Ceramic alloys

Название книги: Engineering Materials 2 An Introduction to Microstructures, Processing and Design .  Pagina 173

Автор: F. Ashby, Michael  -

Ключевые слова: aleaciones, ceramicas, diagrama de fases

Ceramics form alloys with each other, just as metals do. But the reasons for alloying are quite different: in metals it is usually to increase the yield strength, fatigue strength or corrosion resistance; in ceramics it is generally to allow sintering to full density, or to improve the fracture toughness. But for the moment this is irrelevant; the point here is that one deals with ceramic alloys just as one did with metallic alloys. Molten oxides, for the most part, have large solubilities for other oxides (that is why they make good fluxes, dissolving undesirable impurities into a harmless slag). On cooling, they solidify as one or more phases: solid solutions or new compounds. Just as for metals, the constitution of a ceramic alloy is described by the appropriate phase diagram. Take the silica–alumina system as an example. It is convenient to treat the components as the two pure oxides SiO2 and Al2O3 (instead of the three elements Si, Al and O). Then the phase diagram is particularly simple, as shown in Fig. 16.6. There is a compound, mullite, with the composition (SiO2)2 (Al2O3)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. The phase diagram for MgO and Al2O3 is similar, with a central compound, spinel, with the composition MgOAl2O3. That for MgO and SiO2, too, is simple, with a compound, forsterite, having the composition (MgO)2 SiO2. Given the composition, the equilibrium constitution of the alloy is read off the diagram in exactly the way described in Chapter 3.