NANOCRYSTALLINE MATERIALS

The current status of research and development on the structure and pr operties of nanocrystalline materials is reviewed. Nanocrystalline mat erials are polycrystalline materials with grain sizes of up to about 1 00 nm. Because of the extremely small dimensions, a large volume fract ion of the atoms is located at the grain boundaries, and this confers special attributes to these materials. Nanocrystalline materials can b e prepared by inert gas condensation, mechanical alloying, plasma depo sition, spray conversion processing, and many other methods. These are briefly reviewed. A clear picture of the structure of nanocrystalline materials is only now emerging. Whereas the earlier workers had concl uded that the structure of grain boundaries in nanocrystalline materia ls was quite different from that in coarse grained materials, recent s tudies have shown unambiguously that the structure of the grain bounda ries is the same in both nanocrystalline and coarse grained materials. The properties of nanocrystalline materials are very often superior t o those of conventional polycrystalline coarse grained materials. Nano crystalline materials exhibit increased strength/hardness, enhanced di ffusivity, improved ductility/toughness, reduced density, reduced elas tic modulus, higher electrical resistivity, increased specific heat, h igher thermal expansion coefficient, lower thermal conductivity, and s uperior soft magnetic properties in comparison with conventional coars e grained materials. New concepts of nanocomposites and nanoglasses ar e also being investigated with special emphasis on ceramic composites to increase their strength and toughness. There appears to be a great potential for applications in the near future for nanocrystalline mate rials. The extensive investigations in recent years on structure-prope rty correlations in nanocrystalline materials have begun to unravel th e complexities of these materials, and pave the way for successful exp loitation of alloy design principles to synthesise better materials th an hitherto available.