Metals

An example of a grain size dependent phase transition in metals is provided by nickel. In the bulk solid, this metal has fcc structure. However, recent experiments have brought to light a grain size dependent phase transition for very small Ni nanoparticles, chemically synthesised in solution. Indeed for diameters less than 4 nm, the structure is still crystalline, but it becomes hexagonal close packed (hcp) rather than fcc. In the bulk material, an hcp structure is metastable, although it can be obtained in thin films in certain conditions. It should also be noted that the melting temperature of various metals is well known to depend on grain size.