Achieving superplasticity through severe plastic deformation

The processing of metals through the application of severe plastic deformation (SPD) provides an opportunity for refining the grains to the submicrometer or even the nanometer range in bulk materials. In principle, these ultrafine-grained materials should be ideal for achieving excellent superplastic properties but in practice this requires also that the microstructure has reasonable stability when testing in tension at elevated temperatures. Attaining superplastic elongations is an important prerequisite for using metals in commercial superplastic forming applications. Accordingly, this review briefly summarizes the principles of grain refinement in f.c.c. and h.c.p. metals where the mechanisms of grain refinement are different in these different crystal structures. The exceptional grain refinement produced by SPD techniques leads to excellent superplastic properties in many different materials. Especially, controlling the processing parameters of SPD techniques and using two-phase alloys are useful strategies for achieving excellent superplastic properties in metallic materials. This report presents numbers of examples of superplastic flow in several different metals and alloys processed by two representative SPD processing techniques of equal-channel angular pressing (ECAP) and high-pressure torsion (HPT). Finally, it is now possible to evaluate the flow process in superplasticity and this provides an opportunity to present the experimental data in terms of a deformation mechanism map.