Microstructural and microchemical analysis of thecreep damage in mullite tested in flexure

Creep damage of mullite, with nominal composition of 3Al2O32SiO2 and total impurity amount 0.63 wt.% including 0.11 wt.% Na2O, was characterised in four point bending. Stresses from 40 to 130 MPa during 20 h were applied at 1200 °C. The samples tested at 130 MPa failed for shorter testing times (8–18 h). All samples experienced small strains (<0.14%) even before fracture. Microstructural characterisation of the post test samples was performed using SEM–EDX. Cracks that separated dense areas from the rest of the sample were observed close to the compression and tension surfaces. Quantitative evaluation of the size and distribution of these areas demonstrated that the observed damage increased systematically with the applied stress. Microchemical (EDX) analysis at the dense areas and cracks gave the phase compositions expected from the isothermal section at the testing temperature of the Al2O3–SiO2–Na2O ternary phase equilibrium diagram. In particular, high silica glasses with impurity accumulation and pure alumina were found. Microchemical analysis through the samples did not indicate that liquid migration occurred during testing. The reported data agree with enhanced dissolution of the mullite grains to take place at the stressed areas during creep tests.