Nanostructured Materials Synthesized by Mechanical Attrition
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A wide variety of techniques are being used to synthesize nanostructured materials including inert gas condensation, rapid solidification, electrodeposition, sputtering, crystallization of amorphous phases, and chemical processing. Mechanical attrition—ball milling of powders—is a technique that has also been used widely for preparation of nanostructured materials. The term “mechanical attrition” can be subdivided into “mechanical milling,” which is the milling of single composition powders, often elements, and “mechanical alloying,” which involves milling of dissimilar powders such that material transfer occurs during milling. Unlike many of the methods for synthesis of nanostructured materials, mechanical attrition produces its nanostructures not by cluster assembly, but by the structural decomposition of coarser-grained structures as the result of severe plastic deformation. This has become a popular method to fabricate nanocrystalline materials because of several factors: the simplicity of the process, the relatively inexpensive equipment (on the laboratory scale) needed, and the applicability to essentially all classes of materials. A major advantage of mechanical attrition is the possibility for easily scaling up to tonnage quantities of material for various applications. The disadvantages that are often cited are contamination from the milling media and/or atmosphere, and the need, for many applications, to consolidate the powder product without coarsening the nanocrystalline microstructure.
A number of reviews concerned with the synthesis of nanostructured materials by mechanical attrition have been published in recent years, e.g., those of Koch, Fecht, Suryanarayana and Koch, and Fecht. This article will serve to update these previous reports. The details of the mechanical attrition processes, equipment used, etc. has been covered in a number of reviews (e.g., Ref. ), and will not be repeated here. The phenomenology of the development of nanoscale microstructures by mechanical attrition will be reviewed for the various classes of materials including single-phase metals, metallic alloys, brittle materials, and multiphase materials. Several mechanisms proposed for the development of a nanoscale microstructure will then be described with reference to recent experimental observations. Finally, a brief discussion of the problems associated with mechanical attrition as a processing method will be presented. These are powder contamination and the need to consolidate the powders into bulk form without coarsening the nanoscale microstructure.