Room-temperature mechanical behavior of cryomilled al alloys
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DUCTION
GRAIN size refinement is widely regarded as a straightforward approach to strengthening materials. The widespread interest in and development of nanostructured materials for structural applications undoubtedly derives from early studies,[1–5] which demonstrated high strength achieved in nanocrystalline materials. Indeed, the empirical relation between strength and grain size, the Hall–Petch equation sy s0 k y d 1/2
[1]
has been demonstrated for metals with grain sizes as small as 10 nm.[1,4–8] Two basic approaches have been used to fabricate bulk nanostructured materials. In the first, nanostructured powders are produced and then consolidated, while the second reduces the microstructure of an already-bulk sample to the nanostructured scale by means of, for example, equal channel angular pressing (ECAP) or high-pressure torsion.[9] Powder production itself can yield fundamentally different powders depending on the processing method. Individual powder particles with nanometer-scale dimensions may be synthesized by gas condensation,[10] while larger powder particles composed of nanocrystalline grains are formed by mechanical attrition, i.e., ball milling.[11,12,13] For nanocrystalline powders, the challenge that arises is how to consolidate them into bulk forms with sufficient density such that the characterization and properties of the bulk material are not severely compromised by internal porosity. At the same time, consolidation conditions have the potential to lead to grain growth that eliminates the nanostructured character. The consolidation of a metal powder can be conceived in two different ways. In a sintering model, diffusive growth of necks between particles leads first to isolated pore channels, which are then reduced to isolated pores.[14] Such
DAVID WITKIN, was formerly Graduate Research Assistant, Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575. BING Q. HAN, Materials Scientist, and ENRIQUE J. LAVERNIA, Professor, are with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95606. Manuscript submitted October 21, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
small pores may serve to inhibit grain growth by limiting grain boundary mobility due to Zener drag.[15] In a compaction model, increasing density results from plastic flow of the material in the powder particles. The limiting density can be expressed as a function of both the compaction stress and the yield strength of the material.[16] In the case of inert gas condensed nanocrystalline metals, for example, the small grain or particle size, resulting in higher yield strength, would require much higher compaction pressures. In addition to porosity, delamination microcracks have been cited as contributing to the brittle nature of fine-grained materials.[17] Consolidation of nanostructured powders may be improved by a combination of elevated temperature and pressure. The elevated temperature reduces the flow stress of the material
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