Review: achieving superplastic properties in ultrafine-grained materials at high temperatures

The mechanisms of superplasticity occurring in conventional materials, having grains sizes of the order of a few microns, are now understood reasonably well. However, very recent advances in the processing of ultrafine-grained (UFG) metals have provided an opportunity to extend the understanding of flow behavior to include UFG materials with submicrometer grain sizes. In practice, processing through the application of severe plastic deformation (SPD), as in equal-channel angular pressing (ECAP) and high-pressure torsion (HPT), has permitted the fabrication of relatively large samples having UFG microstructures. Since the occurrence of superplastic flow generally requires a grain size smaller than ~10 μm, it is reasonable to anticipate that materials processed by SPD will exhibit superplastic ductilities when pulled in tension at elevated temperatures. This review examines recent results that demonstrate the occurrence of exceptional superplastic flow in a series of UFG aluminum and magnesium alloys after ECAP and HPT. The results are analyzed to evaluate the superplastic flow mechanism and to compare with materials processed using different techniques. The critical issue of microstructural inhomogeneity is examined in two-phase UFG materials after SPD processing and the influence of microstructural homogeneity on the superplastic properties is also demonstrated.