DIFFRACTION STUDIES OF THE STRUCTURE OF AMORPHOUS AND NANOCRYSTALLINE METALS AND ALLOYS - A REVIEW

Amorphous and nanocrystalline materials exhibit broad or diffuse peaks in their X-ray, neutron, or electron diffraction patterns. A detailed analysis of these patterns permits evaluation of the topological and chemical short-range order in amorphous materials, the size of the coherently diffracting domains (particle size), the elastic strains within these domains, and the dynamic and static atomic displacements in nanocrystalline materials. Metallic, ceramic, and semiconducting materials can be produced in the amorphous state and in the nanocrystalline form by various techniques. 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, clusters of particles, or grains of nanometer size, produced by quenching metallic vapor into an inert gas, by precipitation from chemical solutions, and by mechanical grinding of micron-size particles. As examples of metallic glasses, the structures of vapor-quenched Co-Gd alloys, liquid-quenched Be-Ti-Zr alloys and mechanically alloyed W-Fe alloys are discussed. The structure of nanocrystalline metal powders of NiAl and W(Fe), produced by mechanical grinding in a high-energy ball-mill, is 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.