Nanostructured Materials

Nanostructured materials are materials with at least one dimension in the nanometer range (1 nm = 10"9 m), generally iii 100 nm. Because of their novel combinations of mechanical, physical, and magnetic properties they have received considerable attention in the past few years (Ref 27, 28, 29, and 30). The nanostructures can be one-dimensional (layered), two-dimensional (fibrous), or three-dimensional (crystallites). However, the vast majority of work to date has been on the third type, which is generally produced using a P/M approach, and this article only considers this type of nanostructured material. A schematic representation of a nanostructured crystallite material is shown in Fig. 9 (Ref 27). The large fraction of atoms located in the grain boundary regions results in novel processing possibilities and enhanced combinations of mechanical, physical, and magnetic behavior compared to materials, with a more conventional grain size (>1 /'m). Because of the "far-from-equilibrium" nature of nanostructured powder, compaction is a major concern, particularly retention of nanometer-sized (S 100 nm) grains.

Fig. 9 Schematic of nanostructed material. Black circles represent atoms in normal lattice positions within the grains; white circles indicate atoms that are "relaxed" in grain-boundary regions. Source: Ref 27

Methods of production of nanostructured materials have concentrated on gas condensation methods, but chemical, mechanical (e.g. mechanical alloying) and plasma (see earlier) techniques have also been used (Ref 27, 28, 29, and 30). During the MA process the powder particle size and also the grain size are continuously refined, and very often nanometer-sized grains are produced (Fig. 10) (Ref 1, 2). Not only can nanostructured grains be mechanically alloyed, but these grains do not show excessive growth during the compaction cycle (Ref 1, 2).

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Fig. 10 Reduction in grain size with MA time determined from x-ray diffraction line broadening, for a number of TiAl-type alloys

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