Low-cost manufacturing process for nanostructured metals and alloys
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In spite of their interesting properties, nanostructured materials have found limited uses because of the cost of preparation and the limited range of materials that can be synthesized. It has been shown that most of these limitations can be overcome by subjecting a material to large-scale deformation, as occurs during common machining operations. The chips produced during lathe machining of a variety of pure metals, steels, and other alloys are shown to be nanostructured with grain (crystal) sizes between 100 and 800 nm. The hardness of the chips is found to be significantly greater than that of the bulk material.
Nanostructured materials, composed of submicronsized grains (crystals), have novel attributes not typically found in conventional materials.1,2 Furthermore, these attributes can be varied by changing the grain size. Nanostructured solids appear to have high hardness, strength, and ductility,1,3 in addition to possessing interesting electrical and magnetic properties.4 Superplasticity has been observed at relatively low temperatures in these materials.5,6 While many new and exciting applications for nanostructured materials have been identified, a principal barrier to their widespread use has been cost, typically in the range of hundreds of dollars/pound.7 This paper reports on a process for making nanostructured materials that will be approximately two orders less expensive. There have been two broad approaches for producing nanostructured materials, one for the production of fine powders and one for bulk materials. The most widely used techniques for synthesizing nanostructured metals in particulate form are condensation of metal atoms from the vapor phase1,2 and high-energy ball milling.8 The particulates can then be compacted and sintered to bulk form, often at a sintering temperature lower than that for microcrystalline powders and under conditions that suppress grain growth.1 These processes for making particulate and small compacted samples provide excellent control over particle size, but costs are estimated as being in excess of 100 dollars/pound.7 Methods to make nanostructured metals and alloys directly in bulk form have relied on the use of very large strain deformation or severe plastic deformation (SPD) to achieve microstructure refinement.9–12 The general experimental approach involves large-scale deformation a)
Contributed equally to the work.
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http://journals.cambridge.org
J. Mater. Res., Vol. 17, No. 10, Oct 2002 Downloaded: 22 Jan 2015
using processes such as rolling, drawing, equal channel angular extrusion (ECAE), or high-pressure torsional straining. Very large plastic strains, typically in excess of four, are imposed in a sample by the cumulative application of plastic deformation in multiple stages, the effective plastic strain in each stage of deformation being approximately one.10 Using this approach, nanostructured bulk materials have been produced from ductile metals and alloys of initial low to moderate strength.11 However, high-strength metals and alloys are difficu
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