Nanocrystalline materials

Abstract

The current status of research and development on the structure and properties of nanocrystalline materials is reviewed. Nanocrystalline materials are polycrystalline materials with grain sizes of up to about 100 nm. Because of the extremely small dimensions, a large volume fraction of the atoms is located at the grain boundaries, and this confers special attributes to these materials. Nanocrystalline materials can be prepared by inert gas condensation, mechanical alloying, plasma deposition, 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 concluded that the structure of grain boundaries in nanocrystalline materials was quite different from that in coarse grained materials, recent studies have shown unambiguously that the structure of the grain boundaries is the same in both nanocrystalline and coarse grained materials. The properties of nanocrystalline materials are very often superior to those of conventional polycrystalline coarse grained materials. Nanocrystalline materials exhibit increased strength/hardness, enhanced diffusivity, improved ductility/toughness, reduced density, reduced elastic modulus, higher electrical resistivity, increased specific heat, higher thermal expansion coefficient, lower thermal conductivity, and superior soft magnetic properties in comparison with conventional coarse grained materials. New concepts of nanocomposites and nanoglasses are 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 materials. The extensive investigations in recent years on structure-property correlations in nanocrystalline materials have begun to unravel the complexities of these materials, and pave the way for successful exploitation of alloy design principles to synthesise better materials than hitherto available.