the discovery of novel QC/ACs. 

All the phase diagrams reported above show a complete mutual solubility in the liquid state. The formation of a single phase in the liquid state corresponds to behaviour frequently observed in intermetallic (binary and complex) systems. Examples, however, of a degree of immiscibility in the liquid state are also found in selected intermetallic systems. Fig. 2.16 shows a few binary systems in which such immiscibility can be observed (existence of miscibility gaps in the liquid state). All the three... 

Figure 2.17. Liquid-liquid and solid-gas equilibria in intermetallic systems. In a map based on the so-called Mendeleev number coordinates the different binary combinations are represented. Only those combinations have been coded for which the existence of liquid miscibility gaps (or of solid-gas equilibria) is known. (In the same systems, other equilibria, the formation of compounds, etc. may be present). For many systems data are lacking probably in the bottom-left comer of the figure many more boxes could be added to those representing miscibility gap. Notice that the solid-gas equilibria are relevant to systems formed by metals with a large difference between their boiling temperatures.

Stability maps and correlation diagrams. As a concluding remark to some topics considered in this chapter and a recapitulation of procedures often employed in the description and classification of intermetallic systems, a little additional information about correlation diagrams is included here. 

Historical Development of Interstitial Hydrides in Other Intermetallic Systems... 

Finally, new catalytic processes (e.g., multi-step treatments, new supports, and intermetallic systems) which avoid the metallic contamination (mainly by V, Ni, Fe, Ti and the alkalis) must be explored, just as non-catalytic processes should not be neglected in order to prevent shortage of catalyst from limiting progress. 

In contrast to the simple additive volume behavior of Au—Sn, there are other intermetallic systems in which the molecular volumes do not correspond to the sum of the atomic volumes characteristic of the constituent elements. Dilute Sb in Ni (31) is a case in point the Sb sites exhibit substantial core level shifts, apparently a concomitant of substantial charge flow. Only small shifts are observed at Ni sites, in part because the solution alloys are quite rich in Ni but also because of the valence d level pinning discussed early in this Section IV. Non-additivity of volume in such systems may indeed be a manifestation of charging. 

Because intermetallic systems undoubtedly display certain special features that follow from their metallic binding forces, considerable importance attached to the growing evidence that the chalcogenides, the essentially ionic oxides, the nitrides, and other representative binary compounds of the transition metals were, not infrequently, both variable and irrational in composition. Schenck and Ding-mann s equilibrium study of the iron-oxygen system (39) was notable in this connection They showed that stoichiometric ferrous oxide, FeOi 000, the oxide of an important and typical valence state, did not exist. It lay outside the broad existence field of a nonstoichiometric phase. It is, perhaps, still not certain... 

QuasicrystaUine phases form at compositions close to the related crystalline phases. When solidified, the resultant strucmre has icosahedra threaded by a network of wedge disclinations, having resisted reconstruction into crystalline units with three-dimensional translational periodicity. The most well-known examples of quasicrystals are inorganic phases from the ternary intermetallic systems Al-Li-Cu, Al-Pd-Mn, Zn-Mg-Ln, Al-Ni-Co, Al-Cu-Co, and Al-Mn-Pd. In 2007, certain blends of polyisoprene, polystyrene, and poly(2-vinylpyridine) were found to form star-shaped copolymers that assemble into the first known organic quasicrystals (Hayashida et al., 2007). 

Results for titanium-boron and several intermetallic systems (Ni-Al, Ti-Al, Ni-Ti, Co-Ti) are presented in Fig. 49. In all cases, the measured combustion tern-... 

Experimental data for the combustion velocity dependence on initial temperature for different systems are presented in Fig. 52. Examples of intermetallic systems (Al-Ni and Co-Ti) with melting of both reactants show a general trend in which U increases with increasing Tq. The same behavior was obtained for the Mo-Si (Kumar et ai, 1988b), Ti-C (Kirdyashkin et al., 1981) and 3Ni-Al (Lebrat and Varma, 1992a) systems, which are characterized by melting of only one reactant. 

Fto. 52. Combustion velocity for intermetallic systems as a function of initial temperature. 

Anti-phase domain boundaries, are three-dimensional mistakes typical of several intermetallic systems undergoing disorder/order transformations. APBs also act on the apparent domain size, but selection rules are totally different... 

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