Nanostructure Synthesis During Devitrification and Deformation

Nanostructured materials are often viewed in terms of isolated nanocrystalline particles, but an equally important form is based upon a dispersion of a high number density of nanocrystals within an amorphous matrix. For example, from amorphous precursors, based upon marginal glass forming alloys with compositions that limit the solute content to <15at.%, primary crystallization can yield nanocrystals with sizes of 10-20nm and densities of 1021 - 1022 m-3. In comparison, typical devitrification reactions yield product phase densities of about 1018m-3. Since the unusually high nanocrystal densities offer exceptional performance in magnetic and structural applications, the genesis of the nanostructure is important. It is now established in Fe and Al base systems that the primary nanocrystallization reaction yields single crystal nanocrystals with little retained solute and that the nanocrystals are enveloped in a solute-rich layer that develops during diffusion-controlled growth. 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-based clusters or phase separation have been advanced to account for the high nanocrystal density, but recent crystallization kinetics analysis and microstructure examination provide new evidence for the role of the structural heterogeneities based upon medium range order (MRO) that can act as nucleation sites for nanocrystal synthesis and promote transient kinetics behavior. At the same time, the primary crystallization reaction can be controlleded by suitable solute doping to enhance nanocrystal densities up to 1023m-3 at refined sizes. The heterogeneous nature of the local atomic arrangements is reflected also in the response of amorphous alloys shear band formation and to deformation-induced synthesis of nanocrystals.

Alternatively, other synthesis approaches involving alloying by intense cold rolling of elemental multilayers reveals that a deformation-induced amorphization can be achieved for marginal glass forming alloy compositions. These features point to a central role for heterogeneous kinetics in the nucleation and the evolution of nanoscale microstructures.