Principles of superplasticity in ultrafine-grained materials

Ultrafine-grained materials are attractive for achieving superplastic elongations provided the grains are reasonably stable at elevated temperatures. Since the strain rate in superplasticity varies inversely with the grain size raised to a power of two, a reduction in grain size to the submicrometer level leads to the occurrence of superplastic flow within the region of high strain rate superplasticity at strain rates >10−2 s−1. This paper tabulates and examines the various reports of superplasticity in ultrafine-grained materials. It is shown that these materials exhibit many characteristics similar to conventional superplastic alloys including strain rates that are consistent with the standard model for superplastic flow and the development of internal cavitation during the flow process.