Deformation Mechanisms of Nanostructured Al Alloys

Cryomilling has emerged in recent years as an effective approach to process large amounts of nanostructured Al alloys. In the present review, progress in our understanding of the microstructural characteristics and deformation behavior of nanostructured Al alloys processed via consolidation of cryomilled nanostructured powders is reviewed in an effort to elucidate salient findings and point out additional studies needed. A survey of published papers show that the microstructure of consolidated cryomilled Al alloys is comprised of equiaxed grains in the range of ∼ 50 nm to 200 nm, depending primarily on process parameters. Moreover, in many cases the microstructure consists of a supersaturated solid solution, despite the presence of significant amounts of alloying elements. As far as mechanical properties, the tensile behavior of these nanostructured Al systems is characterized by high strength and low strain hardening. The high strength was primarily attributed to three types of strengthening: grain size effect, solid solution hardening and Orowan strengthening. The low strain hardening or work softening behavior was accompanied with the occurrence of Lüders banding. Recent work suggests that there are two approaches that may be effectively used to address the lack of dislocation activity that typically accompany the presence of nanocrystalline grains < 10–50 nm. The first approach involves selective blending of powders to achieve a microstructure that contains multiple length scales (i.e., nanostructured, 50–200 nm, and submicron, 200–750 nm). The second approach involves thermal annealing in an effort to introduce small amounts of submicron grains for improvement of ductility. Recent related work is discussed herein.