Phase diagrams homogeneity range

Figure 3.19 Carbon-titanium phase diagram. Homogeneity range shown in shaded section.1 1...

In Fig. 2.19, on the contrary, we observe that intermediate solid phases with a variable composition are formed (non-stoichiometric phases). In the diagrams shown here we see therefore examples both of terminal and intermediate phases. (For instance, the Hf-Ru diagram shows the terminal solid solutions of Ru in a and (3Hf and of Hf in Ru and the intermediate compound containing about 50 at.% Ru). These phases are characterized by homogeneity ranges (solid solubility ranges), which, in the case of the terminal phases, include the pure components and which, generally, have a variable temperature-dependent extension. 

Several phase diagrams of binary alloy systems have been shown (see for instance Fig. 2.18) in which the existence of intermediate phases may be noticed. In these systems we have seen the formation of AmB phases, which generally crystallize with structures other than those of the constituent elements, and which have negligible homogeneity ranges. Thermodynamically, the composition of any such phase is variable. In a number of cases, as those exemplified in Fig. 2.19, the possible variation in composition is very small (invariant composition phases or... 

As for the thermodynamic modelling of Laves phases, this has been successfully performed for several alloy systems. Especially using the sublattice model (see 2.4.2.2) a number of phase diagrams containing Laves phases have been calculated taking into account homogeneity ranges and polytypism. 

Figure 12-8. Schematic A-B phase diagram with compound A, rfBn which exhibits a narrow range of homogeneity (e.g., Ag-S).

Revised proposals for the phase diagrams of the Group 5 transition metal-carbon systems are given in Figure 4.3(a-c). In our opinion they are preferable with respect to the composition and decomposition temperature of the t, phases (within the limits given in Table 4.1) as well as the homogeneity ranges of the fi and 6 phases. 

Figure 4.7(a,b) shows EPMA scans across phase bands shown in Figure 4.10(a,b) which reveal the homogeneity ranges of the occurring phases. The phase diagram of the Ta-N system was corrected for the present findings (Figure 4.8). These are in excellent agreement with published data.10...

As early as 1943, Sommer (101) reported the existence of a stoichiometric compound CsAu, exhibiting nonmetallic properties. Later reports (53, 102, 103,123) confirmed its existence and described the crystal structure, as well as the electrical and optical properties of this compound. The lattice constant of its CsCl-type structure is reported (103) to be 4.263 0.001 A. Band structure calculations are consistent with observed experimental results that the material is a semiconductor with a band gap of 2.6 eV (102). The phase diagram of the Cs-Au system shows the existence of a discrete CsAu phase (32) of melting point 590°C and a very narrow range of homogeneity (42). 

Firsly, the concentration dependence of the diffusion coefficient can be neglected. Secondly, the concentration of components at the interfaces of any growing layer can be assumed to be equal to the limits of the homogeneity range of a compound according to the equilibrium phase diagram of the A-B binary system. Thirdly, the concentration distribution of the components across a compound layer at any moment of time can reasonably be assumed to be close to linear (Fig. 1.22a), so that... 

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