Grain refinement is a well-established means of strengthening metallic alloys, and is expressed in the well-known Hall-Petch relation. In the past decade, severe plastic deformation (SPD) has been employed to refine grains beyond what was previously possible. While SPD has multiple variants (high-pressure torsion, equi-channel angular extrusion, accumulative roll-bonding, etc.), most of these result in ultra-fine grain structures with grain sizes > 100 nm. Achieving structures with grain sizes <100nm that are also stable and resistant to grain growth is challenging. One SPD technique that works well in this regard is cryo-milling of powders followed by consolidation using conventional means. In a joint project with UC Davis and more recently, Johns Hopkins University, we are investigating processing-structure relations and structure-property relations in Al-Mg alloys and composites. The effort at Davis focuses on processing, while the effort at USC focuses on microstructural analysis, and all partners are investigating aspects of deformation. Because of the powder metallurgical (P/M) process, we are able to create bimodal structures comprised of nanocrystalline (nc) and coarse grains (CG) simply by blending milled and unmilled powders prior to consolidation. This approach affords the opportunity to design microstructures with different proportions of hard and soft components, resulting in an ability to achieve various combinations of toughness, strength, and ductility. In more recent work, we have blended ceramic particles with the powders to achieve trimodal MMCs, and we are focusing on the mechanical behavior of these composites at dynamic strain rates.
Comparison of tensile properties
Bimodal structures and corresponding digital image correlation results
Multi-scale concept of trimodal composite
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