Substituted systems phase diagram

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

Phase diagrams from freezing point depressions show true compound formations for simpler amides—e.g., water-N-methylacetamide forms a compound at a mole ratio of 2 to 1, water-N,N-dimethylacetamide at 3 to 2 and 3 to 1, and water-N-methylpyrrolidone at 2 to 1. The heats of mixing and heat capacities at 25°C. of a number of water-amide systems were determined. All mixing curves were exothermic and possess maxima at definite mole ratios, while the heat capacities for the most part show distinct curvature changes at the characteristic mole ratios. Both experimental results point to the stability of the particular complexes even at room temperature. This is further supported by absolute viscosity studies over the whole concentration range where large maxima occur at these same mole ratios for disubsti-tuted amides and N-substituted pyrrolidones. 

Assume, for example, that two metals A and B are completely soluble in the solid state, as illustrated by the phase diagram of Fig. 12-1. The solid phase a, called a continuous solid solution, is of the substitutional type it varies in composition, but not in crystal structure, from pure A to pure B, which must necessarily have the same structure. The lattice parameter of a also varies continuously from that of pure A to that of pure B. Since all alloys in a system of this kind consist of the same single phase, their powder patterns appear quite similar, the only effect of a change in composition being to shift the diffraction-line positions in accordance with the change in lattice parameter. 

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. 

One objective of this section is to qualitatively describe the relationship between these various outcomes and the resulting phase diagrams. First, however, it is important to appreciate what is meant by a solid solution in a ceramic system and the types of solid solutions that occur — a topic that was dealt with indirectly and briefly in Chap. 6. The two main types of solid solutions, described below, are substitutional and interstitial. 

If the hydrocarbon in such a system is replaced by a compound in whose hydrocarbon chain is substituted a group with weak hydrophilic properties, such as an aldehyde or ester group, we obtain equilibrium diagrams of a slightly different type. Figure 26 shows the phase diagram... 

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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