IN-SITU TRANSMISSION ELECTRON-MICROSCOPY OBSERVATION OF THE INITIATION AND GROWTH OF MICROCRACKS IN AN SIC WHISKER AL COMPOSITE

The initiation and growth of microcracks in a SiC whisker (SiCw)-Al co mposite have been studied by means of the transmission electron micros cope equipped with a tensile stage. It was found that microcrack initi ation sources mainly included (1) the whiskers damaged and/or fracture d during the preparation of the composite, (2) uninfiltrated zones, (3 ) SiCw-Al interfaces and (4) the matrix. Only the microcrack that nucl eated in the matrix grew to become the major crack leading to specimen failure although many microcracks occurred in the early stages of str aining. The whiskers and grain boundaries in the composite played an i mportant role in impeding the growth of the cracks. When a crack was p erpendicular to the frontal whisker, the crack tip would blunt near th e longitudinal interface and it then advanced by bypassing the whisker end; when a crack was parallel to the frontal whisker, the crack woul d blunt near the whisker end and then propagated along the near-interf ace region of one side of this whisker.