Creep damage in different 3Al(2)O(3)center dot 2SiO(2) mullites tested in 4-point bending

The creep behaviour in 4-point bending at 1200 degreesC of two nominally 3A l(2)O(3)2SiO(2) 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 degreesC have been studied. All materials presented very little or no glassy phase at the grain boundaries. Special preparation of the sample s for microstructural analysis by scanning electron microscopy has been use d in order to characterise the "as crept" parts of the samples subjected to tension as well as compression during the creep experiments. In all materi als highly vitrified dense zones have been observed close to the tension an d compression surfaces. Viscous cavity growth has been detected at depths ( approximate to 20-50 mum) from the tension and compression surfaces that va ry from one material to the other and increase with the applied stress. Str ain values for all materials are very low and do not reflect the extreme mi crostructural modifications observed in the samples. Densification and cavi ty 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 or igin of the large amounts of vitreous phase present in the samples after te sting. (C) 2001 Elsevier Science Ltd. All rights reserved.