FUNDAMENTAL RESEARCH OF HIGH-STRAIN-RATE SUPERPLASTICITY IN ADVANCED COMPOSITES PRODUCED BY MECHANICAL ALLOYING

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Positive Exponent Superplasticity, (High strain rate superplasticity, i.e., superplastic behavioe at strain rates over 10^<-2> s^<-1>) has been characterized in very fine grained aluminum alloys and composites, which have been developed by powder metallurgical processing and/or new advanced processing methods to have grain sizes of 50nm-3mum. The experimental results on high strain rate superplastic materials are reviewed and related to the temperature and strain rate dependencies of superplastic behavior, i.e., tensile elongation, strain rate sensitivity, activation energy, and cavitation. The optimum superplastic strain rates aer found to be strongly dependent upon the refinement of grain structures. Specifically, marked changes in these properties and superplastic flow are closely related to incipient melting points in the materials. A maximum value in elongation, for example, is found at temperatures near or slightly above the incipient melting points. These observations confirm the suggestion that the presence of a small amount of liquid phase at grain boundaries in the alloys and interfaces in the composites not only enhances the strain rate for superplasticity, but also has a strong influence on the deformation mechanisms. A model is proposed in which superplasticity is critically controlled by the accommodation process to relax the stress concentration resulting from the sliding at grain boundaries and/or interfaces, involving an accommodation helper such as a liquid phase.