Eutectic phase diagram

Figure 6-34. Generalized schematic Cr-X phase diagram (at.%) for X = Nb, Ta, Zr, and Hf. Eutectic reactions exist for producing in-situ composites based on Cr s or X,s (ss = solid solution) phases and the XCr2 Laves phase. Note that the specific details of the phase diagrams (eutectic compositions, eutectic temperatures, solid solution ranges, etc., vary depending on X).

The lead—copper phase diagram (1) is shown in Figure 9. Copper is an alloying element as well as an impurity in lead. The lead—copper system has a eutectic point at 0.06% copper and 326°C. In lead refining, the copper content can thus be reduced to about 0.08% merely by cooling. Further refining requites chemical treatment. The solubiUty of copper in lead decreases to about 0.005% at 0°C. 

Binary Alloys. Aluminum-rich binary phase diagrams show tliree types of reaction between liquid alloy, aluminum solid solution, and otlier phases eutectic, peritectic, and monotectic. Table 16 gives representative data for reactions in tlie systems Al—Al. Diagrams are shown in Figures 10—19. Compilations of phase diagrams may be found in reference 41. 

Tellurium Sulfide. In the hquid state, teUurium is completely miscible with sulfur. The Te—S phase diagram shows a eutectic at 105—110°C when the sulfur content is 98—99 atom % (94—98 wt %). TeUurium—sulfur aUoys have semiconductor properties (see Semiconductors). Bands attributed to teUurium sulfide [16608-21 -2] TeS, molecules have been observed. 

FIG. 22-2 Simple eutectic-phase diagram at constant pressure. (Zief and Wilcox, Fractional Solidification, i>c/. 1, Marcel Dekker, New York, 1967, p. 24.)... 

The distribution-coefficient concept is commonly applied to fractional solidification of eutectic systems in the ultrapure portion of the phase diagram. If the quantity of impurity entrapped in the solid phase for whatever reason is proportional to that contained in the melt, then assumption of a constant k is valid. It should be noted that the theoretical yield of a component exhibiting binary eutectic behavior is fixed by the feed composition and position of the eutectic. Also, in contrast to the case of a solid solution, only one component can be obtained in a pure form. 

There are many types of phase diagrams in addition to the two cases presented here these are summarized in detail by Zief and Wilcox (op. cit., p. 21). Solid-liquid phase equilibria must be determined experimentally for most binaiy and multicomponent systems. Predictive methods are based mostly on ideal phase behavior and have limited accuracy near eutectics. A predic tive technique based on extracting liquid-phase activity coefficients from vapor-liquid equilib-... 

These data underline the phase diagrams for tire pairs of elements which are in contact in microchips, where Pd-Si and Pt-Si form stable inter-metallic compounds, and An-Si forms a eutectic system. 

The copper-antimony phase diagram contains two eutectic reactions and one eutectoid reaction. For each reaction ... 

The cloudiness of ordinary ice cubes is caused by thousands of tiny air bubbles. Air dissolves in water, and tap water at 10°C can - and usually does - contain 0.0030 wt% of air. In order to follow what this air does when we make an ice cube, we need to look at the phase diagram for the HjO-air system (Fig. 4.9). As we cool our liquid solution of water -i- air the first change takes place at about -0.002°C when the composition line hits the liquidus line. At this temperature ice crystals will begin to form and, as the temperature is lowered still further, they will grow. By the time we reach the eutectic three-phase horizontal at -0.0024°C we will have 20 wt% ice (called primary ice) in our two-phase mixture, leaving 80 wt% liquid (Fig. 4.9). This liquid will contain the maximum possible amount of dissolved air (0.0038 wt%). As latent heat of freezing is removed at -0.0024°C the three-phase eutectic reaction of... 

Carbon steels as received "off the shelf" have been worked at high temperature (usually by rolling) and have then been cooled slowly to room temperature ("normalised"). The room-temperature microstructure should then be close to equilibrium and can be inferred from the Fe-C phase diagram (Fig. 11.1) which we have already come across in the Phase Diagrams course (p. 342). Table 11.1 lists the phases in the Fe-FejC system and Table 11.2 gives details of the composite eutectoid and eutectic structures that occur during slow cooling. 

Take the silica-alumina system as an example. It is convenient to treat the components as the two pure oxides SiOj and AI2O3 (instead of the three elements Si, A1 and O). Then the phase diagram is particularly simple, as shown in Fig. 16.6. There is a compound, mullite, with the composition (Si02)2 (Al203)3, which is slightly more stable than the simple solid solution, so the alloys break up into mixtures of mullite and alumina, or mullite and silica. The phase diagram has two eutectics, but is otherwise straightforward. 

Teaching yourself phase diagrams, part 3 eutectics, eutectoids and peritectics... 

Eutectics and eutectoids are important. They are common in engineering alloys, and allow the production of special, strong, microstructures. Peritectics are less important. But you should know what they are and what they look like, to avoid confusing them with other features of phase diagrams. 

The Pb-Sn system has a eutectic. Look at the Pb-Sn phase diagram (Fig. AT. 26). Above i27°C., liquid lead and liquid tin are completely miscible, that is, the one dissolves in the other completely. On cooling, solid first starts to appear when the lines (or boundaries) which limit the bottom of the liquid field are reached. 

Figure A1.27 shows the unusual silver-strontium phase diagram. It has four inter-metallic compounds. Note that it is just five simple phase diagrams, like the Pb-Sn diagram, stuck together. The first is the Ag-SrAgj diagram, the second is the SrAgj-Sr.Agj diagram, and so on. Each has a eutectic. You can always dissect complicated diagrams in this way.

The three phase diagrams, or parts of diagrams, shown in Fig. A1.28, all have a eutectic point. Mark the point with an arrow and list the eutectic temperature and composition in wt% (the co-ordinates of the point). 

Not all alloys in the lead-tin system show a eutectic pure lead, for example, does not. Examine the Pb-Sn phase diagram and list the composition range for which a eutectic reaction is possible. 

The aluminium casting alloys are mostly based on the Al-Si system (phase diagram Fig. A1.31). It is a classic eutectic system, with a eutectic point at about 11% Si and... 

Attachment of a hot or cold stage to the ordinary microscope stage allows the specimen to be observed while the temperature is changed slowly, rapidly, or held constant somewhere other than ambient. This technique is used to determine melting and freezing points, but is especially useful for the study of polymorphs, the determination of eutectics, and the preparation of phase diagrams. 

Eutectic growth is a special mode of solidification for a two-component system. Operating near a specific point in the phase diagram, it shows some unique features [121,137]. 

The phase-diagram (temperature vs concentration) for a eutectic two-component alloy shows at low temperatures a central two-phase region and two solid one-phase regions at low and high relative concentrations. At the eutectic temperature the liquid phase at an intermediate concentration can all of a sudden coexist with the two solid phases. Upon further increase of temperature, the liquidus lines open up a V-shaped liquid... 

The curious phase relations between phosphorus, sulfur and their binai compounds are worth noting. Because both P4 and Sg are stable molecules the phase diagram, if studied below 100°, shows only solid solutions with a simple eutectic at 10° (75 atom % P). By contrast, when the mixtures are heated above 200° the elements react and an entirely different phase diagram is obtained however, as only the most stable compounds P4S3, P4S7 and P4S10... 

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