THE STRUCTURE OF AMORPHOUS AND NANOCRYSTALLINE METALS AND ALLOYS

The structures of amorphous and nanocrystalline materials are characterized by broad or diffuse peaks in their X-ray, neutron, or electron diffraction patterns. A detailed analysis of these patterns permits us to evaluate the topological and chemical short-range order in amorphous materials and the size of the coherently diffracting domains, the elastic strains within these domains, stacking faults, and the dynamic and static atomic displacements in nanocrystalline materials. Amorphous metallic alloys have been prepared by rapid quenching from the vapor and liquid phases, by electron, proton or ion irradiation, by solid state reactions in multilayer films, and by mechanical alloying and mechanical working in high-energy ball mills. The term nanocrystalline metal has been introduced to describe compacted samples of metallic particles or clusters of particles produced from the vapor phase by quenching in an inert gas. Nanocrystalline alloy films have been prepared by vapor-quenching onto a cold substrate. Fine particles of metals and compounds can also be produced by chemical methods and by mechanical grinding. A comparison will be given of the structure of amorphous metallic alloys prepared by liquid-quenching, irradiation, and mechanical alloying. It will be shown that the structure of the amorphous alloy of a particular chemical composition does not depend on the method of preparation. All amorphous metal-metal alloys are characterized by chemical short-range order, i.e. a preference for unlike nearest neighbors. The structure of nanocrystalline metal powders and thin alloy films will be described in terms of particle size and strains, determined from the broadening of the powder pattern peaks, and from the analysis of the atomic distribution functions.