Mullite composites AI2O3 phase

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. 

The effect of lime- (CaO) rich phases penetrating the brick is to render the muUite binder in the refractory as a nonequiUbrium phase, i.e., the binder is dissolved into the penetrating slag phases and corrosion products. In 70% AI2O3 brick, the mullite is stable until the local composition reaches 20% CaO, while in 90% AI2O3 brick, the mullite disappears when the local composition reaches 8% CaO. 

---