SOME BASIC NOTIONS ON NANOSTRUCTURED SOLIDS

Understanding the physical origin of the properties of nanostructured solids requires characterization of the atomic-scale structure in the interfaces, of the microstructure, and of the interactions between the individual structural elements, e.g. chemical components and topological defects. The first part of the paper points out some of the essential parameters: the atomic short-range order and energetics of the interfaces may depend on the thermomechanical history of a sample. Interfaces may be atomically sharp, as in nanocrystalline Pd, or they may be delocalized on a scale comparable with the particle size, as in nanostructured amorphous Ti-Pd. In nanocrystalline alloys, the degree of equilibration between the particle volume and the interfaces is an important parameter, which affects both the nanometer-scale microstructure and the overall thermodynamic functions. The second part of the paper is devoted to a discussion of the thermodynamic properties of nanostructured alloys. It is predicted that alloys with a strong tendency for grain boundary segregation can be metastable with respect to grain growth. The concentration dependence of the grain size in these alloys enables control of the grain size down to values as low as 2 nm.