High and low temperature superplasticity in nanocrystalline materials

Superplasticity, the ability of a crystalline material to deform to hundreds of percent strain, has been demonstrated at elevated temperatures for several nanocrystalline metal and ceramic systems. Nanocrystalline materials manifest superplasticity at lower temperatures and faster strain rates than their larger-grained counterparts; however, their enhanced superplasticity can easily disappear during deformation due to a combination of static and dynamic grain growth. Despite this limitation, applications such as near net shape forming, diffusion bonding, thermally mismatched composite structures, and flaw-free processing are already under development. In contrast to conventional superplasticity, low (room temperature) superplasticity has yet to be demonstrated conclusively in nanocrystalline materials. Early measurements of a room temperature ductility/superplasticity effect can be largely attributed to the presence of porosity. Unusual trends in room temperature strain rate sensitivity may reflect thermally activated dislocation glide past synthesis-generated defects, rather than a true change in deformation mechanism at ultrafine grain sizes. (C) 1997 Acta Metallurgica Inc.