Stable and Metastable Equilibria in the System SiO2‐Al2O3

Abstract

Concentration profiles of Al2O3 in diffusion couples made from sapphire and fused silica were used to determine the stable equilibrium phase diagram of the system SiO2‐Al2O3. The intermediate compound mullite, 3Al2O3· 2SiO2, melts incongruently at 1828°± 10°C; its stable solid‐solution region ranges from 70.5 to 74.0 wt% Al2O3 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: SiO2‐Al2O3, in the absence of mullite, SiO2‐“ordered”‐mullite in the absence of alumina, and SiO2‐“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% Al2O3. The solid‐solution range of disordered mullite extends to ∼ 83 wt% Al2O3 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.