| Nanocrystalline or ultrafine-grained Al alloys are often produced by severe plastic deformation methods and exhibit remarkably enhanced strength and hardness compared to conventional coarse-grained materials, resulting in great potential for structural applications. To achieve nanocrystalline structure, grains were refined by cryomilling (mechanical milling at cryogenic temperature) pre-alloyed powders. Cryomilling provides capability for rapid grain refinement and synthesis of commercial quantities (30-40 kg). The cryomilled powder was primarily consolidated by hot or cold isostatic pressing in general. Secondary consolidation was achieved by extrusion or forging. Alternatively, quasi-isostatic forging was applied either as an initial consolidation or as a further deformation step.;To improve insufficient ductility and toughness of nanocrystalline materials, an intelligent design with microstructural modification was introduced by generation of multiple size scales. A bimodal grain structure consisting of nanocrystalline grains and inclusions of coarse-grained material was produced by consolidation of blended powders. The resulting materials exhibited enhanced ductility compared to 100% nanocrystalline materials, with only moderate decreases in strength. A similar process was used to produce hybrid trimodal microstructures comprised of regions of nanocrystalline and coarse grains, as well as hard ceramic particles, providing super-high compressive strength.;For cryomilled nanocrystalline Al alloys, effects of degassing temperature were investigated in terms of microstructural evolution. Higher degassing temperatures resulted in higher density and lower hydrogen content, which can reduce loss of toughness in consolidated materials. Different consolidation methods were compared with regard to the relation between the microstructures and mechanical properties. Quasi-isostatic forging led to greater and more isotropic fracture toughness, compared with other processing routes. Strain rate sensitivity in room temperature deformation was examined as a function of grain size using nanoindentation. Negative strain rate sensitivity was observed in nanocrystalline and ultrafine-grained materials, while a conventional alloy was strain rate insensitive.;For multi-scale materials, local displacements in bimodal materials during tensile deformation were measured by digital image correlation. Inhomogeneous strain behavior was observed between nanocrystalline and coarse-grained regions and attributed to differences in dislocation plasticity. In the Al matrix nanocomposite with hybrid microstructures, microstructural evolution of the composite powder with boron-carbide reinforcements was investigated as a function of milling time. |
