Stable and Metastable Equilibria in the System SiO ₂ ‐Al ₂ O ₃

Concentration profiles of Al 2 O 3 in diffusion couples made from sapphire and fused silica were used to determine the stable equilibrium phase diagram of the system SiO 2 ‐Al 2 O 3 . The intermediate compound mullite, 3Al 2 O 3 · 2SiO 2 , melts incongruently at 1828°± 10°C; its stable solid‐solution region ranges from 70.5 to 74.0 wt% Al 2 O 3 below 1753°C and from 71.6 to 74.0 wt% at 1813°C. The microstructures of diffusion zones and heat‐treated specimens also indicate the incongruency of mullite. Additional information is given for 3 metastable systems: SiO 2 ‐Al 2 O 3 , in the absence of mullite, SiO 2 ‐“ordered”‐mullite in the absence of alumina, and SiO 2 ‐“disordered”‐mullite in the absence of alumina. Under metastable conditions, ordered mullite melts congruently at = 1880°C and its solid‐solution range extends up to = 77 wt% Al 2 O 3 . The solid‐solution range of disordered mullite extends to ∼ 83 wt% Al 2 O 3 with an estimated congruent melting temperature of ∼ 1900°C. The existence of metastable systems is associated with superheating of mullite above the incongruent melting temperature and with nucleation of alumina and mullite in supercooled aluminum‐silicate liquids.