Understanding plasticity of nanostructured metals through advanced micromechanical testing

Nanostructured metals are materials with feature sizes below 100 nm. These features can take various forms such as nanograins, nanolayers and nanopores. The resulting nanocrystalline, nanolayered and nanoporous materials show outstanding strength due to HallPetch strengthening, which makes them very promising as structural materials of future. However, most nanostructured materials are also brittle due to the small number of dislocations available for mediating plasticity. In order to enable the use of these new generation materials in engineering applications, novel approaches that can extend their plastic limits are needed. Recently, we have investigated the mechanical behavior of a wide range of nanomaterials under different loading conditions. The nanomaterials that we considered include nanocrystalline alloys, nanolayered metals, nanoporous metals and metallic glass-crystalline composites. We have employed different loading conditions through nanoindentation, micropillar compression and nanoscratch testing, to gain insight to the plastic behavior of these materials. Our findings indicate that one of the most promising methods to improve ductility is to combine metallic glasses and crystalline metals at the correct length scale. Nanolayered metallic glass-crystalline composites provide a simple model system to investigate this route. In these nanolaminated materials, when the layer thickness is comparable to that of shear band spacing, the crystalline layers start to effectively impede the catastrophic propagation of shear bands. The optimized amorphouscrystalline nanocomposite, as a result, provides an excellent combination of high strength and ductility. When it comes to nanoporous metals and nanocrystalline alloys, an analogous route can be taken through grain boundary engineering, which is currently the most promising approach for improving the ductility of these brittle materials