THE SIGNIFICANCE OF THE STRUCTURE OF INTERNAL INTERFACES FOR THE PROPERTIES OF MATERIALS

Abstract
The methods available to study the correlation between properties and atomic structure of interfaces are critically assessed by considering the specific limitations. Studies of the behaviour of grain boundaries by means of the plate/sphere method are reported indicating that a few boundaries exhibit special properties which were observed for a particular interface independently of the property investigated (e.g. energy, corrosion, embrittlement). Most special boundaries were of high coincidence type. But not all high coincidence boundaries showed special properties. It seems the interatomic interaction which selects between special and "non-special" high coincidence boundaries. The properties of boundaries deviating from special misorientations were found to be controlled by the presence of (secondary) boundary dislocations. systematic studies of the properties of interphase boundaries between ionic crystals and noble metals showed that coincidence misorientations do not result in special behaviour which was observed only if close packed rows of atoms at the "surfaces" of the metal crystals "locked" into the "valleys" between close packed atomic rows at the "surfaces" of the ionic crystals ("lock-in" model of interphase boundaries). The basic idea of nanocrystalline materials is to generate a new type of solids by exploiting the highly distorted atomic structure existing in the core of grain (interphase) boundaries. This is achieved by reducing the crystal size of a polycrystalline material to a few nanometers (nanocrystalline material) so that the volume fraction occupied by interfaces (interfacial component) and crystals are comparable. The interfacial component of such a material consists of many (typically 10¹⁹ cm^-3) boundaries. As the structures of these boundaries are all different, the interfacial component (i.e. the sum of all boundary structures) resembles a frozen gas. Experimental studies of nanocrystalline materials support this hypothesis and suggest that interfaces may be used as a structural component for generating a gas-like solid state structure.