Although the precise mechanics involved remain unclear, it is likely that during this phase, the high defect boundaries serve as attractors for any imperfections that already existed in the crystal lattice and that the nanocrystals which are, by contrast, relatively defect -- free, become increasingly refined and near crystalline perfection.
3. The final step in the process involves random reorientations of the single-crystalline grains with regards to their neighboring grains. At the point where the grain structure achieves its limiting size (this size limit relates to the particles' crystal symmetry and the energy and amount of mechanical milling employed), the material become amenable to plastic deformation through grain boundary sliding. In fact, this type of deformation mechanism has been discerned in superplasticity in which a high diffusion rate stage is capable of accommodating such forces at any strain rate. Researchers have posited that in the case of the nanocrystalline, the high defect-density crystal interfaces are responsible for producing the rapid diffusion paths to provide the means by which the self-organization and rotation of the grains is achieved, thereby increasing the ability of the grain boundaries to store energy based on the reorientation of the grains with respect to their neighboring particles and the boundary's excess volume. The research to date suggests that to the extent that this reorientation process is allowed to continue is the extent to which it eventually releases some of the strain as the grains relax during the reorientation stage .
In order to use the foregoing steps to manufacture bulk nanostrucutured materials, it is frequently necessary to consolidate nanopowders in various ways. This is typically accomplished through the use of compaction or sintering; both of these processes apply energy in order to create a dense body which can also result in grain growth . Likewise, compaction and sintering can also result in defects in the final body as a result of impurities in the surface and the porosity of the substances involved. Consequently, it is important to establish careful monitoring of the consolidation stage in order to achieve successful outcomes . Sintering takes place through surface diffusion at higher temperatures that produces contact points in the processed particles in ways that they are then capable of aggregating resulting in an increasing densification of the powder. As temperatures become more and more elevated, sintering rates also increase; in addition, sintering also increases as particle size decreases. The research to date suggests that nanomaterials can result in densification at reduced temperatures in ways that restrict grain growth .
The research showed that the future of materials and applications of nanotechnology appears unlimited and these technologies represent an important dimension in technological innovation. The research also showed that mechanical alloying represents one such innovative method which has been successfully used to produce an increasingly varied range of commercially useful and materials that are of scientific interest, including intermetallics and amorphous, nanocrystalline, and nanocomposite materials. In addition, mechanical milling and mechanical alloying methods have also been employed successfully to refine the grain size and to synthesize non-equilibrium structures in ways that have not been possible in the past. Finally, the research also showed that the mechanical milling processes that are used to create innovative nanomaterials use many of the same mechanical processes that have been used in the past, but in ways that produce exceedingly small particles in the nanorange that represent a fundamental leap forward in the production of nanomaterials that hold an enormous amount of promise in a wide range of commercial applications in the future.
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