STRUCTURE AND PROPERTIES OF NANOCRYSTALLINE MATERIALS

The present article reviews the current status of research and develop ment on the structure and properties of nanocrystalline materials. Nan ocrystalline materials are polycrystalline materials with grain sizes of up to about 100 nm. Because of the extremely small dimensions, a la rge fraction of the atoms in these materials is located at the grain b oundaries, and this confers special attributes. Nanocrystalline materi als can be prepared by inert gas-condensation, mechanical alloying, pl asma deposition, spray conversion processing, and many other methods. These have been briefly reviewed. A clear picture of the structure of nanocrystalline materials is emerging only now. Whereas the earlier st udies reasoned out that the structure of grain boundaries in nanocryst alline materials was quite different from that in coarse-grained mater ials, recent studies using spectroscopy, high-resolution electron micr oscopy, and computer simulation techniques showed unambiguously that t he structure of the grain boundaries is the same in both nanocrystalli ne and coarse-grained materials. A critical analysis of this aspect an d grain growth is presented. The properties of nanocrystalline materia ls are very often superior to those of conventional polycrystalline co arse-grained materials. Nanocrystalline materials exhibit increased st rength/hardness, enhanced diffusivity, improved ductility/toughness, r educed density, reduced elastic modulus, higher, electrical resistivit y, increased specific heat, higher thermal expansion coefficient, lowe r thermal conductivity, and superior soft magnetic properties in compa rison to conventional coarse-grained materials. Recent results on thes e properties, with special emphasis on mechanical properties, have bee n discussed. New concepts of nanocomposites and nanoglasses are also b eing investigated with special emphasis on ceramic composites to incre ase their strength and toughness. Even though no components made of na nocrystalline materials are in use in any application now, there appea rs to be a great potential for applications in the near future. The ex tensive investigations in recent years on structure-property correlati ons in nanocrystalline materials have begun to unravel the complexitie s of these materials, and paved the way for successful exploitation of the alloy design principles to synthesize better materials than hithe rto available.