Aluminum matrix (6061) composites reinforced with either whiskers or particulates of Si3N4 were processed under the same hot extrusion conditions, and their superplastic properties have been compared in constant strain rate tension tests at a constant temperature of 818K. Large elongations from 400 to 450% were observed at a high strain rate range from 10-1 to 1s-1 in the Si3N4p/6061Al composite. For the Si3N4w/6061Al composite, on the other hand, a similar value of about 450% elongation was obtained at a lower strain rate of 4×10-2s-1 than that for the Si3N4p/6061Al composite. Grain sizes were about 2μm for the Si3N4p/6061Al composite and about 3μm for the Si3N4w/6061Al composite. High strain rate superplasticity of both composite may be attributed to fine grained structures. On the true stress-true strain curves of the Si3N4w/6061Al composite, strain hardening was observed over almost entire part of elongation during superplastic flow. For the Si3N4p/6061Al composite, on the other hand, the near steady state of flow stress or strain softening was found, suggesting a difference in microstructural evolution during superplastic flow between these composites.
