Superplasticity at high strain rates in metals and their composites

Over the last decade, superplasticity at high strain rates (> 10^-1 s^-1) has been reported in many fine-grained structural materials, including Al-base and Mg-base composites, mechanically-alloyed materials, and modified conventional alloys. Up to the present time, there exists an extensive data base indicating that the maximum superplastic strain rate increases with decreasing grain size. The phenomenon of high strain rate superplasticity (HSRS), however, is not observed in all materials containing fine grain sizes, suggesting that a fine grain size is a necessary but insufficient condition. Some recent experimental evidence suggests that the presence of a small amount of liquid phase at interfaces or grain boundaries, or both, not only enhances the strain rate for superplasticity, but also has a strong influence on the deformation mechanisms. In the present paper, two alloy systems - 2124 Al and ZK60 Mg, and their corresponding metal matrix composites are examined. The microstructure-superplastic property relationships for each alloy matrix and its corresponding composite are characterized and compared to illustrate the optimal microstructural conditions that are required for achieving HSRS. The selection of appropriate alloy compositions, and of appropriate processing steps to create economical superplastic materials, are discussed.