Synthesis of nanostructures from amorphous and crystalline phases

摘要

The nanocrystalline state is often viewed in terms of isolated nanocrystalline particles, but an equally important form of nanostructured alloys is based upon a dispersion of a high number density of nanocrystals or upon a bulk nanocrystalline microstructure. In a number of marginal glass-forming alloys with compositions that limit the solute content to <15 at.%, nanocrystal densities of 10~(21) to 10~(23) m~(-3) can develop during primary crystallization and offer exceptional magnetic and structural performance. There is also a remarkable thermal stability of the dispersed nanocrystal and amorphous matrix microstructure to significant change in size scale. Several proposals involving solute effects, phase separation or quenched-in nuclei and heterogeneous nucleation have been advanced to account for the high nanocrystal density. Alternatively, other approaches involving alloying by intense cold-rolling reveal that a deformation-induced amorphization can be achieved for marginal glass-forming alloy compositions. In other systems, a deformation-induced nanocrystal synthesis can be observed during the cold-rolling of amorphous ribbons. These developments represent a major level of microstructure control.