Solvus diagrams

In aging, the alloy is heated below the solvus to permit precipitation of fine particles of a second phase 9 (CuAl ). The solvus represents the boundary on a phase diagram between the soHd-solution region and a region consisting of a second phase in addition to the soHd solution. 

Figure 10.48 TTT diagrams for U720 and U720LI based on calculated

The temperature range of formation of the various carbides and the solvus temperatures of 6 are well established (Ferrer et al. 1991, Vemot-Loier and Cortial 1991) and the calculated diagram is in excellent agreement with this experimental information. However, during processing it is usually the 7" phase which is formed instead of 6. This is due to much faster transformation kinetics of this phase and hardening Ni,Fe-based siqreralloys is usually due to a combination of 7 and 7". 

The other single phase region is the liquid L. In addition to the two phase a+0 region, there are two other two phase regions L + a and L + 0. Just as in the isomorphous diagram the solidus and liquidus lines are connected by tie-lines of constant temperature. In a like manner, the a + 0 region is also considered to possess tie-lines joining the two solid-solution or solvus curves. 

The Cu-Zn system (see Figure 2.7) displays a number of intermediate solid solutions that arise due to limited solubility between the two elements. For example, at low wt% Zn, which incidently is the composition of alloys known as brass, the relatively pure copper a phase is able to accommodate small amounts of Zn as an impurity in the crystal structure. This is known as a terminal solid phase, and the solubility limit where intermediate solid solutions (such as a + /S) begin to occur is called the solvus line. Some of the three-phase transformations that are found in this diagram include a peritectic (5 - - L -> e) and a eutectoid (5 -> y - - e). Remember that these three-phase transformations are defined for equilibrium coohng processes, not heating or nonequihbrium conditions. 

In many cases, there is partial solid solubility between the pure components of a binary system, as in the Pb-Sn phase diagram of Figure 11.5, for example. The solubility limits of one component in the other are given by solvus lines. Note that the solid solubility limits are not reciprocal. Lead will dissolve up to 18.3 percent Sn, but Sn will dissolve only up to 2.2 percent Pb. In Figure 11.5, there are two two-phase fields. Each is bounded by a distinct solvus and liquidus line, and the common sofidus line. One two-phase field consists of a mixmre of eutectic crystals and crystals containing Sn solute dissolved in Pb solvent. The other two-phase field consists of a mixture of eutectic crystals and crystals containing Pb solute dissolved in Sn solvent. 

When the stable boundaries of an equilibrium phase diagram are extended as, for example, in Figure 11.14, regions of metastability are shown. In eutectic systems (Fig. 11.14fl), metastable equilibria of the solvus lines usually form a liquid miscibility dome. On the other hand, as illustrated in Figure 11.14/7, metastable extensions of... 

In general, the various experimental techniques differ in sensitivity, and therefore in usefulness, from one portion of the phase diagram to another. Thus, thermal analysis is the best method for determining the liquidus and solidus, including eutectic and peritectic horizontals, but it may fail to reveal the existence of eutectoid and peritectoid horizontals because of the sluggishness of some solid-state reactions or the small heat effects involved. Such features of the diagram are best determined by microscopic examination or x-ray diffraction, and the same applies to the determination of solvus (solid solubility) curves. 

To return to the main subject of this chapter, we might now consider the methods used for determining the position of a solvus curve (solid solubility curve) on a phase diagram. Such a curve forms the boundary between a single-phase solid... 

Besides the homogeneity ranges of the silicon borides mainly phase diagram data concerning the liquidus and the silicon solvus were published. The eutectic temperature between SiBg and silicon was calculated to 1658 K [36] (1657 K in the Scheil schemes) in good agreement with measurements with experimental data [52, 56, 61]. 

Fig. 10 Thermodynamic and kinetic basis for solute depletion in the case of a binary alloy consisting of solvent A and solute B. (a) Binary equilibrium phase diagram with complete miscibility in the liquid state, partial miscibility in the solid state given by existence of a terminal solid solution. Cs is the composition along the solvus line. is the overall composition of the alloy, (b) Time-temperature-transformation diagram for precipitation of in an a matrix for the alloy shown in (a) with overall composition,...

FIGURE 8.2 Patametric method for detetmination of the solvus lines in a binary phase diagram. 

The series provides consistent phase diagram deseriptions for individual ternary systems. The representation of the equihbria of ternary systems as a function of temperature results in spaeial diagrams whose sections and projections arc generally published in the literature. Phase equilibria are deseribed in terms of hquidus, soUdus and solvus projections, isothermal and quasibinary sections data on invariant equilibria are generally given in the form of tables. 

Figure 17.21 The two basic binary diagram elements. In the phase transition loop (left diagram) solution 1 and solution 2 can be solid and solid, solid and liquid, or liquid and vapor, respectively. and are melting temperatures, boiling temperatures, or polymorphic phase transition temperatures for pure A and B respectively. Three representative tie-lines are shown. In the solvus (right diagram), solution 1 and solution 2 can be two solids or two liquids. Two representative tie-lines are shown.

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