OVERVIEW

The difference in the properties of nanocrystalline and coarse-grained polycrystalline substances is associated with the small size of crystallites and extremely developed interfaces of the nanocrystalline substances. The interfaces can include up to 50 % of atoms of the nanocrystal. At present, many researchers of nanocrystalline compacted materials assume that the physical properties of these materials (in particular, mechanical properties) are determined primarily by the large area and special structure of the interfaces [1, 2]. For this reason, the examination of nanocrystalline compacted substances is concentrated mainly on the analysis and describing of the special features of the structure of grain boundaries.

Before we proceed to discuss the microstructure of nanocrystalline materials, i. e. polycrystalline substance, let us define more exactly some notions, which will be used below. Three types of the grain contact are possible in a polycrystalline substance. They include contact surfaces, contact lines, and contact points. Surfaces of two grains, which contact one another, are called interfaces. A contact line may represent a common line for three or more adjacent grains. The contact line of three grains is called triple junction. The triple junctions are most often appearing contact lines among different contact lines, which exist in polycrystals. Usually about six grains meet at a contact point. The boundary of a grain is its surface. Grain boundaries, which are seen in metallographic slides, represent the section of interfaces by the slide plane. A triple point is the section of the triple junction (contact line...