Bulk Nanostructured Materials
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Bulk Nanostructured Materials C.C. KOCH, T.G. LANGDON, and E.J. LAVERNIA This paper will address three topics of importance to bulk nanostructured materials. Bulk nanostructured materials are defined as bulk solids with nanoscale or partly nanoscale microstructures. This category of nanostructured materials has historical roots going back many decades but has relatively recent focus due to new discoveries of unique properties of some nanoscale materials. Bulk nanostructured materials are prepared by a variety of severe plastic deformation methods, and these will be reviewed. Powder processing to prepare bulk nanostructured materials requires that the powders be consolidated by typical combinations of pressure and temperature, the latter leading to coarsening of the microstructure. The thermal stability of nanostructured materials will also be discussed. An example of bringing nanostructured materials to applications as structural materials will be described in terms of the cryomilling of powders and their consolidation. DOI: 10.1007/s11661-017-4298-0 The Minerals, Metals & Materials Society and ASM International 2017
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INTRODUCTION
BULK nanostructured materials are defined as bulk solids with nanoscale or partly nanoscale microstructures. This category of nanostructured materials has historical roots going back many decades but has relatively recent focus due to new discoveries of unique properties of some nanoscale materials. Early in the last century, when ‘‘microstructures’’ were revealed primarily with the optical microscope, it was recognized that refined microstructures, for example, small grain sizes, often provided attractive properties such as increased strength and toughness in structural materials. A classic example of property enhancement due to a refined microstructure—with features too small to resolve with the optical microscope—was age hardening of aluminum alloys. The field of nanocrystalline (or nanostructured) materials as a major identifiable activity in modern materials science results to a large degree from the work of Gleiter and co-workers in 1980s[1] who synthesized nanoscale (1 lm) polycrystalline or single crystal materials of the same chemical composition. This is the stimulus for the tremendous appeal of these materials. There are a number of processing methods to produce bulk nanostructured materials. These include the inert gas condensation method pioneered by Gleiter,[1] electrodeposition,[2] crystallization of amorphous precursors,[3] severe plastic deformation methods,[4] or the consolidation of nanoscale powder precursors.[5] The nanoscale powder precursors may be nanoscale powders produced by a variety of chemical methods[6] or powders which are micron or tens of microns in size but with a nanoscale grain size produced by ball milling of powders. The methods to be emphasized in this short review will be those that involve either severe plastic deformation of bulk materials, or the severe plastic deformation (mechanical attrition) of powders followed by consolidation into bulk. Th
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