Achieving superplastic behavior in fcc and hcp metals processed by equal-channel angular pressing

Processing through the application of severe plastic deformation provides an opportunity for producing polycrystalline metals with ultrafine grain sizes in the submicrometer or nanometer ranges. If these small grains are reasonably stable at elevated temperatures, the materials will exhibit large elongations to failure when tested in tension without the development of any significant necking within the gauge lengths. Examples of superplastic flow are presented for ultrafine-grained aluminum and magnesium alloys. The results available to date reveal several similarities with conventional superplastic materials in terms of both the mechanical properties and the development of internal cavities during deformation. An important difference visible especially for Al-based alloys is that, because of the exceptionally small grain size after processing, optimum superplastic flow occurs at strain rates which are significantly faster than in conventional alloys.