Processing-structure-property relationships in ultrafine grain and nanocrystalline materials

This paper will review selected aspects of the processingstructureproperty relationships in ultrafine grained (ufg- grain sizes 100 to 500 nm) and nanocrystalline (nc- grain sizes < 100 nm) materials. Of the various processing methods to obtain fine grain size materials, the two that have provided bulk artifactfree samples are electrodeposition and severe plastic deformation. The processing methods and important variables will be described for these techniques. Since the stability of the nanocrystalline microstructure is important for both processing (e.g. consolidation of powders) and elevated temperature mechanical property studies, the stability of nanocrystalline grain sizes as influenced by solute additions will be discussed. While hardness and strength usually increase with decreasing grain size, ductility is typically poor. There are now, however, a number of examples of nanocrystalline materials which combine high strength with good ductility. An example from the author's laboratory on nanocrystalline Cu with optimized mechanical properties will be presented.