Creep damage in different 3Al2O3·2SiO2 mullites tested in 4-point bending

The creep behaviour in 4-point bending at 1200°C of two nominally 3Al2O32SiO2 materials and that of other materials prepared by annealing one of these two materials at the sintering temperature as well as at 900 and 1200°C have been studied. All materials presented very little or no glassy phase at the grain boundaries. Special preparation of the samples for microstructural analysis by scanning electron microscopy has been used in order to characterise the “as crept” parts of the samples subjected to tension as well as compression during the creep experiments. In all materials highly vitrified dense zones have been observed close to the tension and compression surfaces. Viscous cavity growth has been detected at depths (≈20–50 μm) from the tension and compression surfaces that vary from one material to the other and increase with the applied stress. Strain values for all materials are very low and do not reflect the extreme microstructural modifications observed in the samples. Densification and cavity growth can be explained on the basis of the viscous cavity growth model. Stress-enhanced dissolution of the mullite grains is proposed to be the origin of the large amounts of vitreous phase present in the samples after testing.