Microstructural changes during creep of an SiC/Al2O3 composite

The creep properties of an SiC/Al2O3 composite were studied under constant-load, four-point flexure tests. The initial stresses ranged from 50 to 350 MPa at temperatures in the range 1000–1300 °C in an air atmosphere. The final strain was as high as 3% at the highest test temperature. A subsequent transmission electron microscopy (TEM) investigation revealed no dislocation generation in the matrix; the few dislocations present were no different than those in the as-made samples. A certain amount of cavitation was observed on the tension face of the specimens tested at 1300 °C. The cavitation was seen to be generated at triple junctions in the matrix and along the whisker/matrix interface. Such cavities then grew into the cracks and, under load, propagated in a mixed mode across the sample. Unaccommodated grain boundary sliding was seen to generate voids along the grain boundaries which then joined into a microcrack and lowered the load-bearing capacity of the composite; this caused a marked drop in the strain to failure of the composite compared with the unreinforced matrix.