HIGH AND LOW-TEMPERATURE SUPERPLASTICITY IN NANOCRYSTALLINE MATERIALS

Superplasticity, the ability of a crystalline material to deform to hu ndreds of percent strain, has been demonstrated at elevated temperatur es for several nanocrystalline metal and ceramic systems. Nanocrystall ine materials manifest superplasticity at lower temperatures and faste r strain rates than their larger-grained counterparts; however, their enhanced superplasticity can easily disappear during deformation due t o a combination of static and dynamic grain growth. Despite this limit ation, applications such as near net shape forming, diffusion bonding, thermally mismatched composite structures, and flaw-free processing a re already under development. In contrast to conventional superplastic ity, low (room temperature) superplasticity has yet to be demonstrated conclusively in nanocrystalline materials. Early measurements of a ro om temperature ductility/superplasticity effect can be largely attribu ted to the presence of porosity. Unusual trends in room temperature st rain rate sensitivity may reflect thermally activated dislocation glid e past synthesis-generated defects, rather than a true change in defor mation mechanism at ultrafine grain sizes. (C) 1997 Acta Metallurgica Inc.