JOURNAL OF MATERIALS SCIENCE 38 (2 0 0 3 ) 3319 – 3324 Tensile behavior of bulk nanostructured and ultraﬁne grained aluminum alloys B. Q. HAN, F. A. MOHAMED Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575, USA E-mail: famohame@uci.edu E. J. LAVERNIA Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575, USA; University of California, Davis, CA 95616-5294, USA In the present study, data on tensile behavior of bulk nanostructured aluminum alloys processed via consolidation of mechanically milled powders and severe plastic deformation are analyzed. High strength and low strain hardening were observed in bulk nanostructured and ultraﬁne-grained Al alloys. The ductility of aluminum alloys decreases with decreasing grain size. The high amount of intercrystalline components may have an inﬂuence on tensile properties of bulk nanostructured materials when grain sizes are less than 100 nm. The high strength in bulk nanostructured Al-Mg alloy may be attributed to contributions arising from grain size strengthening, the presence of high dislocation densities, Orowan strengthening, precipitation hardening and solid-solution hardening. The large and sudden stress drops in the stress-strain curves of cryomilled Al alloys are most probably indicative of the dislocation annihilation in the vicinity of or breakaway from the strong pinning role of dispersoids. C  2003 Kluwer Academic Publishers 1. Introduction Bulk nanostructured materials (BNMs) exhibit unique microstructures [1, 2] in which the volume of grain boundary is signiﬁcant. For example, a 5-nm mate- rial has approximately 50% of its volume as grain boundaries. BNMs are emerging as a new class of ma- terials with unusual structures and, as a result, have attracted considerable attention in recent years. They offer interesting possibilities related to many structural applications. The successful synthesis of large-scale BNMs with a grain size in the range of 10–200 nm represents a ma- jor achievement in the wide ﬁeld of nanotechnology. The fact that it is now possible to synthesize large- scale BNMs with dimensions in the 10 2 –10 4 mm is of technological and scientiﬁc signiﬁcance. From a tech- nological point of view, it will be feasible to obtain engi- neering materials that retain the structural and chemical attributes of particles in the nanometer size range. From scientiﬁc point of view, large-scale BNMs will permit careful studies of the physical and mechanical behav- ior, using standardized testing. In addition, BNMs, by virtue of their microstructure, will allow systematic in- vestigations of the inﬂuence of multiple-length scales (from the nanometer to micrometer) on the fundamental physical mechanisms that govern the materials. Severe plastic deformation (SPD) and consolidation of mechanically milled powders represent the two most widely used techniques for synthesizing bulk nanos- tructured aluminum alloys [2–5]. There are two main differences between these techniques in terms of the characteristics of the materials produced. First, the min- imum grain size of BNMs processed via consolidation of mechanically milled powders is typically smaller than that via SPD. For instance, the minimum grain size in BNMs processed via consolidation of mechani- cally milled powders is about 20–30 nm (reported in a MA pure Al [6]), compared to about 100–300 nm via SPD [2]. This difference most likely is the result of: (a) the higher level of severely plastic deformation intro- duced during milling procedure, and (b) the limitation of grain growth by the Zener pinning of dispersoids in the former approach. Also, this difference is consistent with the general observation that the ﬁnal grain size of BNMs is largely determined by the inherent thermal stability of the microstructure in combination with the parametric space used during processing [7]. Second, the tensile ductility of BNMs processed using consol- idation of mechanically milled powders is lower than that of SPD, probably because of the absence of defects in the microstructure characterizing the material pro- duced by the latter technique. As reported elsewhere [8], the low tensile ductility of many nanostructured materials is often attributed to defects and ﬂaws. Data reported for the mechanical behavior of bulk nanostructured aluminum alloys have shown two trends. First, for a grain size ranging from 20 to 300 nm, the grain size softening phenomenon (e.g., reverse Hall-Petch relationship [9]), which is sometimes re- ported, is absent, and the ﬂow strength follows the 0022–2461 C  2003 Kluwer Academic Publishers 3319