Elemental Phase Diagrams

This section has attempted an overview of the key conceptual features involved in computing elemental phase diagrams on the basis of microscopic insights. The... 

Oxygen, a halogen, and a main group element. Phase diagrams for LaOI-AI(A = Na Rb) systems determined by Tupoleva et al. (1988) by DTA and X-ray diffraction suggest formation of the peritectic phases A2LaOl3, A = Na-Rb, and Rb2La303l5. However, structural data were not presented. 

Young D A 1991 Phase Diagrams of the Elements (Los Angeies University of Caiifornia Press)... 

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. 

E. A. Brandes and R. E. Flint, Manganese Phase Diagrams, The Manganese Centre, Paris, 1980 L. B. Pankratz, Thermodynamic Properties of Elements and Oxides, Bull. 672, U.S. Bureau of Mines, Washington, D.C., 1982. 

Heat Treatment of Steel. Steels are alloys having up to about 2% carbon in iron plus other alloying elements. The vast application of steels is mainly owing to their ability to be heat treated to produce a wide spectmm of properties. This occurs because of a crystallographic or aHotropic transformation which takes place upon quenching. This transformation and its role in heat treatment can be explained by the crystal stmcture of iron and by the appropriate phase diagram for steels (see Steel). 

Fig. 6. Effect of alloying elements on the phase diagram of titanium (a) a-stabilized system, (b) P-isomorphous system, and (c) P-eutectoid system.

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. 

Young, D.A., Phase Diagrams of the Elements, University of California Press, Berkeley, CA, 1991. 

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. 

Prove this result to yourself. Notice that, for the first time in these notes, we have not given a parallel result for atomic (or mol) fraction. You cannot have an atom fraction of a phase because a phase (as distinct from a pure element, or a chemical compound of a speeifie composition) does not have an atomic weight. Its composition can vary within the limits given by the phase diagram. 

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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