Abstract
High-temperature strength and fracture behaviour were investigated by flexural test, fracture toughness test, microstructure and fractographic observations by SEM and TEM. In the case of the flexural test, at below 1250 C fracture starts from internal defects but at higher temperatures the crack starts from the tensile surface. Flexural strength is affected by temperature and shows S-curve behaviour with a peak near 1250 C. The actual fracture toughness increases with increasing temperature over 1300 C. The high-temperature fracture toughness with slow crack growth depends on the cross-head speed because of the viscous flow of the glass phase. The microstructure of the sample at 1350 C had a large amount of lattice dislocations whereas the sample at 1500 C had much cavitation and a small amount of dislocations. 7 refs.