Equilibrium Binary Phase Diagrams of Alumina with Other Oxides
The most important binary oxide and ceramic phase diagram is the alumina-silica (Al2O3-SiO2) diagram, shown in Fig. 2, as determined by Klug . Important features in this diagram are the very low solid solubility of SiO2 in Al2O3 and Al2O3 in SiO2 and the single stable intermediate solid phase of mullite, which has the composition 3Al2O3-2SiO2; at higher temperatures, the amount of alumina in mullite increases. In contrast to binary metal systems, which usually have considerable solid solubility in the pure components and limited solubility in intermetallic phases, there is some solid solubility in mullite and very little in the end members of SiO2 (cristobalite) and Al2O3 (corundum).
There is complete solid solubility in the system Al2O3-Cr2O3; both end members have the corundum structure [11, 12]. There is also subsolidus phase separation in this system . There is also considerable solid solubility in the end-member oxides in the Al2O3-Fe2O3, Al2O3-Y2O3, and Al2O3-Ga2O3 systems . Thus, the solid solubility results because the three-valent ions of Cr, Fe, Y, and Ga can substitute for aluminum in the corundum structure, and aluminum substitutes for these ions in their oxides. Alternatively, the solubilities of oxides with cation valences other than plus three are usually very low. For example, the solubility of magnesia in alumina is about 1 ppm atom fraction (Mg/Al) at 1,200°C . Thus in mixtures of Al2O3 with higher concentrations of MgO than 1 ppm, second phases containing MgO can form on grain boundaries at 1,200°C and lower temperatures, but the Mg atoms do not dissolve or substitute for Al in the Al2O3. In the literature, there are many reports of higher solubilities of ions with valences different from three in
Fig. 2 The alumina-silica phase diagram. From 
alumina, but in view of the very careful work of Greskovitch and Brewer  with extremely pure alumina, these higher solubilities are unlikely.
In  there is a table (XI, on page 63) of minimum melting temperatures for a variety of binary alumina-oxide systems. With most oxides, these eutectic or peri – tectic temperatures vary from about 1,500 to 2,000°C. The Al2O3-V2O5 system has an anomalously low eutectic of 660°C at 99% V2O5; other low melting mixtures are Al2O3-Bi2O3 of 1,070°C, Al2O3-WO3 of 1,230°C, and Al2O3-B2O3 of 1,440°C.