ACOUSTIC MICROSCOPY OF CERAMIC-FIBER COMPOSITES .2. METAL-MATRIX COMPOSITES

Scanning acoustic microscopy (SAM) has been used to study metal matrix composites (MMCs) reinforced with silicon-carbide monofilaments, For most of the specimens the matrix was Ti-6Al-4V, but Ti3Al and 6061 Al matrices were also examined. The titanium-matrix specimens were subjec ted to a range of thermal ageing treatments to investigate potential i n-service degradation. The main effect was progressive deterioration o f the fibre-matrix interface. In the as-received material the carbon-r ich coating protected the SiC by forming a reaction layer with the tit anium. As a result of ageing, the reaction layer was penetrated adjace nt to the beta-phase titanium grains. More extensive ageing caused the carbon-rich coating to degrade and eventually disappear. The final st age of deterioration was direct attack on the SiC. Cracks and porosity between fibres were observed in some specimens, probably due to poor diffusion bonding during fabrication. Fine radial microcracks were obs erved in an annular region inside the mid-radius of some fibres; they are believed to be a consequence of stress relief during thermal agein g. These cracks could not be observed optically. The extra sensitivity of acoustic microscopy is due to the reflection of Rayleigh waves by tight closed cracks.