Ductility of nanostructured materials

The experimental results on the ductility of single-phase nanocrystalline materials indicated little ductility in tension for grain sizes less than about 25 nm. This was true for both materials that are ductile when coarse-grained, such as elemental metals, and those that are brittle when coarse-grained. The predicted increased ductility of brittle intermetallics or ceramics by reduction of their grain size to the nanoscale has not been realized. While many of these disappointing experimental results can be attributed to artifacts present in consolidated particulate samples, similar results have also been observed for nc materials made by one-step processes such as electrodeposition. The low ductility in tension is presumably attributable to the deformation mechanism - still not understood - in nc materials where mechanical instability due to lack of strain hardening must be important. In this sense, the deformation and fracture behavior of nc material is analogous to that of metallic glasses. Very encouraging results have been observed however regarding the combination of high strength and good ductility in a number of multiphase alloys with nanoscale microstructure. These include nc/amorphous, nc/crystalline, and nanoquasi-crystalline/crystalline alloys. Perhaps this conclusion is not surprising, given that the well-established important structural materials - tempered steel and age-hardened aluminum alloys - commonly contain nanoscale microstructural features for optimum mechanical properties.