| This work investigates the synthesis of nanocrystalline NiAl from elemental powders by cryogenic mechanical alloying, or cryomilling. Particle size, lattice parameter, and transformation kinetics were studied using X-ray diffraction and transmission electron microscopy (TEM). Nanocrystalline Ni-rich phase with particle size ;The author suggests a picture of the synthesis in which cryomilling results in severe particle size reduction of both Ni and Al to the nanometer scale level. Due to the local agitation introduced into the structure, an exothermic reaction occurs between the Ni and Al within the intercrystalline regions, whose atomic configuration resemble a B2 NiAl. Because of the energy release from the formation of B2 NiAl, this in turn triggers the same reaction in neighboring atoms to form a B2 NiAl nucleus. This reaction terminates when it grows to nickel or aluminum nanocrystals, and is quenched by the cryogenic environment. The amount of the B2 nuclei increases with increasing milling time, and gradually saturates as the volume fraction of suitable material is reduced. A sigmodal curve may be used to describe this transformation.;Cryomilling in liquid nitrogen results in a reaction Al + 1/2 N... |
